Projects and ponderings for film photographers

Archive for September, 2008

Bokeh: What it is and isn’t

The word “bokeh” is the Japanese word for blurry; and based on skimming Flickr comments recently, it seems to be the buzzword of the moment in photography circles. There’s a puzzle, though. Since English already has the word “blur,” why did anyone feel a need to start using the Japanese one? It’s also odd for the word to be transliterated with a final ‘h’; after all, we write sake and not sakeh.

I had been taking photographs for over 30 years before ever hearing the term; and at first it confused me too. However it turns out that bokeh refers to quite a specific aspect of lens blur—calling attention to subtle phenomena that might otherwise be overlooked.

Unfortunately though, the meaning of “bokeh” has been getting rather blurred itself lately. We ought to make a stand to preserve its specific technical meaning, before this useful term degenerates into just another name for “fuzzy.”

toycam bokeh sample

A corny flower shot shows nice bokeh from a rotten plastic lens (on a vintage Diana)

Now don’t get me wrong—photographs which use selective focus to give nicely blurred backgrounds can be very pleasing. I like this effect, and have written about how to get it. And because many of today’s digital cameras limit your ability to achieve this look, a photo with shallow focus and a creamy blurred background will often attract many admiring comments about “great bokeh!”

But bokeh is NOT a synonym for “blurry background,” or “shallow depth of field.” It actually has little to do with the amount of blur. The degree of blur you see in out-of-focus areas is essentially a function of geometry—the relationship between the aperture’s diameter and its distance from the subject. Lets say you’re taking a portrait from 4 feet away using a 50mm lens at f/4. Every brand and every design of 50mm lens will render the background with the same amount of blur. But to the connoisseur, two different lenses may yield violently different bokeh.

Bokeh refers to the subjective quality of the blur. Is it “jangly” and busy-looking, or creamy and smooth? Do out-of-focus highlights have odd, distracting shapes, or are they unobtrusive circles? Does the blurred area seem to “swirl” around the center of the photo in arcs? These are some of the factors which might be mentioned as aspects of the bokeh for a particular lens. And these may be the reasons why a serious bokeh geek would chose one particular lens over a different brand with otherwise identical specs.

The word “bokeh” officially entered the English language in 1997, in an issue of the magazine Photo Techniques—whose editor Mike Johnston decided to add the final ‘h’ to make the pronunciation less ambiguous. He tells the story here, and includes some interesting photos showing different subjective effects in various blurred backgrounds.

Where does Bokeh come from?

But WHY might different lenses have different bokeh signatures? Well, there are two effects.

Each point of light from an unfocused area of the subject forms an extended bright patch at the image plane. Conventionally we call this a ‘blur disk,’ as if these were always circular; but really the blur spot takes on the same shape as the lens’s aperture stop. If the diaphragm blades form a 6- or 8-sided “stop-sign” shape (as SLR lenses typically do), so will the blur spot.

A most extreme example of this happens with mirror telephoto lenses, which have a central obstruction: Their blur disks are fuzzy doughnuts. This creates exceptionally distracting bokeh, if there are pinpoint highlights to accentuate it.diamond bokeh

This crop from an Olympus XA shot shows busy diamond-pattern bokeh, matching the shape of the camera’s simple 2-blade aperture stop

Also, if a lens’s barrel design obstructs the more oblique light rays, the effective aperture opening becomes progressively more football-shaped towards the corners of the frame. This often leads to a “swirly” background effect if the lens is used at wide apertures.

The other issue has to do with a subtlety of optical design; namely, whether the blurred light ends up more concentrated at the middle of the blur disk or at its edges. A bright rim to the blur disk generally leads to distracting, jangly-patterned bokeh. But note that this effect often reverses depending on whether the subject is in front or behind of the focus point.

Both these effects are discussed in much detail in this excellent article (it is actually one of the original 1997 Photo Techniques articles mentioned above).

A blur disk with the light concentrated more towards its center will generally lead to smoother, creamier bokeh—and ironically one way to achieve this is to create a lens design which leaves some uncorrected spherical aberration. That compromises overall sharpness, so lens designers usually avoid it.

But there have been some specialized soft-focus lenses manufactured that exploit the effect; and it’s the reason why a plastic piece-of-junk camera often gives such dreamily smooth blur where the subject is out of focus, like in the Diana daisies shot I posted above.


Update: This page from Rick Denny compares the bokeh from several lenses of similar focal lengths; it illustrates very well how differently each renders out-of-focus highlights (scroll down the page to the photographs).

Polaroid Addendum: Eames Film about SX-70

I’m a fan of the design work of Charles & Ray Eames, so I was tickled to find a film on YouTube they did for the introduction of the SX-70. It’s part advertisement, and part user guide; but also gives a cool inside look at how the mechanism works. The Polaroid images shown are nicely inspiring, though there’s a bit of a pang of loss viewing them today (especially seeing all that original SX-70 film with its turquoise “opacifying layer” being shot).

The film ends with some rather cosmic commentary by Philip Morrison, who also collaborated on the classic Eames film Powers of Ten.

Vox sez, “check it out.”

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