Fresnel Lens Tips

Presumably, this same approach could be used with other cameras wherever a fresnel lens could provide added light to the user. Mr. May cites a Speed Graphic and Omega 45E as two cameras updated using this method.

My guess is the same light concentrating tricks might work on a slide table or even battery powered viewer too.

Another use for this fresnel lens is in your darkroom enlarger, where it serves as an ''auxiliary lens''. Mr. May gives an example using his Omega B22 medium format enlarger. Here again, the light re-directing power of the fresnel lens makes it easier to direct more light through your enlarger and onto the paper.

Another use for a fresnel lens is as a cheap telephoto adapter for a strobe. The fresnel lens will concentrate the strobe's light output into a narrower beam. You will be able to improve your strobe's reach by simply mounting the fresnel lens in front of the strobe head.

You can use a conventional loupe magnifier with the fresnel without problems too.

Mr. May suggested the following assembly technique:

Most cameras use clips to hold the ground glass against small bands so that the matte focusing surface duplicates the film plane. If the screws holding the clips in place are loosened, the thin field lens can be slipped under the clips without affecting focus quality. Maximum protection is given to the Fresnel surface by mounting that side against the ground glass. It is not necessary to have exact contact between the glass and the plastic.

If you camera does not permit this simple assembly method, it may be practical to cut the field lens to a convenient size and hold it in place with two pieces of double sticky tape in the corners.

The plastic field lens is durable enough to allow a focusing aid to be pressed against it to examine the focus. The faint lines of the field lens can be ignored when studying the image; they have no effect on the accuracy of the focus setting.

Mr. May's article is accompanied by two photos of the 4x5 ground glass of the same scene, identically exposed and processed. The non-fresnel ground glass is bright only in the center. The fresnel equipped photo is brighter and illuminated to the corners. Very impressive performance improvement for under a dollar!

Homebrew Strobe Fresnel Lens for Distance Shots

Most strobes are designed to cover 28mm or 35mm lenses at their rated distances. If you are using a 300mm lens, that wastes a lot of light on the sides! Using a fresnel lens, you can get up to 3 stops more light from the same strobe while using the telephoto lens (e.g., 300mm here). The distance between strobe and fresnel is adjusted to provide the desired coverage angles (and format shape - square fresnel for square format, rectangular for rectangular). Make sense?

Frankly, the biggest use I can see for this setup is providing a new capability for use with long telephoto lenses (esp. 200 to 300mm range). Where you might not be able to reach 60 feet before, now you might cover 120 to 150 feet or more easily! No extra battery power, while using the same low cost consumer or pro class strobe as normal.

Cost is just a few dollars for a fresnel lens (see above notes) and a light, homebrew strobe attachment (go high-tech, use rubber bands) with bright white insides. Adjust mounting on strobe, forward and backwards, while taking sample exposures and making notes. Determine which position on the strobe provides the best coverage and least light loss or drop-off. Cut the final holder to match this distance. You can also mark off a 200mm and a 300mm setting, so the same setup can be used with either long lens. Use your imagination! Enjoy!

Postings:

From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel lens question - answer from experience
Date: Wed, 04 Feb 1998

bladsnot@aol.com (BladSnot) wrote:

Just to further enlighten/confuse everyone: I just bought my first 4x5 and found the freznel and GG to be one piece (GG on one side, freznel on the other). It is installed with GG toward the lens, freznel toward the rear, with a piece of cear glass over the freznel. This arrangement sounds a little contradictory to the above advice. Additional enlightenment appreciated...

TIA,
Corky
bladsnot@aol.com

Some clarification, I hope:-)

The ideal, and what you would do if designing such a back from scratch, is to mount the field lens on the lens side of the ground glass. The ground surface of the of the ground glass and the grooved side of the field lens are incontact in the middle. Where a field lens is added to an existing back it is used on the viewing side of the GG so that the reference plane for focusing isn't changed. Generally, the grooved side of the field lens is set facing the ground surface so it will be separated by only the thickness of the ground glass. If you find that it works better the other way then use it that way.

The main idea is to minimize the separation of the fild lens and ground glass and to columnate the light from the lens before it gets to the GG. None of this is a big deal as long as the focus is accurate and the screen bright.

The combined screen you have may be intended to retrofit in existing cameras. The cover glass is a puzzle unless the screen itself is made of plastic. If that is so the glass is there to protect it from scratching. I would like to know what kind of camera this is.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

From: Joe McCary - Photo Response mccary@erols.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel lens question - answer from experience
Date: Fri, 30 Jan 1998

Richard Knoppow wrote:

... If the camera design is such that the Fresnel must go on the outside if the GG the ridged surface should still face inward, again to minimise the distance to the ground surface and to minimise internal reflections. Harald Finster wrote:

That's exactly what I did. As I mentioned in an earlyer posting, I had problems with internal reflections with this configuration. (Super-Angulon 47mm XL, Boss-Screen, Linhof fresnel lens) After turning the ridget surface to the outside (facing my eye) the problem of internal reflections almost disappeared. Is this due to the extreme wide angle of my lens? Any other explanation?

Harald

I have the Sinar Fresnel attachment to for the gg. It works well and is located on the outside of the GG. This is nice becaue sometimes you will want to remove it to see the edges with a wide angle Lens.

Joe

Ground Glass Sources:

From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Ground glass source?
Date: Wed, 04 Feb 1998

hermcam@mindspring.com wrote:

I am looking for a source for Ground Glass for large format camera use.

Thanks,

Henry

Edmund Scientific has ground glass up to 8x10 at reasonable prices. its fine grain and the right thickness.

They have a web site at http://www.edsci.com

If you don't have a catalogue they will help you identify the item on the phone. I boght some 8x10 sheets recently, I think they were about $20 US each.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

4x5 Ground Glass

I have had good luck with getting ground glass in 4x5 with or without corners cut for $7 plus s/h from Pacific Rim

bobm

[Ed. note: this is posted to show that separate commercial fresnels are available, for those who prefer this approach, and the relative costs..]

rec.photo.marketplace
From: rgraf@gate.net (Bob Graf)
[1] FS: 4x5 fresnel lens - Wisner
Date: Wed Feb 25 1998
FOR SALE
4x5 removable fresnel lens for Wisner field camera. Has attached bracket to insert and remove from behind ground glass. Cost $80. new direct from Wisner.

Sell for $35. including shipping.

If interested, please reply by email for address to mail check.

Bob G.

(gturco) wrote:
Hi,
Looking to buy a medium, (Approx 2 foot Dia.) Fresnel lens.

one out of an overhead projector? Check with a local repair shop, might have just what you need or could use.

Ed Eagleton
edeagle@worldnet.att.net

Date: Sun, 07 Jun 1998
From: Richard Knoppow dickburk@ix.netcom.com
Subject: Re: [Rollei] 50mm Distagon and Beattie Screens

At 02:32 PM 6/7/98 EDT, you wrote:

>Rolleisofts,
>
>It appears to me that my 50mm/SL66 Distagon produces considerably darker
>images at f/4 than my 80mm Planar. I use a Beattie screen.
>
>Does anyone know if this has something to do with the way light is projected
>on the screen and its construction?
>
>Thank you and Best of Light,
>
>Chris 

If the Beattie is a field lens it will work best with lenses of a particular focal length and some range around that. When the field lens is focused at the exit pupil of the taking lens it columates the light emerging from it and gives an evenly illuminated field when viewed on axis.

If the exit pupil is much in front or back of this point the illumination darkens in the corners. This is why cameras like the Nikon have a series of inserts for different focal length ranges of lenses.

----
Richard Knoppow
Los Angeles,Ca.
dickburk@ix.netcom.com

rec.photo.equipment.large-format
From: djm@my-dejanews.com
[1] Re: Fresnel or not?
Date: Mon Jul 20 18:19:03 CDT 1998

On my 5X7 Linhof I have plain ground glass. I purchased a flexible plastic fresnel that you buy for reading fine print. They cost about $2:00 here in Australia. I attached that fresnel using 6 small pieces of transparent double sided tape on the outside of the GG (ie on the side of the GG that I view from, not between lens and GG). It improves the viewing greatly with all lenses I use. (120mm to 305mm). At first I intended to remove the fresnel when required for critical focussing with a loup but have found this is not neccesary if I adjust the loup to focus on the GG surface the fresnel lens lines don't intrude too much. ( I made my own adjustable focus loup from a single eyepiece magnifier lens and some 1" threaded electrical conduit fittings. It does all I need.) Note that some of these plastic fresnel are better than others so try a few types as they are so cheap. I could not afford a Brightscreen so I did some experimenting. I found I could brighten the screen by about 1 stop by applying "Armorall" (tm.)to the GG screen on the ground side. (apply then polish off.) Armorall is a product used to improve the shine on plastic dashboards etc in cars. These 2 improvements have made a great difference to my comfort under the dark cloth.

Des
Melbourne Australlia

From: Jean-David Beyer jdbeyer@exit109.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel or not?
Date: Thu, 23 Jul 1998

Marcus Carlsson wrote:

> I´m building my own camera and naturally I need a ground glass.
> I havn´t looked through one yet and therefor I wonder if I should buy an
> ordinary or one with a fresnel.
>
> I know that the one with a fresnel makes the corner lighter, but is the
> other one actually so bad? (don´t you see the corners even with a black
> cloth over your head?)
>
> I have also heard that the fresnel makes it hard to focus, and since that
> is actually the most important bit I wonder if it is so much better to buy
> a fresnel?
>
> I would be very gratefull if anyone could give me a hint of what kind of
> glass I should buy. After all the fresnel is a lot more expensive.
>
> NB Money aren´t important...

I put a Fresnel lens on my Deardorff and it worked pretty well. However, when I switched to the Wisner T.F., I found it was unsatisfactory. This has nothing to do with the Wisner camera, of course. I bought the bag bellows for it so I could use a S.A.90mm f.8 lens. That is the real cause of the problems. On axis, the Fresnel lens provides more even illumination for focusing, and this is an advantage. But if the movements are used much, the illumination appears worse and dimmer than the ground glass alone. I later took it off. The reason I did not notice a problem with the Fresnel on the Deardorff is that it would not accept a bag bellows, so it was not practical to use lenses shorter than about 135mm.

--
Jean-David Beyer
Shrewsbury, New Jersey

Date: Tue, 8 Sep 1998
From: Kizhavana Unnikrishnan kunnikris@cas.org
To: rollei@mejac.palo-alto.ca.us
Subject: Re: [Rollei] Rolleicord III f/stop adjustment/brighter screen

Edmund Scientific sells a fresnel lens which fits Yashicamat and Rolleicord and Rolleiflex and can improve the brightness of the viewing screen.

Date: Wed, 9 Sep 1998
From: Kizhavana Unnikrishnan kunnikris@cas.org
Subject: Re: [Rollei] Rolleicord III / Rolleiflex Fresnel Lens

I believe it is the 2 1/8 x 2 1/4, cat. no. D30389, which they used to show in their old catalogs "fits YashicaMat." I bought this from their New Jersey store a few years ago to replace the original Rollei fresnel lens that cracked and broke in my Rolleiflex 2.8C. May I suggest that you call them and ask them whether this is the right one that fits 6x6 viewfinder.

From: wi0r@aol.com (WI0R)
Newsgroups: rec.photo.equipment.large-format
Subject: A groundglass/fresnal question
Date: 9 Sep 1998 20:04:02 GMT

I'm getting conflicting advice on where and how to place the fresnal lens in relation to the groundglass. Some say the frosted side of the groundglass goes closest to the camera lens, the concentric circle side of the fresnal lens goes next to the clear side of the ground glass. But by measuring and comparing this to my film holders, I don't see how this can be right. The Graflex website says the opposite, that for any graflok back, the fresnal lens goes in first (closest to the camera lens) with its clear side to the front, and the ground glass goes in next with its frosted side to the front and next to the concentric circle side of the fresnal lens. My measurements say this is the right way, but why am I getting advice to the contrary? Is there a difference of philosophy on this point?

Thanks
Troy

From: "skgrimes" skgrimes@ma.ultranet.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: A groundglass/fresnal question
Date: Wed, 9 Sep 1998

There is some difference between manufacturers. The Graflok type backs were original manufacture with the fresnel screen installed towards the lens from the groundglass. Many fresnel screens are aftermarket additions. The important thing that should be the same on all installations is that the frosted side of the groundglass should face the lens and the frosted side of the groundglass must be exactly the same distance from the lens as the film. Another way to say this is that the frosted surface of the glass must be in the same place as the film.

A measurement from the lensboard (with no lens in it) to the ground glass surface should be exactly the same as the same measurement to a piece of film in a filmholder inserted into the back.

In an aftermarket fresnel addition the easiest way to avoid displacing the ground glass is to install the fresnel behind (away from the lens) the groundglass.

--- S.K. Grimes -- Feinmechanik ----
http://www.skgrimes.com

From: stevehof@home.com (Steve Hoffmann)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel lens and focus shift
Date: Fri, 16 Apr 1999

Actually, on the Toyo Cameras the Fresnel is in the light path inside the camera. At least this is the way the store's 45AX was setup. That setup is what we copied to my CX. The Fresnel lens is clamped to the lens side of the ground glass with the ringed side of the Fresnel touching the ground side of the ground glass. I'll double check my loupes using your suggestion. Both are adjustable for focus.

Thanks, Steve

From: B Peters bill.peters@home.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel lens and focus shift
Date: Fri, 16 Apr 1999

Steve,

Reinstall the Fresnel so the grooves face the plain side - non lens side - of the ground glass. A Fresnel should never be placed on the lens side. To do this is respacing of the focusing screen would be required to correct the focus shift that you noticed. Even then the Fresnel in the light path causes an uneven focus shift across the ground glass.

It is EXTREMELY common to find Fresnels installed incorrectly by camera stores and others who should know better.

Bob's advice about correctly adjusting the focusing loupe is right on the mark as well. Do what we both suggest and you will get sharp images.

Bill Peters
Calgary, Canada

From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel lens and focus shift
Date: Fri, 16 Apr 1999

....

But, some cameras are intended for the field lens to be on the lens side of the ground glass. This is the way Graflok backs are made. The reference surfaces of the back are designed for the extra thickness and optical path length of the field lens. If the Toyo is designed this way this is the correct location for the field lens.

This is the ideal location for a field lens since it minimises the light scattering (halation) in the combination.

For other cameras where the fresnel is an addition you are right and the lens must be installed on the outside of the g-g. Installing it wiht the grooves facing the smooth surface of the g-g will eliminate any possibility of Newton rings from the two smooth surfaces being together.

>> Actually, on the Toyo Cameras the Fresnel is in the light path inside
>> the camera. At least this is the way the store's 45AX was setup. That
>> setup is what we copied to my CX. The Fresnel lens is clamped to the
>> lens side of the ground glass with the ringed side of the Fresnel
>> touching the ground side of the ground glass. I'll double check my
>> loupes using your suggestion.  Both are adjustable for focus.
>>
>> Thanks, Steve
>>
>> >How are you determining sharp focus? A fresnel placed on the back of a
>> >ground glass obviously can not effect the focus since the focus plane is the
>> >front side of the ground glass and the fresnel is no where near the  ground
>> >surface of the ground glass.
>> >
>> >Therefore the problem has to be how you focus.\
>> >For instance, have you adjusted the focus on your loupe so it is  focused on
>> >the ground side of the ground glass? Or are you focusing with a non
>> >adjustable loupe placed directly on the top of the fresnel?
>> >
>> >If so you are automatically out of focus by the combined thickness of the
>> >ground glass and the fresnel.
>> >
>> >The proper procedure is to take a quality loupe with adjustable  focus, and
>> >only a quality loupe that can be focused, remove the lens. Point the  camera
>> >at a light source. Focus the loupe till the grain of the ground glass is
>> >sharp.
>> >
>> >Now you are focused at the film plane. Put on a lens and you should have
>> >accurate focus.
>>
>> Steve Hoffmann

From: "Bob Salomon" bobsalomon@mindspring.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel lens and focus shift
Date: Fri, 16 Apr 1999

(Richard Knoppow) wrote:

>  If the Toyo is designed
> this way this is the correct location for the field lens.
>   This is the ideal location for a field lens since it minimises the
> light scattering (halation) in the combination.

Actually this is not the ideal position as some want to be able to focus without the intrusion of the fresnel rings so they can remove the fresnel for focusing without the rings intrusion and then replace it when they need the benefits of the fresnel.

Early fresnel lenses were easily damaged and were frequently mounted under the groundglass to protect them. Today that is not a problem.

--

HP Marketing Corp. U.S. distributor for Amazon, Braun, Gepe, Giottos, GO Light, Heliopan, HP Combi Plan T, Kaiser fototechnik, KoPho cases, Linhof, Pro Release, Rimowa, Rodenstock,Sirostar 2000

[Ed.note: cheap bright screen alternative?]
Date: Thu, 17 Jun 1999
From: ron baker rlbaker@midusa.net
Subject: RE: [BRONICA] WTB: Split Image Screen

If you want a really bright screen without spending any money try putting some vasaline on a standard screen, making sure it is distributed evenly and then with a dry cloth wipe it until it looks like it's all off. I do this with all my large format ground glass's even a beattie screen on my 4x5. I learned this little trick back in the 50's before the bright screens were invented. If you don't like the way it works simply wash it off with soap and water.

Ron

rec.photo.equipment.large-format
From: "Bob Salomon" robertsalomon@mindspring.com
[1] Re: fresnel placement
Date: Wed Aug 18 13:20:02 CDT 1999

No they don't!

Many decades ago they placed the fresnel inside. All recent Linhof cameras have clips to place the fresnel on the outside side of the ground glass. That way it can be removed if the concentric rings are annoying with a loupe and both the ground glass and the fresnel are user replaceable.

--
bobsalomon@mindspring.com http://www.hpmarketingcorp.com/
HP Marketing Corp. Amazon, Braun, Gepe, Giottos, Heliopan

From Rollei Mailing List:
Date: Wed, 24 Nov 1999
From: Jack Casner jackinkc@earthlink.net
Subject: [Rollei] TLR Brighter Viewfinder UPDATE

roland wrote:
A few months ago in Canera Shopper, Mike Roskin indicated that a fresnel  screen was available from Edmund Scientific.   Since my 2.8D suffered  from a dark viewfinder, I contacted Mike to find out about the screen.  He indicated that you just plop it into the viewfinder, grooves down, on top of the present focusing screen.  I did so and was pleasantly surprised with the result.   The brightness is enhanced enough to make focusing in dim light and in bright sun a successful experience.

The screen sells for $12.75 plus S&H from Edmund Scientific 856 573-6250.

The stock number is H30389 and the size is 2-7/8 X 2-1/4.

Jack L. Casner
Kansas City, Missouri

From ROllei Mailing List:
Date: Tue, 01 Feb 2000
From: Roland Schregle schregle@ise.fhg.de
Subject: Re: [Rollei] Fresnel lenses

Jakob Sigurdsson wrote:

> Hi,
>  I bought a prewar Rolleicord a few days ago,
> and have been using it. I was wondering if
> I could brighten up the screen around the edges
> with a fresnel lense. I know of Beattie screens, which
> cost +100$ .
> I also came across http://www.fresneltech.com/ which
> make a lot of fresnel lenses at a reasonable price.
> Can I brighten up the edges of my screen with a
> normal fresnel? 
>
> Any thoughts, info on this matter appreciated

My 2.8C came with a lousy fresnel lens. Sure, it was bright, but totally mushy and difficult to focus. I chucked it out! I prefer a sharp viewfinder - even if it it's dim.

--
Roland Schregle
Fraunhofer Institute for Solar Energy Systems

From Rollei Mailing List:
Date: Sun, 30 Jan 2000
From: Jakob Sigurdsson jakobs@decode.is
Subject: [Rollei] Fresnel lenses

Hi,

I bought a prewar Rolleicord a few days ago, and have been using it. I was wondering if I could brighten up the screen around the edges with a fresnel lense. I know of Beattie screens, which cost +100$ .

I also came across http://www.fresneltech.com/ which make a lot of fresnel lenses at a reasonable price. Can I brighten up the edges of my screen with a normal fresnel?

Any thoughts, info on this matter appreciated

Regards,
Jakob S.
jakobs@decode.is

From Rollei Mailing List:
Date: Sun, 30 Jan 2000
From: Richard Knoppow dickburk@ix.netcom.com
Subject: Re: [Rollei] Fresnel lenses

A fresnel lens dropped on top of the ground glass can make a big difference in corner brightness. Rollei used to sell such a fresnel under the name Rolleigrid, they turn up used sometimes. No modification of the camera is needed to use one and its easlily removable when you don't want it. Similar fresnel lenses were sold by Kodak (Ektalite screen as used on Speed Graphics) and by another company who's name is escaping me at the moment (Parker's effect?).

The Rollei version has a clear spot in the center which is helpful for precise focussing. I can't tell if this company makes a similar type of screen.

The PDF catalogue BTW is about as good a primer on Fresnel lenses as I have ever seen. I recommend downloading it (its a PDF document) and printing it out.

They also appear to make sizes suitable for large-format cameras. The prices are very low compared to Edmund. I can't make out whether they sell in single quantities.

----
Richard Knoppow
Los Angeles,Ca.
dickburk@ix.netcom.com

From Rollei Mailing List:
Date: Sun, 30 Jan 2000
From: Roland Smith roland@dnai.com
Subject: Re: [Rollei] Fresnel lenses

I bought some fresnel lenses from Edmund Scientific that are the size to drop into a Rolleiflex with the grooves down but on top of the existing lens. It does brighten the viewfinder. The stock number is 303.89 and they sell for $12.95 plus shipping making a landed cost of around $20.00.

Edmund Scientific is reached at 800-728-6999 or www.scientificsonline.com.

Roland Smith

Date: Thu, 17 Feb 2000
From: "Heinz Grau" heinz.grau@bluewin.ch
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Focus shift with fresnel?

You are right, the correct place for a conventional fresnel lens is on the inner face (towards the lens), but it is important that the ground glass surface is not moved away by the fresnel thickness, i.e., the fresnel must be mounted so that it sits on top of the ground glass which is left at its defined location.

Otherwise, you could use a combination glass which has a groundglass side to be mounted towards the lens (as usual) and a fresnel side towards your eye. Since it is not mineral glass, you better protect the fresnel (eye-)side with a plain glass on which you can have the grid etc., to avoid scratches from your loupe.

I bought my 4x5 glass from Waltuch Basel @ Sfrancs 250.00 incl. protective glass (waltuch@compuserve.com).

Good luck

Heinz

...

> I have a tough old MPP 4x5 folder and am about to have the old
> murky ground glass replaced with brighter, gridded material. I
> am also considering fitting a fresnel to help with viewing in
> the corners. I have been told that the correct place for the
> fresnel is on the inner face of the glass, between the ground
> glass and the lens. If the ground glass and film planes
> coincide at the moment, will the introduction of a fresnel
> cause a focus shift that will disturb this coincidence?
>
> Keith Davison  

Date: Thu, 17 Feb 2000
From: James Meckley jmeckley@pegasus.cc.ucf.edu
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Focus shift with fresnel?

....

Yes it will. In your situation the correct place for the Fresnel is on the outer face of the ground glass (non-lens side).

If you place a Fresnel on the lens side of a properly mounted ground glass, you will introduce a focus shift of about 1/3 the thickness of the Fresnel into the system... the GG will no longer be in the right place. The only time a Fresnel should be on the lens side of the GG is when the entire system was designed for it to be there.

Jim Meckley

Date: Thu, 27 Jan 2000
From: Robert Monaghan rmonagha@post.cis.smu.edu
To: Robert Monaghan rmonagha@post.cis.smu.edu
Subject: [Rollei] TLR Brighter Viewfinder UPDATE (fwd)

roland wrote:
A few months ago in Canera Shopper, Mike Roskin indicated that a fresnel  screen was available from Edmund Scientific.   Since my 2.8D suffered  from a dark viewfinder, I contacted Mike to find out about the screen.  He indicated that you just plop it into the viewfinder, grooves down, on top of the present focusing screen.  I did so and was pleasantly surprised with the result.   The brightness is enhanced enough to make focusing in dim light and in bright sun a successful experience.

The screen sells for $12.75 plus S&H from Edmund Scientific 856 573-6250.

The stock number is H30389 and the size is 2-7/8 X 2-1/4.

If you are interested in doing this, you better order now!  This item is discontinued but a few are still available.

Jack L. Casner
Kansas City, Missouri

From Rollei Mailing List:
Date: Tue, 28 Dec 1999
From: Richard Knoppow dickburk@ix.netcom.com
Subject: Re: [Rollei] Focusing screens

you wrote:

>At 05:13 PM 12/28/1999 +0000, Niels Vandrup wrote:
>>
>>   A little while ago, I thought that I was going to buy a Beattie or
>>Maxwell screen. But now I'm confused. Can anyone explain why some people
>>are having trouble focusing on a "modern" screen?
>
>It isn't "some people":  bright screens, through the laws of optics, do not
>focus as accurately as do "dim" screens.  Some folks prefer a
>fuzzier-focusing but brighter image, others do not.  It is a matter of
>choice as to which you use.
>
>The basic bright-screen technology is nothing new:  it has been known for
>sixty or seventy years, or more.  But, in the days of really slow films,
>precise focusing was more important due to the very slow shutter speeds
>mandated.  Now, with faster films, you can often accomodate a faster
>shutter speed and smaller aperture for a similar picture which would have
>been a wide-open tripod shot a generation back.
>
>Marc
>
>msmall@roanoke.infi.net  FAX:  +540/343-7315

I don't know what type of technology is used in the Maxwell screen so my comments may not apply to it.

It may be that some types of screens don't have a definite plane of focus, i.e. they have some diffusion. I suspect the type which uses a layer of wax may do this.

Fresnel lenses may also have some amount of diffusion, or at least confusion due to the discontinuous surface. This would make focussing more difficult since there might not be a definite plane of focus.

The simple overlay type fresnels like the Rolleigrid have a clear spot in the center for focussing directly on the ground glass.

The grain of the ground glass can also make a difference. The finer the grain the more even the illumination due to more scattering of light from the lens. Fine grain screens are also easier to focus on because the grain structure doesn't interfere so much with the detail of the image. IN theory, a coarse screen will be brighter in the center but dimmer at the edges.

The most accurate focus is gotten by "parallax" focussing on a clear screen with a mark at the focal plane, not a practical method where focussing must be done quickly.

The effect of a fresnel lens, or an actual field lens as found on Exakta cameras, is to bend the light from the lens back to an angle where it will enter the eye. The image is not actually brighter at the center, in fact it may be a little dimmer, but the corners are much brighter because light, which would normally follow the direction of the cone of light from the lens, is bent back toward the eye.

The focal length of the field lens must be right to focus light from the avarage position of the camera lens toward the eye or the corners will get even dimmer, which is why a selection of screens is usually available for cameras with interchangable lenses. The use of a "normal" fresnel with a wide angle lens may make things worse rather than better, than with a plain ground glass.

----
Richard Knoppow
Los Angeles,Ca.
dickburk@ix.netcom.com

Date: Wed, 18 Aug 1999
From: "Ralph W. Lambrecht" lambrecht@btinternet.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: fresnel placement

lam wrote:

> Hi all
>
> Just wondering, if a fresnel lens is used, it's usual placement would
> be between the groundglass and the taking lens right ?
>
> Wouldn't this effectively move the groundglass further away from the
lens
> ( as the fresnel would displace the groundglass )
>
> Is the focusing image still formed on the groundglass
> or is it now formed on the fresnel ?
>
> if it is formed on the groundglass, which has been displaced by the
> fresnel, wouldn't the focused image on the groundglass be slightly
> out from the image on the film plane ? ( since the groundglass has
> been displaced by the fresnel )
>
> Thanks
> Lam

A very interesting and also confusing subject.

1. Linhof suggests to place the fresnel lens between the ground glass and the viewer. 2. Toyo places the fresnel lens between the ground glass and the taking lens.

In any case, the image is always formed on the ground glass and that needs to be in the same plane as the film in the film holder. The function of the fresnel lens is to even out the illumination differences between the center and the corners of the groundglass image which occur mostly in wide angle applications. Its function is not to focus on.

There are some pros and cons for both methods and both methods are used by major camera manufactures.

In case 1 (Linhof suggestion) is is very easy to take the fresnel lens off when not needed or when it needs to be replaced since the groundglass is not moved and stays in the film plane. For the same reason it is the easiest way to add a fresnel lens since no groundglass adjustment is required. However, focus may be confused since it is possible to focus on the ridges of the fresnel lens. (I did not find this to be a problem)

In case 2 (Toyo suggestion) the groundglass protects the more fragile fresnel lens and a focus confusion is not possible. However if a fresnel lens is added this way then the groundglass may be pushed back by the fresnel lens and is therefore moving away from the film plane.

Here is my suggestion, if your camera came with a fresnel lens, leave it where the manufacturer wants it. If you are adding one to your camera place it according to Linhof's suggestion.

Both methods work if the groundglass is at the proper film plane location.

Date: Mon, 18 Oct 1999
From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel Question

"Jon Judson" jjudson@spamstuff.erols.com wrote:

>Richard Knoppow said:
>
>"If the focal length isn't right for the focal length of taking lens and
>viewing distance it will focus at the wrong distance and make the screen
>margins even darker than with a plain ground glass."
>
>"The closer then screen comes to a true diffusing surface the more even the
>illumination and the less variation with focal length and lens postion
>relative to the screen, but the darker the overall image will be."
>
>Richard,
>
>Considering that the image is already pretty dark (except for the middle  hot
>spot) with the Wista fresnel/groundglass combo, would plain ground glass be
>a better choice for using the 90mm?  Would I get even illumination from edge
>to edge (or, at least, somewhat even)?  Are there fresnels that are matched
>to lens sizes?  Today's photomaking was at an 1800's abandoned military
>fortress.  The sky was heavy overcast and the area I shot was very dim.
>Composition was a nightmare with my setup.  Do you have any advice for
>something that would be a little better?  Dark I already have.  Especially
>outside of the center 3cm of the groundglass.  Anything would be an
>improvement at this point.
>
>Also, would just any groundglass/fresnel/bright-screen work?  I've heard
>something that the Bosscreens don't work with Wistas.  Something about the
>thickness.  I would assume that the screens would need to be of the same
>thickness as the current one.  Are all focusing screens of a standard
>thickness?  Also, where does the image actually focus?  Is it on the surface
>of the groundglass nearest the lens?  Or is it on the fresnell in the
>grooves?  How does that impact changing screens?
>
>Thanks again.
>Jon

A fesnel lens should have a focal length about equal to the distance the lens is going to be from it most of the time, usually a bit more than the focal length of the taking lens. Light originating at exit pupil of the lens is then columated or made somwhat convergent by the fresnel. The effect is to make the screen evenly bright all over provided both the eye and the taking lens are centered on the optical axis of the fresnel. If either is off, as when you view the screen at an angle, the screen becomes rapidly dimmer. (This is the effect of the new style traffic signals which use a focussed beam to limit the angle over which they are visible).

When a wide angle lens is used its too close so the fresnel lens does not focus its light where its supposed to go. As a result the corners look even dimmer than with a regular ground glass. Sometimes if you get very close to the screen the corners fill out a bit but the fresnel must match the lens, at least approximately. This is why many SLR cameras have interchangable screens to match different ranges of lens focal length.

For some uses a very fine grain conventional ground glass is the best choice. Its gong to be relatively dim all over but the ratio of center to corners will be better than for a mismatched fresnel or for a coarse grain ground glass. The fine grain glass is also the best choice when lens movements are used. The dim image is just something that comes with the territory. I don't have a Boss Screen but understand it is a wax layer between glass layers so it should act as a very fine ground glass. Likely someone out there knows exactly how they are constructed and can post someting more authoritative (plus I'm curious).

A Fresnel lens is made up of sections of a lens in the form of annular sections of a prism i.e. circular prisims. The angle of the prisims increases as you move from the center to the periphery of the lens. Although they are "flat" they emulate a three dimensional lens although the image quality isn't very good. Fresnal lenses were first devised for use on lighhouses. The construction is much lighter and thinner than a conventional lens. Field lenses (to give it its proper functional name) for ground glass finders can go on either side of the ground glass. The closer the ridges are to the ground surface the better since that tends to minimize internal reflections. A few cameras are designed for this, the Graflok back for the Speed/Crown Graphic being an example.

However, quite satisfactory results can be had by mounting the field lens on the outside of the ground glass with the ridges facing the ground surface. In both cases the ridges are facing the ground surface although the lens itself is facing in opposite directions with regard to the front of the camera in the two examples.

This type of lens can be made very cheaply of plastic by molding. The condensers in overhead projectors are made this way. Often they are made of two fresnel lenses cemented face to face, just as glass lenses are used in pairs to reduce spherical aberration in condensers, which can result in uneven illumination. I've even seen compound fresnel condensers which were bent slightly, presumably to reduce coma.

The image is always formed on the diffusing surface of the ground glass. However, when the lens is placed between the ground glass and the taking lens it has two effects. One is to mechanically displace the ground glass if its mounted directly in place of it. Obviously, this is not a problem if the back is designed for the combination, nor is a problem if the fresnel is simply placed in contact with the ground glass without removing it.

The fresnel should also extend the light path slightly. A plane thickness of a transparent substance will extend the effective light path a distance depending on its index of refraction compared with air. Since most glass has an index of around 1.5 it extends the distance by about 1/3rd of its thickness. Plastic has a lower indes than most glass so the displacement due to the thickness of a plastic fresnel is less than for a piece of glass the same thickness.

None of this is of consequence when the fresnel is on the viewing side.

Boy I am getting wordy:-( I hope that among them somewhere is something helpful.

I think you need to find a good dark cloth:-)

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

Date: Sat, 08 Apr 2000
From: "B. Buckles" buckles@home.com
Newsgroups: rec.photo.technique.nature
Subject: Re: Lepp-project-a-flash

The Better Beamer can be purchased direct from the maker by calling: 847-438-3587. I'm holding one in my hand. It's great and so much better the Lepp cardboard construction.

Bob

Roy wrote:

> I sold my Lepp Project-a-Flash and bought a Better Beamer. Much superior.
> Very portable (folds flat) and much easier to align. You can get a Better
> Beamer through Arthur Morris' web site at http://www.birdsasart.com.
>
> Roy
> http://bresfam.tripod.com
>
> Renedeheer  wrote in message
> Renedeheer renedeheer@aol.com wrote in message
> > I know of the existence of the above, but i can recall the existence of
> another
> > look-a-like flash enhancement. Its more portable than the Lepp one. Does
> anyone
> > have any idea what the name of the the other make is and where can i get
> > them....was it really right stuff or Kirk enterprises???
> > many thsk
> > Rene de Heer

From: flexaret2@aol.com (FLEXARET2)
Newsgroups: rec.photo.equipment.medium-format
Subject: Re: Bronica Focusing
Date: 06 May 2000

from: flexaret2@aol.com (Sam Sherman) 5-6-2000

You can remove the fresnel and the image will be sharper on the groundglass.

You will have to calibrate your camera accordingly without the fresnel, which gives a brighter image in the corners. I have kept all of my fresnels in.

If you haven't already -- see my related articles on Bob Monaghan's Classic Bronica website-

http://www.smu.edu/~rmonagha/bronica.html

[Ed. note: Mr. Knoppow is a noted camera repair expert...]
d From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Date: Thu, 27 Jul 2000
Subject: Re: Burke & James questions

brownt@flash.net wrote:

>I have a chance to purchase a B&J; 5x7 view camera from a colleague. He
>bought it used long ago (longer than he can remember) and never used
>it. My last experience with LF was 30 years ago when I was about 10,
>assisting my uncle, but I digress...
>
>This camera is the grey painted wood and seems to have had little use.
>The bellows is red and in great shape. Controls work smoothly. It has a
>Wollensack 190mm f4.5 Enlarging Raptor on a sticky Packard shutter. I
>want to fit modern lenses and shutters. The lens board measures 5 1/8"
>square. Is this the actual size of standard 6x6 boards? There are no
>film holders. I want to use a roll film back. Are 5x7 backs common? Is
>there a reducer to use 4x5 backs? There is a viewing hood but you can't
>see the whole image at once on the GG. Suggestions for a better GG?
>Fresnel? I would like to get this up and running if it doesn't take too
>much work and $$.
>
>Tim

The lens board is the size you measure. 6x6" boards are found on 8x10 B&J; cameras. Lens boards are not hard to make if you can't find any.

You can probably unstick the Packard by cleaning it. The shutter blades are hard rubber so be careful of the solvent you use. Trichlorethane or pure Isopropyl alcohol is OK. Don't heat the blades, they will melt.

There are all sorts of viewing screens on the market. Once cheap way to get improved uniformity is to make a Fresnel screen out of a Fresnel desk magnifier. You can get them from places like Office Depot or Staples. Cut it down to size by scoring the plastic and snapping. Place it over the existing ground glass with the grooved side facing the glass. You will want to be able to take it off for fine focusing or when using movements. If the GG is very coarse you will get a much brighter image if you replace it with finer glass. B&J; used pretty fine glass but this may not be the original glass.

Better Fresnels are available from Edmund Scientific.

B&J; made 4x5 reducing backs for these cameras, check around with LF specialist dealers like Steven Shuart to see if you can find one.

If you want to use a roll film adaptor you will have to make a back with a Graflok back. This is simply a sheet of wood to fit the back of the camera (see the back you have for how to make it) with a graflok mounted on it. This will give you a 4x5 back which is compatible with all sorts of accessories. The Graflok back comes wth a Fresnel field lens built-in.

5x7 holders are not as common as 4x5 but there are plenty of them out there. If you buy used holders look for Riteway holders. As with 4x5 they are the best of the older holders by a lot. Because 5x7 is not as popular as 4x5 used holders sell for about the same price as 4x5.

A 5x7 camera with a 4x5 back can be very useful since it usually has considerably more bellows draw than a 4x5 camera without being a whole lot larger.

B&J; cameras were well made. They have a reputation for being a little sloppy but usually can be tightened up without much trouble.

BTW, under the battleship gray paint you will find beautiful Maple.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Date: Wed, 09 Aug 2000
Subject: Re: Ground Glass / Fresnel Placement

"Kevin C. Kelly" kckelly@dallas.net wrote:

>I am planning to make my own view camera.
>
>The information I have on film holders indicates that the depth from the
>outer surface of the holder to the film 'backplate' should be the
>following:
>
>                    4x5       0.197    ±0.007
>                    5x7       0.228    ±0.010
>                    8x10     0.260    ±0.016
>
>Can anyone confirm that these distances are ANSI standards?
>
>With a plain ground glass focusing screen you would  want your focus
>surface placed at the same distance minus your film thickness ( 0.007
>approx.), right?
>
>But what about using a Fresnel, Ektalite, etc. before the ground glass
>to improve ground glass viewing?  Is the lens side of the ground glass
>still your focus point or do you need to compensate for the Fresnel
>lens?
>
>What about using a Beattie Intenscreen - are they set up like a plain
>ground glass?

These are the ISO/ANSI standards. They do NOT include the film, which subtracts .007" from the distance for most sheet film.

The ground surface of the ground glass should be the distance of the emulsion from the film, i.e., the above distances minus the thickness of the film.

When a fresnel lens is used on the lens side of the optical path its thickness must be compensated for. The effective thickness is dependant on the index of refraction of the material. For most plastics it will be between 1/3 and 1/5 the physical thickness. The lens may also have some effect.

When mounted on the viewing side of the GG there is no effect. I can't answer for the Beattie screen, check with them about mounting.

Fresnel lenses will make the GG image brighter and more uniform _provided_ the focal length is right for the lens being used _and_ the whole path is aligned. When camera movements are used a Fresnel may actually make things worse.

While in theory the ideal place for the Fresnel is inside the camera in practice its often convenient to be able to remove it easily, so outside mounting is often better in actual use. The inside mounting, with the prism surface against the ground surface, results in the least internal reflections but the actual difference in performance between inside and outside mounting is slight.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

Date: Sat, 12 Aug 2000
From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Ground Glass / Fresnel Placement

John Hennessy northbay@directcon.net wrote:

>With the fresnel lens on the outside, one removes it for focussing I
>presume?

Focusing is often easier without it. Being on the outside gives you the option of removing it.

When using wide angle lenses where the focal length of the fresnel may not match very well the corners may be brighter without it, or you may want two fresnel lenses of different focal lengths to approximately match the taking lens FL.

When using swings or tilts the image may be brighter wihtout a fresnel since it will tend to concentate the light in the direction of the taking lens optical axis, which will no longer be the viewing axis. Although the performance of the fresnel may be a little better when under the GG there are good practical reasons for placing outside.

Note that the texture of the ground glass makes a big difference in corner brightness. Coarse glass will have a very definite hot spot and dark corners. Very fine grained glass may make critical focusing a little more difficult but the best compromise is toward finer grain glass.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

From: flexaret2@aol.com (FLEXARET2)
Newsgroups: rec.photo.equipment.large-format
Date: 10 Aug 2000
Subject: Re: Graflex slr's and fresnel screens

You can certainly put a fresnel screen (shiny side up) on the top of your groundglass in a Graflex Reflex. I did this with my Graflex Reflex B- 4x5, years ago and it is sharp as a tack.

The exact plane of focus is on the underside of your groundglass screen nothing above it will change anything. I just taped the screen in place with some black masking tape. It was a wonderful improvement to the brilliance of my screen and really aids in focusing.

- Sam Sherman

From: "Peter De Smidt" pdesmidt@fdldotnet.com
Newsgroups: rec.photo.equipment.large-format
Date: Wed, 16 Aug 2000
Subject: Re: Fresnel Lens Revisited

As far as I understand it, the less diffuse your ground glass, the brighter the image and the less even the illumination will be from a given viewing point. "Bright screens" use less diffusion, and, as a result, have to have a fresnel built in to even out the light. They are definitely brighter than regular screens. I replaced the screen in my old Rollei TLR with one from Beattie, and the difference was amazing. The problem with bright screens, though, is with the fresnel. Under some conditions (fl of lens) they can obscure detail, with makes focusing harder. With the Rollei, though, this trade off was definitely worth it.

Here's a cheap way to see if you'd like a bright screen. Get an appropriately sized piece of anti reflection glass, the kind that's acid etched on one side. An 8x10 piece cost me around $5. Then get a cheap fresnel page magnifier from walgreens, walmart... Replace the ground glass with the anti reflection glass. If you take a look under the darkcloth, you'll see a very bright circular image about the size of the rear element. Next, put the fresnel on the outside of the glass--with a little cutting, you can make it so that you can pressure fit the fresnel, and take it off at will. With the cheap fresnel that I bought, this evened the light out considerably. At around 12" distance from the gg, roughly 75% was brightly illuminated. A short movement of the head lightened up the other areas. By moving about 3 feet back, the whole ground glass was evenly illuminated and *very* bright! Obviously, this distance is dependant on the FL of the Fresnel. There are probably significantly better ones than I got for a few bucks, such as the ones sold by camera manufacturers or Edmund Scientific. Experimentation is in order.

While the brightness of this setup was spectacular, it did make critical focus with a loupe a little harder. First, the cheap fresnel was quite coarse and obscured detail a little. That's why I think that you should compose with it on, and then take it off for critical focusing. Second, while pretty fine grained, the picture frame anti reflection class as a fine pattern on it: it looks *very* much like expensive anti newton glass. Still, I thought that I could focus well enough. I'll report back latter a little more extensively since the lens that I had mounted on the camera was a Verito soft focus lens at F4, that might have obscured detail too. OK, I'm sure that it did.

Here's another possibility: as I said, the glass looks remarkably like Anti-newton glass. If you are having problems with Newton's rings, you might want to try a sheet of this cheap stuff before (or instead of) the really expensive anti newton glass sold for enlargers. I haven't tried this yet, but I will.

Peter De Smidt

Eric Woodbury woodbury@ix.netcom.com wrote

> I used a good darkcloth velcro'd all the way around the groundglass.  That has
> worked better than any fresnel.  Works with all lenses and with all tilts,
> swings, and the like.
>
> E
>
> Terry Jon Lisansky wrote:
>
> > Hello All,
> >         I've just started using my 8x10 and I would like to brighten up
> > the image.  The lens has a max opening of 6.3.  My 4x5 has an internal
> > fresnel screen and works great.  Edmund Scientific is about 45 minutes
> > away and I thought I'd run up there and buy one.  There is one on  their
> > web site that's the right size and has a bunch of lines/inch for less than
> > $60 bucks.  Of course it's going on the outside.  I'm interested in
> > anyone's opinion about what they used to brighten the gg image....tnx
> > --
> > Terry Lisansky - N3JJB                          http://terry.cx

From: Doug Paramore dougmarydo@alaweb.com
Date: Wed, 16 Aug 2000
Newsgroups: rec.photo.equipment.large-format
Subject: Re: How to install fresnel question

The Fresnel lens usually goes behind the ground glass (toward the photographer) with the lens grooves facing the ground glass, smooth side out. A few of the older cameras put the Fresnel in front, but the modern ones seem to place it in the rear so it can be removed easily. If you place the Fresnel in front of the ground glass in a camera that was not designed that way the focus will be off the distance equal to the thickness of the Fresnel.

Date: Wed, 23 Aug 2000
From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Frenel Question

dkfletcher@aol.com (DKFletcher) wrote:

>I cane across a 4x5 fernel lens and an interested in using it on my Cambo-Wide.
> My stupid question is I can just pull my ground glass, and instal the frenel
>and replace the GG?  Is that right?  Will that change my focus at all?
>
>Seems too easy....
>
>thanks, please email to dkfletcher@aol.com

No, the Fresnel does not replace the GG. For cameras designed for a ground glass the fresnel should go on the outside with the grooves facing the GG. There will be no difference in focus that way.

Ideally, the Fresnel should go inside with the grooves facing the ground surface but that does change the focal plane and is not easy to correct. The difference in performance between the two positions is negligible. Having the Fresnel on the outside has the advantage of making it easily removable when its use makes things worse as when using camera movements or very wide angle lenses.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

From Rollei Mailing List:
Date: Thu, 21 Sep 2000
From: Jonathan Prescott jonpre@uswest.net
Subject: [Rollei] Fresnel Screen

From: ShadCat11@aol.com

> I prefer the MX-EVS, but the unaided viewfinder is too dim in all but the
> best light. Rollei made a Fresnel screen that fit over the ground glass, and
> similar ones have been reported available from Edmund's Scientific.

I placed an Edmund's Scientific fresnel over my 2.8C screen and it did a wonderful job brightening things up. But I kept noticing that my pictures were misfocused. I did some testing by focusing on a fixed object with and without the screen installed. The object was focused when the knob read 1.2m w/screen and 1.1m w/o screen. Since removing the screen the focus problems have disappeared but I can't see diddly around the edges of the ground glass. Looks like its time to save my pennies for a decent screen.

Jonathan

From Rollei Mailing List:
Date: Fri, 22 Sep 2000
From: Richard Knoppow dickburk@ix.netcom.com
Subject: Re: [Rollei] Fresnel Screen

....

Putting a Fresnel _over_ the ground glass should not change the focus. I suspect what happened is that the ground glass got pushed down against its loading springs and popped back when you took the Fresnel out again. Try the screen again but pay close attention to how it fits against the ground glass.

Rollei screens are undercut just a little where they go under the little spring things that hold them in place. You might have to file some tapered areas where the screen fits under them to make it fit.

I have also found that very occasionally I can push the ground glass down accidently when installing my Rollei screen and for some reason it gets stuck. Simply pushing it gently with my finger gets it back where it belongs.

I find with the Rolleigrid that it brightens up the image, especially the corners, but also increases reflections from outside light under some conditions unless a hood is used.

I think the Edmund Fresnel has a clear spot in the center. The Rolleigrid does and it makes precise focusing easier.

----
Richard Knoppow
Los Angeles,Ca.
dickburk@ix.netcom.com

From: ncongdon@my-deja.com
Newsgroups: rec.photo.equipment.large-format
Date: Wed, 29 Nov 2000
Subject: Re: Large Fresnel

Bill Maxwell of Maxwell Precision Optics can certainly handle this, and you will be guaranteed to be delighted with the results, though he is neither cheap nor quick:

1-404-244-0095

Maxwell
3158 caintal ct
decatur ga 30033

Alternatively, Wisner (http://wisner.com/,(800) 848-0448) has a source for Fresnels up to 20X24, which I gather are not as fine-grained, but probably a fair amount cheaper (I think the 16 X 20 was ca $200, or maybe that was the 20 X 24.)

Good luck,

Nathan

sanking@hubcap.clemson.edu (Sandy King) wrote:

> Anyone know of a source for large fresnel lens? I am looking for one of
> about 16X20" size of approximately 14" focal length.
>
> Sandy King

From: jess4203@aol.com (Jess4203)
Newsgroups: rec.photo.equipment.large-format
Date: 30 Nov 2000
Subject: Re: Large Fresnel

Sandy:

I see a lot of RV's running around with something that looks like a large fresnel lens mounted to the back window, I guess to see a wider angle. You might try an RV supply place and see if they have such and if it is appropriate.

HTH,
Roy

From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Date: Sat, 02 Dec 2000
Subject: Re: Large Fresnel

jess4203@aol.com (Jess4203) wrote:

>Sandy:
>
>I see a lot of RV's running around with something that looks like a large
>fresnel lens mounted to the back window, I guess to see a wider angle.  You
>might try an RV supply place and see if they have such and if it is
>appropriate.
>
>HTH,
>Roy

You can also get fairly large, but rather coarse, fresnel lenses at office supply places. They are sold as large magnifiers. I tried one for an 8x10 camera, it worked- sort of. Couldn't loose for a couple of three bucks.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

From Nikon MF Mailing List:
Date: Wed, 28 Feb 2001
From: Rick Housh rick@housh.net
Subject: Re: Home brew flash extender

Lars wrote:

>I have been thinking about making a flash extender by fitting a fresnel  lens
>infront of my flash (like the BETTER BEAMER) - but cheaper of course and
>perhaps even better yet...
>I have been looking for a source of fresnel lenses and wondered whether  an
>Apollo full page magnifier is what I am looking for. Office Depot have  them
>(http://www.officedepot.com/shop/catalog/sku.asp?ID=327016&LEVEL;=SK&SID;=PE6L
>PRSMBVVR9GSRD0PACQRXF4K2BJ9F&PP;=14) but there is no info on whether this  is
>indeed a fresnel lens type design (and contacting Apollo
>http://www.apollopresentations.com have not helped). The customer support
>lady either did not now or did nott understand was I was asking. Another
>question is whether the focal lenght is suitable.

Edmund Scientific is the place to get things like this. Here's their fresnel magnifier lens page:

http://www.edmundscientific.com/Products/ListProducts.cfm?catid=152

They don't have as many things as they did at one time. The larger products can be cut down. As you will see there are some specifications available.

- Rick Housh -

From Nikon MF Mailing List;
Date: Thu, 1 Mar 2001
From: "Hansen, Lars Holst" LHHansen@zi.ku.dk
Subject: RE: Home brew flash extender

Hi Dag, Henry, List members,

I am aware of the Metz teleattachment but @ US$77.95 compared to US$4.75 for a fresnel lens @ Edmund Scientific (thanks Rick) I think, for me, home brewing is the way to go! After all the the Metz extender does not seem to fold nicely as mine is going to... and it is set for only one coverage.. I do see at least one advantage, however. It is a closed unit and will therefore give no stray light. This could be useful for night photography using IR filter and film.

Lars

...

From Nikon Mailing List;
Date: Mon, 7 May 2001
From: DT886@aol.com
Subject: Re: [NIKON] Fresnel Flash Extenders??

Try George Lepp's site www.leppphoto.com for a version him and his son use. Art Morris uses one called the Better Beamer/Walt Anderson flash extender. I don't have Morris's web address, maybe someone else will. Sorry I couldn't be more helpful.

Good luck,

Dave

From Nikon Mailing List;
Date: Tue, 8 May 2001
From: "Alan Wakeman" awakeman2@home.com
Subject: [NIKON] RE: Fresnel Flash Extenders??

Thanks, To those who responded to my request for info on flash extenders. The two that I found that look to be quality made were. The Better Beemer http://www.birdsasart.com/accs.html#BEAMER and the Flash X tender http://www.kirkphoto.com/accessories.html#xtender.

Thanks Again!

Al
A. Wakeman Photography
http://www.citytel.net/sunchaser/alan

From Minolta Mailing List;
Date: Mon, 28 May 2001
From: diakonos@juno.com
Subject: Re: Fresnel Flash unit adapters

I own the Lepp version Project-A-Flash, 6 oz, which does not collapse. I have not used the Kirk model, which is lighter and folds flat. The concept appears to be the same and should function the same regardless of the weight. It seems to me a question may be the durability of the 3 oz Kirk version. However, I like the idea of being able to fold the device flat to fit my camera bag.

--- In minolta@y..., gremlins1@e... wrote:

> Thanks for the quick response. The Fl extender is only 3oz vs. 6 oz.
> for the Fresenel unit. Is the 3 oz. version still good enough to use?

From Minolta Mailing List:
Date: Tue, 29 May 2001
From: "William Cassing" wcassing@thegrid.net
Subject: Tele-Flash

For another option for tele-flash, See Moose Petersons device at

http://www.moose395.net/gear/bb.html

Bill Cassing

From Minolta Mailing List:
Date: Tue, 29 May 2001
From: diakonos@juno.com
Subject: Re: Tele-Flash

All the Fresnel attachments are basically the same design and operate on the same principle. You attach a fresnel panel to the flash head. In addition to the Kirk, Lepp, and Better Beamer, there is also the one designed by Newton Brackets:

http://www.newtoncamerabrackets.com/newton.html

There, you have at least 4 options, perhaps you can same your money by making one yourself :-)

From Nikon MF Mailing List:
Date: Tue, 17 Apr 2001
From: LHHansen@zi.ku.dk
Subject: Re: Home brew flash extender

Dear members,

I have found a good and competitive European source of fresnel lenses:

http://www.magnifyingglasses.co.uk

I bought their HL-1W sheet magnifier value pack (consisting of a large and small flexible lens) @ £3.95 and their HL-P full page magnifier (a non-flexible lens) @ £4.95 They seem to have thrown in an extra free credit card sized magnifier too (that might be just right for a small extender for my SB-15).

Best regards,<
Lars.

PS. I must admit that I also found a good European source for the original Metz extender 45-33 (108.50 DM @ http://www.technikdirekt.de/english/) but I think it is much more fun to home brew...

William Gartin wrote:

> on 3/1/01, Hansen, Lars Holst wrote:

>> ... After all the the Metz extender does not seem to fold nicely
as mine is going to... and it is set for only one coverage..  I do see
at least one advantage, however. It is a closed unit and will
therefore give no stray light. This could be useful for night
photography using IR filter and film.

> I'd like to see some pics of your extender in the file section when
you get it done. You might be able to find an old broken down flash
with a fresnel on it that you could cannibalize for not a much
expense. It'd likely already be the right size, and might have some
other useful parts.

From: Roland roland.rashleigh-berry@virgin.net>
Newsgroups: rec.photo.equipment.medium-format
Subject: Re: Cleaning Fresnel ground glass
Date: Thu, 16 Aug 2001 

Leonard Evens wrote:
> 
> I have several Fresnel ground glass focusing screens for medium format
> cameras.  I'm starting to use these cameras after a long gap, and I'm not
> sure I remember how to clean them.  Any advice would be appreciated.
> I do remember that one can ruin such a screen if one goes about it
> improperly.

If it's clean enough to use then don't clean it. If it's too dirty for
use then you should not worry about messing it up entirely. Clean your
hands thoroughly first so there is no trace of normal body grease on
them. Wash them two times at least and use plenty of soap. Then clean
the screens under a flow of warm tap water and use plenty of washing up
liquid several times and rinse after. Use you finger as the scrubbing
brush. The more times you clean it like that thwen the better it will
get. Let it drain to dry after shaking off excess water.

And if you ruin it them don't blame me. That's the way I clean ground
glass screens. But WHATEVER YOU DO - make sure your hands are very clean
first.

Roland



From: wiltw@aol.com (Wilt W)
Newsgroups: rec.photo.equipment.medium-format
Date: 16 Aug 2001 
Subject: Re: Cleaning Fresnel ground glass

Caution, not all 'ground glass' is glass!  If it's really plastic, I suggest to
not try cleaning it, else you run the risk of ruining the surface.

--Wilt


From: wiltw@aol.com (Wilt W)
Newsgroups: rec.photo.equipment.medium-format
Date: 17 Aug 2001 
Subject: Re: Cleaning Fresnel ground glass

>> so washing off your groundglass
(without Goop Off please) shouldn't hurt it a bit.>>

Regardless of plastic or real glass, gentle wash and rinse off with distilled
water, with no attempt to wipe dry with a cloth or tissue, probably will do no
harm.  

Drying with cloth or tissue will, at best, leave lint.  Brushing off the lint
with a brush runs risk of the brush itself leaving dust in the texture.  
At worst, abrasion renders the surface of the ground 'glass' plastic a mess
suitable for the trashbin (been there, done that!)
That's why I think that a rinse in distilled water, and a drip dry or blow dry
with warm air, is best.

--Wilt


From: RHaigh5748@aol.com
Date: Fri, 18 Jan 2002 
Subject: Re: [Rollei] screen (how to?)
To: rollei@mejac.palo-alto.ca.us

If your screen is the same one that came in my 6008i, take care in cleaning 
it. Simply wiping it carefully with a microfiber cloth can scratch the 
delicate surface which has small ridges or grooves. ( I believe these reduce 
glare and increase finder brightness) Marflex told me to remove the screen 
and clean it with warm soapy water and carefully airdry/light wipe. Its a 
tough proposition at best and I replaced one already. My only complaint with 
this camera is the focusing screen collects dust in use under the best care 
and what you can't blowoff is pretty much there. It is, though, a nuisance 
which does not affect image quality.

Bob Haight


Date: Wed, 19 Sep 2001 
From: "Paul Shinkawa" paul.shinkawa@lcra.org>
To: rmonagha@post.cis.smu.edu>
Subject: Fresnel lenses for viewfinders

Robert:

Fresnel lenses are available from Barnes & Noble as reading=20
magnifiers. A long thin one that can be cut into a 6X6 screen costs=20
$2.25 while a 3.5 X 5 one which can be cut to fit a Sunpak 544 flash=20
costs $2.50. I use a Vis-a-Vis marking pen to mark the center spot and the =
cut-lines. The marks rub off.

Also Edmund Scientific has a professional equipment catalog (not the one =
with all the fun science projects) which has many many sizes and kinds of =
very expensive fresnel lenses.

-Paul M. Shinkawa



From: rankbeginner@pppweb-solutions.net (Ryan)
Newsgroups: rec.photo.equipment.medium-format
Subject: Re: Cleaning the ground glass
Date: Sun, 23 Sep 2001 

I concur, some focissing screens have a fresnel screen attached
underneath, too much water and it can get between the two and may
never be the same again. I have seen two screens ruined this way, so
very careful cleaning!! 

R.

"Q.G. de Bakker" qnu@worldonline.nl> wrote:

>Mike wrote:
>
>> Assuming you can remove it.....warm water and soap
>
>Would depend on the type of screen (assuming we're talking about focussing
>screens). Some ground glass screens are in fact two parts held together in a
>frame. Rinsing it in water would allow water to get in between these parts.
>Not a good thing. So wipe it using a damp, not wet, cloth.
>
>



From: wiltw@aol.com (Wilt W)
Newsgroups: rec.photo.equipment.medium-format
Date: 23 Sep 2001 
Subject: Re: Cleaning the ground glass

And if you use warm water and detergent, just let the thing air dry rather than
try to wipe dry and leave lint embedded in it...and some 'ground glass' is
really plastic and easily damage if wiped.

--Wilt


From: Richard Urmonas rurmonas@senet.com.au>
To: rollei@mejac.palo-alto.ca.us
Subject: Re: [Rollei] plastic ground glass!!!
Date: Wed, 26 Sep 2001 

> I have just discovered that the ground glass focusing screen in my Rollei
> 3.5f is actually plastic!  Is this normal, or have I been diddled!?????

Perfectly normal.  It is easier to mould a fresnel pattern into a plastic
screen (plus split image, microprisms etc. etc.).  Given the screen is fairly
well protected it is generally not an issue.  BTW if you are cleaning the
screen DO NOT use any solvents, the plastic is easily dissolved by many common
solvents.  Just use soapy water and clean fingers to work the soapy water in,
then rinse.

Richard
--------------------------------------------------------------
Richard Urmonas
rurmonas@senet.com.au



From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Fresnel Lens & Focus Accuracy
Date: Thu, 27 Dec 2001 

mkirwan@nospampacbell.net (Mike) wrote:

>I have a number of view cameras a few real oldies with a plain ground
>glass back for focusing. They are dim but work fine with a loupe. I
>have a Speed Graphic and Wista 45 DX and they both have a fresnel lens
>mounted on the bellow side of the ground glass. Now here is my
>confusion, the Wista 45 came with the fresnel lens pre-installed and I
>have never had any problems with focus. However, I have seen some
>conflicting articles stating that the Fresnel should be on the outside
>of the Ground Glass (one article I believe was by the famous Ron
>Wisner).
>
>So any of you lens/focus experts out there have any comments, insights
>etc.
>
>
>Thanks and Happy New Year

  The ideal position for a Fresnel lens is on the textured side of the
ground glass, facing it. This minimises reflections from smooth glass
surfaces. However, the field lens must also not disturb the position
of the ground glass, so placing it in the ideal position is not always
possible. The next best place is on the smooth side of the ground
glass, again facing the textured side. . There is little practical
difference in performance. Having the Fresnel on the outside has the
advantage of making it easily removable if desired. Since the focal
length of the fresnel should be such as to approximately match the
distance of the exit pupil of the lens from the ground glass a Fresnel
lens which works OK with "normal" focal length lenses may not work
with wide angle or telephoto types. It may also make things worse when
camera movements are used. So, its useful to be able to remove it
sometimes. 

  Graflok and similar backs which were built for use with Fresnel
lenses have the reference surfaces for the ground glass-Fresnel lens
combination placed so as to compensate for the Fresnel. Most camera
backs locate the ground glass using a reference surface which contacts
the ground side of the glass. A Fresnel lens can be used only on the
smooth side of the ground glass unless the back is modified (which I
do not recommend). 

  Again, there is little practical difference in the performance of
the two positions as far as eveness of image illumination. Proabably
the ideal position is slightly superior for clearity of focusing. 

---
Richard Knoppow
Los Angeles, CA, USA.
dickburk@ix.netcom.com



From Rollei Mailing List:
Date: Tue, 19 Feb 2002 
From: Matthew Phillips mlphilli@hsc.vcu.edu
Subject: Re: [Rollei] Those Maxwell Screens


There is an issue with using brightscreens with large format cameras that
often gets overlooked. These super bright fresnels are designed for maximum
brightness on the lens axis, but as soon as you start using significant
camera movements, there can be some radical vignetting of whats visible on
the screen. In particular screens that are designed for use with normal
focal length lenses have problems with wide angles. If you order a screen
from Maxwell, he'll first ask you which focal length you work with most,
and send you a screen for that range. If you're working from a variety of
lenses, you may find yourself needing more than one fresnel.



Date: Wed, 06 Feb 2002 
From: Richard Knoppow dickburk@ix.netcom.com
To: rollei@mejac.palo-alto.ca.us
Subject: Re: [Rollei] Kodak TLR lens & Ektalite Field Lens

 you wrote:
>Hi Jerry,
>
>Interesting about the Ektalite Field lens. How does it differ from a "usual"
>fresnel screen? Or does it? I suppose it is more like a Rolleigrid?
>
>todd

  AFAIK, the Ektalite was just a Fresnel lens. On the Kodak Reflex and
Graflok backs for Graphic cameras, it was used under the ground glass,
which is the ideal position. While a Fresnel can be used on the viewing
side of the ground glass its use on the lens side reduces reflections and
results in somewhat higher contrast.

  Rollei was offering the Rolleigrid and there was another Fresnel sold as
the Hartley field lens at about the same time.

  I don't know for sure how any of these were made but suspect it was some
type of injection molding; none of them was expensive.

  Plastic Fresnel lenses are interesting. They can be made aspherical by
making the prisms non-uniform, and the entire sheet can be bent. Fresnel
sheets can also be cemented together to make multiple element lenses. If
the right plastics are available good chromatic correction can be gotten.

  While thery are not very good for image forming purposes quite highly
corrected condenser systems can be made rather cheaply.
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com


From rollei mailing list:
Date: Tue, 09 Apr 2002 
From: Cliff Travis cliff@allcamera.com
Subject: Re: [Rollei] RB screen in a Rolleiflex

Hi,

For what it's worth when Graflex Repairs set up focus on Graphics with
Fresnels screens they used a substitute back on the camera. The back was
the same as any other with the shiny side of Fresnel toward the lens, the
"rough" side toward the focusing surface of the glass leaving the plain
glass surface open for cleaning. However on this particular assembly a hole
was cut in the center of the fresnel to allow more accurate focusing. The
Rolleigrid accessory supplied by Rollei to improve corner brightness was
installed with the Fresnel facing top of glass screen The Rolleigrid had a
clear center for better focusing.

The newer focusing screens should be installed with shiny (or smooth side
up ) to preserve the proper Back focus distance.
The Latest screens I have are Of the "F" vintage and have a Fresnel lens
molded into one side which should face down. The Fresnel
is so fine it is very hard to see but it can be detected without damage by
dragging a fingernail across it.

    Newer screens of various makes claim to have an optically modified
surface and usually incorporate a Fresnel .

  I jus disassembled a Yashica screen ,which I think provides a brighter
image the do most Rollei TLRs and found it to be similar to the Graphic
system... The Fresnel is mounted under the Ground glass with the Fresnel
facing the focusing surface

Incidentally the Yashica Fresnel could be cut down a bit  (fine tooth
jewelers saw or file) and used as a Rolleigrid.. It could also be ised
under a Rollei screen if you are willing to adjust focus.
Cliff

All Seasons Camera
Cliff Travis
5 Harvard Lane PO Box 111
Hastings on Hudson, NY 10706
cliff@allcamera.com

www.allcamera.com
914 478 0931
FAX 914 478 7354



From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Focusing panel upgrades...what's available? How good?
Date: Sun, 05 May 2002

 news.smu.edu rec.photo.equipment.large-format:4811

"Graphic" graphic99@mindspring.com wrote:
>I have a very old 6x9 focusing panel for my XLSW body that is extremely dim,
>becoming dreadfull at the the corners.
>
>Kodak used to have an Ektalite field lens that could be mounted on 4x5
>panels, but I still think that it was not that much of an improvement, even
>if one could be found for a 6x9 unit.
>
>What are modern photographers using on 6x9's for brighter, snappier focusing
>with lenses in the f:8 realm??? I'm even considering cannibalizng 2 6x6cm
>focusing screens to place the awful groundglass currently in the panel......
>
>Thanks,
>Wayne
>graphic99@mindspring.com

 There are a couple of different types of bright screens but a lot of
them are simply Fresnel lenses of the same type as the Ektalite. While
mounting the field lens between the lens and ground glass is ideal
from the standpoint of eliminating internal reflections they work very
well on the outside of the ground glass too and are a lot easier to
mount that way.

  You can get Fresnel lenses from Edmund's among other places.

  Provided the lens is the right focal length and no movements are
used the Fresnel will brighten up the corners by focusing the light
there toward the axis of the ground glass, where your eye normally is.

  However, the focal length of the field lens should be about equal to
the average focal distance of the lens. This is more important for
wide angle lenses than longer than normal lenses. A mismatched field
lens will actually darken the corners compared to no field lens.

  Also, when movements are used, the field lens will continue to focus
the light along the axis of the lens, so the light coming out of the
field lens will not be perpendicular with the ground glass, and again,
the ground glass image will be more uneven than without the field
lens.

 If your ground glass is _very_ dim it may be dirty. Remove it and
soak it in warm water with some diswashing detergent in it. If its
really greasy you might need to wipe it with a solvent. Isopropyl
alcohol or ligter fluid are both safe for glass and plastic. The
degergent should do the job however. Its amazing how much brighter
just cleaning will often make the image.

---
Richard Knoppow
Los Angeles, CA, USA.
dickburk@ix.netcom.com



From: Stefan Patric tootek2@yahoo.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: How good are Beattie screens for 5x4
Date: Sun, 19 May 2002

Wayne Stubbs wrote:

> I recentley purchased a Wisner technical field. Although i love the
> camera the standard viewing screen leaves a lot to be desired. I
> mainly photograph churhes and cathedrals here in England which are
> very dimley lit at the best of times.
>
> I use a 90mm f5.6 a 75mm f8 and a 65mm f8.
>
> Would a Beattie screen offer a big improvement with these lenses ?

20 years ago, I replaced the ground glass and fresnel setup on my Cambo
SCII with one.  (I couldn't afford faster lenses.)  Big improvement (1
to 1.5 stops brighter) for my 90 f8.0, but less so with the 75 f6.8.

It was worth it.  Made composing dark, architectural interiors so much
easier.

--
Stefan Patric
tootek2@yahoo.com


Date: Fri, 24 May 2002
From: "Victor Bazarov" vbazarov@dnai.com
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Screen brightness with different focal lengths at the same f/stop

....

Howard,

Brandishing your (no doubt deep and wide) knowledge of astronomical
terms doesn't make a point.  Implying that non-astronomers are slow
doesn't either.  You should consider that "many more" stars collected
by a longer lens is actually collected from a much smaller area of
the sky (square of the difference in focal lengths).  And, BTW, the
number of stars visible through a lens with a particular focal length
has nothing to do with the brightness.

Now, let me explain.  Take day-time shooting.  Let's assume that you
are taking a picture of an evenly lit 18% infinite grey plane.  What
is the picture going to be?  Grey (given a proper exposure), right?
Does it matter how close you are to that plane (if it's infinite), or
what focal length you are using?  No.  What matters is EXPOSURE, the
combination of APERTURE and SPEED.  Now, let's take speed out of the
equation and assume that it is the same for both shots (with 90 mm
and 210 mm).  Would the aperture be the same?  Now, given that, would
you say that the film should come out exposed the same way (the light
energy collected from the scene should be the same)?  If that's the
case, there must be something other than the focal length that affects
the brightness of the ground glass, mustn't there?

Actually, it is not the amount of light falling ONTO the ground glass,
but the amount of light the ground glass lets through in the direction
of our eyes, what makes the "brightness" of the ground glass.  The cone
of light hitting the GG from the 90 mm lens is shorter which makes the
outer areas of the GG lit at a greater angle from a normal to GG.  The
cone of light coming from a 210 mm lens is taller, which makes the
vector much closer to normal to GG.  The closer the light to normal,
the greater the amount of light passing through, the more light reaches
our eyes to make the perceived brightness of the GG.

                    210mm
                  N  |
                  |  /
   N  90mm        | |
   |  /           | /
 r | /          r ||
  \|/            `|/
 _=========== GG ==========
  /|             /|\
 /                |
m                 m
      \^/           \^/
      Eye           Eye
(r - reflected light, m - main refracted, N - normal to the GG)
(I wish I had a better picture for you...)

HTH

Victor


From rollei mailing list:
Date: Mon, 15 Jul 2002 
From: Richard Knoppow dickburk@ix.netcom.com
Subject: Re: [Rollei] Rolleicord photos

...
  A good way to check coincidence of the taking and finding lenses is to
place a square of ground glass against the film rails. The finder image and
the image on this ground glass should be in focus at the same point.

  Also, make sure the ground glass in the finder is seated against the
inside surface of the hood. Its on a couple of springs and can get cocked a
little. Usually just pushing it gently with your finger will get it seated
correctly.

  A simple Fresnel lens, like the Rolleigrid, on top of the finder ground
glass will make the finder image a lot brighter. Its probably not as good
as a dedicated screen, like the Maxwell screen, but they are inexpensive
and don't require any special installation.

  Rolleigrids turn up used fairly frequently and Edmund Scientific sells a
Fresnel the right size for a few dollars.
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com


From: dickburk@ix.netcom.com (Richard Knoppow)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: One more stupid lens question
Date: Thu, 25 Jul 2002 


"Ed Margiewicz" Edmarg1@comcast.net wrote:

>Hello,
>I have one more stupid question concerning f stops for lenses for 4x5.  I am
>looking  to buy a telephoto soon and a wide angle lens later this year ( I
>currently own a 150mm).  I'm looking in the 250-300mm range with a circle of
>coverage a big as possible.  One lens that interests me is the fujinon T
>300mm f/8.  My question: Does the small apperature affect the brightness in
>the viewing glass for compositon and focusing (ie will the image be darker
>then a faster lens)?
>Thanks,
>Ed

 The ground glass image from f/8 is not as dim as you would think.
  Special screens work mostly by redirecting the light from the lens
toward the eye. They don't make the center of the image any brighter
but make the edges brighter by directing light there toward the eye
rather than going off in the direction it left the lens.

  Since the angle of the light from the telephoto is smaller than for
a "normal" focal length lens the corners will be brighter with a
normal ground glass.

  While a Fresnel field lens will brighten up the corners it will not
have as much effect as it does for shorter FL lenses.

  The angle of the cone of light from the telephoto is the same as for
a conventional lens of the same focal length. This is actually the
principle of the telephoto. The rear element is a negative lens which
makes the front element seem further away to the camera. So, a lens of
relatively long focal length is physically closer to the film than
would be a lens of conventional construction.

---
Richard Knoppow
Los Angeles, CA, USA.
dickburk@ix.netcom.com


From rollei mailing list:
Date: Sun, 25 Aug 2002 
From: Richard Knoppow dickburk@ix.netcom.com
Subject: [Rollei] WARNING about Rolleigrids.

Do NOT use Isopropyl alchol to clean Rolleigrids. It will cloud the
plastic. I just ruined a mint one I bought an hour ago at a local camera
sale. Isopropyl alcohol works on my plastic eye glasses and on lenses but
does not work on whatever Rolleigrids are made of.

  I am sending this warning in the hope that it will save someone else from
making the same mistake.

  Its the sort of thing that makes me feel the gods are out to get me. :-(
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com


From rollei mailing list:
Date: Sun, 25 Aug 2002 
From: Olivia Hibel olivia@infi.net
Subject: Re: [Rollei] WARNING about Rolleigrids.

Thanks for sharing this important information about alcohol (isopropyl).
I recently learned it is actually a solvent for plastic!

  I found out that alcohol will remove the plastic gop when a plastic bread
bag melts onto/against a hot toaster!  It just dissolves it.  So no wonder
it would damage a plastic mechanism on a camera!

    So sorry Richard, that this happened to you;  think of it as a material
sacrifice to the gods instead of a serious one!

    Best wishes, Olivia