Serene Life

by garik

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A friend of mine is thinking of buying a used medium-format film TLR camera (a Mamiya C330), and he showed me some of the test shots he'd taken with it. I was struck by the curiously non-uniform bokeh in some of the photos, like this one:

Photo with non-uniform bokeh
(Subject's face blurred for the sake of privacy, since they're not my kids.)

If you look at the background, especially the trees at the top of the picture, you can clearly see that the bokeh is not circular but elliptic, and that the long axis of the ellipse seems to be orthogonal to the line from the center of the image. It almost looks like circular motion blur, as if the camera was rotated during the shot, but the lack of blur in the foreground makes it clear that it's not.

I rather like the effect, especially the way it draws the eye to the center of the picture. (It's not so effective in this particular shot, since there's no strong central subject for the eye to be drawn to, but in some of the other photos with a more central composition it worked really well.) What I'm wondering, however, is what's causing it, and is there a name for it?

I can sort of see how it might arise from the way the light travels diagonally through the iris near the edges of the image, emphasized by the relatively large film format (my own camera, which I've never really noticed such an effect with, is a Nikon DSLR with a comparatively tiny APS-C sensor), but is that really all there is to it, or is there something more complicated going on? And how could I deliberately achieve the same effect, short of switching to medium format myself?

Ps. Here's a close-up of the top of the photo above (click to enlarge):

Close-up of non-uniform bokeh

The picture was taken with the Mamiya-Sekor 80mm f/2.8 lens. Unfortunately, I don't know the exact aperture and shutter speed settings used.

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During partial solar eclipses, I've seen gaps between tree leaves act as pinhole cameras and project images of the crescent sun on the ground. So I have to wonder whether the shapes of the bokeh are being influenced by the shapes of the gaps between the leaves (which, of course, are not going to be perfect circles), as well as the lens. –  Zack Jun 18 at 20:12
    
@Zack: They surely are (generally, the shape of a bokeh spot is a convolution of the shape of the light source and the camera point spread function), but I'm pretty much sure that the actual gaps between the leaves were not arranged in the circular pattern shown in the photo. –  Ilmari Karonen Jun 18 at 20:18

2 Answers 2

up vote 6 down vote accepted

The shape of the bokeh is related to the apparent shape of the aperture of the lens.

Straight on, this will produce a bokeh that is approximately a circle. As the subject moves away from the center of the field, the bokeh starts to look like a sliver of the circle.

This can be reduced by stoping down the lens.

lenses

(above image from http://toothwalker.org/optics/vignetting.html)

More on this can be read at Shape of the blur patch and Cat's eye effect

To achieve this affect, you need a wider field of view in the lens (a long lens will never see the aperture from a steep angle) and shoot wide open.


The optics of this can be understood by looking at the light rays through the lens system:

light path through lens

However, its not quite this simple as the lens construction itself also plays into the shape of the bokeh.

four lenses, four bokeh

Above images from http://photographylife.com/what-is-vignetting

The only difference between the four images is the lens. Each lens has a different front and rear pupil size and lens blade count and shape. If this effect wasn't caused by the lens, then it would indicate some other phenomena. But instead, we do see a difference between different lenses on the same camera indicating that the lens construction is the cause for the shape and the place one should look to understand the nature of this bokeh.

Ultimately, the cat's eye bokeh is a form of mechanical vignetting similar to when the lens hood is too long for the lens and blocks some of the scene.

This type of bokeh can also be seen in the apparent changing of the shape in DIY bokeh shape filters:

enter image description here

Image from http://www.diyphotography.net/diy_create_your_own_bokeh/

enter image description here

Image from http://www.infobarrel.com/How_to_make_a_bokeh_filter

Note the shape of the heart and mickey mouse at the edge of the frame. If one was to go and look at the lens from those point light sources, one would see that the shape of the heart or mickey mouse on the camera is the shape of the bokeh (with some additional adjustments for the apparent shape of the pupil).

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This seems plausible, but note that as far as I can tell, these references are the only source of this particular claim online. And the author, in his bio page, says "I am not professionally involved in photography [or] optics [....] In daily life I work as an underwater acoustician." Now, I am not a professional in these fields either, but since this contradicts everything else I've read (other than Internet posts traceable back to that one) , I'd like to see further references. –  mattdm Jun 18 at 16:56
    
@mattdm consider the various DIY make your own bokeh. The shape of the apparent lens pupil is the shape of the bokeh (this also has impact on the diaphragm blade count and shape). And when you look at the various DIY bokeh, you often see at the edge of the frame. i.stack.imgur.com/Bh1Pv.png - this is explained by the apparent shape of the lens pupil. (and yea, wikipedia isn't that great of a source but en.wikipedia.org/wiki/Bokeh#cite_ref-11 ) –  MichaelT Jun 18 at 17:08
    
I'm not disputing that you can make shaped bokeh, or even that this effect can happen near the edge of the lens. I just don't think that's what's going on here. –  mattdm Jun 18 at 17:32
    
Do we agree that the shape of the bokeh is the shape of the apparent lens pupil (such as the hexagonal bokeh on 6 bladed diaphragm?) And that mechanical vignetting (a DIY bokeh filter) can change the shape of the bokeh? Why wouldn't these two factors account for the cat's eye bokeh? –  MichaelT Jun 18 at 17:39
    
What you are describing is a form of spherical aberration in the lens. Accepting that this is the case with a lens, one can design a simpler lens if one accepts a not flat focal surface. However, that alone doesn't account for the shape of the cat's eye bokeh, which can be accurately described and understood using the model of mechanical vignetting of the out of focus area. Going beyond this, if one wants a real understanding of the shape, it would likely be a good question to send over to Physics.SE to get the optics and math background on it. –  MichaelT Jun 18 at 17:54

This is field curvature. (And a nice example of it!) Simple lenses naturally project a curved field, not a flat one to match film or digital sensors. Modern lenses attempt to correct for this, but many older designs do not. In fact, it's sometimes called the "Petzval effect" after a classic design famous for this look.

Interestingly, just this week Sony showed prototypes of a curved digital sensor designed specifically for this reason — by curving the sensor, the lens can be much simpler yet still yield across-the-frame sharpness.

There's more information on (and examples of) field curvature in my answer to What characteristics make a lens good or bad?, and more technical details at What does "Flat Field Focus" mean? and Do lenses have focal planes or focal spheres/ellipsoids?

To replicate this effect, you can either find an actual classic lens with the Petzval design, or some more recent but still "vintage" Soviet-era Helios lenses, or pick up Lomography's recently-launched Petzval art lens for Canon EF or Nikon F mounts.

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Are you sure? I can see how field curvature would change the focus between the center and edges of the image, but I don't see how it would change the shape of the bokeh. (Ps. I googled for "field curvature" and found this page illustrating different optical aberrations. What I see in this photo looks more like their illustration of "sagittal astigmatism", although I'm not quite sure if that's the same effect as seen in the photo, either.) –  Ilmari Karonen Jun 18 at 15:56
    
Yes, I'm quite sure, at least as far as practical photography goes. If you get to the physics of the optical design, field curvature and tangential and sagittal astigmatism are all interrelated and it all gets quite complicated. Notice that when searching for those things, you get mostly diagrams and test charts — and pages about astrophotography. That's all well and good, but from the point of view of recognizing this effect and replicating it, petzval field curvature is what you're after. –  mattdm Jun 18 at 16:11

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