This image from the Nikon SU-4 instruction sheet.
This recent gadget makes life easier for those people who wish to use
multiple flashes or use their Nikon flashes with another brand camera.
Its official name is "Wireless Slave Flash Controller" which sort of
implies there may be some radio link, but no, the link is purely the main
flash light triggering the slave. In the specifications there is mention
of "infrared communication" which means the unit will have better
range than one that just works off visible light.
The unit has no batteries and slips onto the foot of the Nikon flash
unit. It has a 120 degree rotation left or right and the sensor has 120
degree rotation up or down. The SU-4 can be mounted on any standard 1/4"
thread tripod head.
It comes with a SG-1 diffuser to slip into a hotshoe and lessen the
effect of the pop-up flash, if that is the main flash.
Starting from the top centre line and going clockwise, the lines point to - The hotshoe, the +/- 120 deg rotatable foot, the holdoff button to stop other flashes triggering the SU-4, the beep or no-beep selection switch, the Auto/Manual selection switch, the flash sensor, the sensor mount with its +/- 120 degree up/down swivel action. Not shown, the underside of the SU-4 has a standard 1/4 inch tripod socket.
The SG-1 diffuser comes with the SU-4 and provides a way of limiting the pop-up flash output on any brand camera. Not checked yet on all brands, so far only on Nikon F70 and Pentax MZ series (ZX series). It slides into the flash shoe and the diffusing screen (milky white plastic) can be flipped up out of the way if you wish to use the pop-up directly.
The SU-4 can be used in either "AUTO" mode or "M" mode.
The AUTO mode has to use a Nikon slave flash that is capable of TTL
operation. The list at July 1999 in the SU-4 instruction sheet includes
SB-28,27,26,25,24,23,22s,22,20,18,16B,15 set to TTL flash mode. The AUTO
range is quoted as 7m (23 ft).
The M mode can use any of the above list set to non-TTL auto where
the slave flashes' own sensor controls the output, or set to Manual where
the slave flash gives the full output and you have to calculate f/stops
from distance and guide numbers. The M mode range is quoted as 40m (131
ft).
The SG-1 diffuser also may make the SU-4 sensor work more reliably
when the slave is off to the side of the camera. To be effective in TTL
mode the SU-4 sensor has to see the flash coming from the main TTL flash.
If the sensor could only see its own slave flashes' output, then it would
never shut off the slave flash resulting in overexposure of the film but
an under exposure indication from the slave flash.
For AUTO mode the master flash can be any brand flash on any brand camera that is TTL controlled. The slave just adds to the general flash illumination and the camera will sense the total light and turn off its flash when enough is received. This main flash turn-off is sensed by the SU-4 and it turns off its slave flash. If not enough illumination was sensed so that the slave unit does not get a turn-off signal before the slave flash capacitor is completely emptied, then the usual Nikon flash underexposure warning noises occur plus the SU-4 will beep for three seconds (if switched to beep mode). If too much light was received then there is no indication of overexposure. Maybe the extremely short flash timing may be a clue, but experience will have to teach you how to set it up properly.
If using two similar flashes with the same output guide number and the zoom setting set the same, one as the master and one as the slave, the illumination can be adjusted to achieve proper lighting ratios by moving the slave either closer to or further away from the subject than the master flash. Experimenting may have to be done to see the effect. Keep notes on setups.
If you wish to use bounce flash with a camera having only a pop-up flash then the SU-4 and suitable Nikon flash can be used aimed in the required bounce direction. The SG-1 can be placed over the pop-up to lessen its effect, if desired.
Another use of the SU-4 and slave flash is to eliminate background shadows. If you require a shot having an over-exposed background then you can set the SU-4 to M mode and set the slave flash to 2 or 3 stops overexposure based on the aperture being used on the camera. More experiments may be required to learn how to best do this, as the effect will vary with film type.
One reason to use the SU-4 may be to free up the movement of the camera user. The SU-4 could fire the main flash(es) and the camera use a pop-up or small TTL flash, not even aimed at the subject. This way the camera operator is not tripping over sync leads.
There is no mention in the instruction sheet whether it is possible
to daisy chain with appropriate cables another TTL flash off the first
slave flash, I will have to try that out and report back later.
Later... I connected the SU-4 to an SB-26 flash, then used the
three wire cable SC-19 to connect the SB-26 to an SB-28. The SB-28 was
used in the base of a cardboard box with a tracing paper covered top (making
a small soft-box light) and of course, in that position the second flash
on a wire with the SU-4 is needed so the trigger flash can be seen by the
SU-4. In this case it was my wife's Pentax MZ-50 camera.
To stop the Pentax pop-up flash from affecting the shot, it was blocked
by a piece of cardboard angled so the light bounced to the SU-4 on the
first cabled flash. The first cabled flash was either used as a second
flash or its output stopped by covering the head with the SU-4 soft black
zipper pouch when I wanted light only from the soft-box. All shots were
successful.
Later again... This time I tried the SU-4 on an SC-17 cable
which then connected to the remote flash. All appears to work OK and has
the advantage that there is more freedom in the position of the slave flash
because the SC-17 cord can be stretched to hold the SU-4 right next to
the master flash, thus avoiding the problem of the SU-4 seeing its own
slave's flash and getting confused.
There is a mention that more than one SU-4 can be used but in practical terms the limit appears to be three SU-4 triggered off the main flash. Maybe this is more the result of the fact that the slave flash outputs may overwhelm the main flash output if the SU-4 sensor not carefully aimed and may result in the SU-4 not sensing the end of the main flash output. Of course this doesn't matter if the SU-4 are to be used in M mode. It would make more sense to have one SU-4 that can be aimed at the main flash output and then cable the rest of the flashes together with SC-18/19 cables.
A simple two flash mini-studio or table top outfit could be set up to work in TTL mode by having the camera wired to the first flash in an umbrella or soft box using the SC-17 cord then the second flash in an umbrella or whatever positioned anywhere and using the SU-4 with the sensor aimed at the first flash. Again I want to experiment with this.
One warning is that the anti red-eye and the monitor pre-flash features
have to be turned off on the camera/main flash combination. This is simply
done by using the camera in manual mode. Putting the camera in manual mode
does not affect flash TTL mode.
This page still being written, more diagrams and photos to be added
later..........................
Conclusions... the SU-4 is a handy gadget, pity it carries a Nikon price.