Bokeh Comments Invited

[xpatUSA]

I don't usually have trouble with getting smooth bokeh because I am either close to the subject or I'm shooting an 'everything more or less in focus' scene. However, today I took a snapshot of some tree damage (rot) about 15ft up the trunk and, therefore, against back-lit foliage. Sigma SD14 with 18-200mm zoom at 120mm:



Shot was made @ f/5.6 (as low as it would go), f=120mm. Lens has 7 rounded blades.

To my untrained eye, the bokeh looks horrible. Is it inevitable with that type of lens at that linear distance of about 6 or 7m from cam to subject?

[Max von MeiselMaus]

Oooh, that's strange.

I initially thought motion blur, but it is not showing that characteristic double edge. What was the shutter speed?

Could it be a result of sharpening (over-sharpening) the background? Does it look like that in the unprocessed image?

Could there be an element of chromatic aberration creeping in, queering the outlines? Does your lens usually suffer from that?

I would be interested to hear what others think. Not seen an out of focus area do that before.

[xpatUSA]

Oooh, that's strange.

I initially thought motion blur, but it is not showing that characteristic double edge. What was the shutter speed?

1/60s

Could it be a result of sharpening (over-sharpening) the background? Does it look like that in the unprocessed image?

Here's a raw composite export from RawDigger which is about as unprocessed as you can get, just cropped to post here:

Could there be an element of chromatic aberration creeping in, queering the outlines? Does your lens usually suffer from that?

Can't answer that - haven't had the lens long and I don't usually make that kind of shot - hence my post here.

I would be interested to hear what others think. Not seen an out of focus area do that before.

Indeed.

[Manfred M]

Ted - your untrained eye seems to be bang on. The out of focus areas are not very pleasing.

The lens designers have to make a lot of trade-offs in coming up with an 18-200mm lens and my understanding is that it is quite a bit less costly than similar lenses from Canon and Nikon, so poor bokeh is likely part of the package for Sigma to hit the price point they were designing this lens to.

[Shadowman]

The closer you look at the bokeh, the more it resembles doughnut shape.

[TonyW]

Can you capture the same scene from a greater distance with a larger focal length? While the depth of field should be the same, the background would be more magnified and might be more pleasing.

[Shadowman]

I have a mirror lens that makes that type of bokeh, but I actually like the look.

[xpatUSA]

Can you capture the same scene from a greater distance with a larger focal length? While the depth of field should be the same, the background would be more magnified and might be more pleasing.

Sorry, Tony, I should have made it clearer that it was a snap for an email. So I'm not looking to get a more pleasing image; I was just interested in why the bokeh had that particular appearance for that lens at that distance. I'll give it try, though, might tell me something, eh?

Would I keep the same aperture?

[xpatUSA]

I have a mirror lens that makes that type of bokeh, but I actually like the look.

[edit] Found this in the Wiki:



I've seen an article in the past that showed various types of bokeh (link lost, of course) and I vaguely remember the appearance, kind of like frogspawn. Also recall 'c' shapes, summat to do with coma or summats.

[Max von MeiselMaus]

Ah! Could be due to a form of lens aberration, then; the same type of aberration that produces swirly bokeh in older lenses. Coma bokeh produces smudged-looking tails to points of light. I wonder if something similar is happening here, enhanced by the high contrast of a backlit background. In which case, it is a characteristic of the lens. Doesn't mean the effect can't be reduced by being able to predict the situations in which it is most likely to occur.

[xpatUSA]

Ah! Could be due to a form of lens aberration, then; the same type of aberration that produces swirly bokeh in older lenses. Coma bokeh produces smudged-looking tails to points of light. I wonder if something similar is happening here, enhanced by the high contrast of a backlit background. In which case, it is a characteristic of the lens.

Thanks,

The lens is a Sigma 'kit' lens provided with their current DSLR, the SD1M. At the focal length selected, the maximum aperture possible is f/5.6. Also, the background is not real far behind the subject which, at f/5.6, means it's not as out-of-focus as it would have been with, say, f/2.8. So the bokeh is "smaller" than what I'm used to.

The much-maligned Ken Rockwell's page seems quite relevant and does mention spherical aberration which can produce circles:

http://www.kenrockwell.com/tech/bokeh.htm



Looks quite familiar . . . . .

[Saorsa]

I suspect that what you are seeing is hundreds of 'pinhole cameras' made up of the gaps between the leave projecting into your lens and being resolved as circles.

[xpatUSA]

I suspect that what you are seeing is hundreds of 'pinhole cameras' made up of the gaps between the leave projecting into your lens and being resolved as circles.

So perhaps I should look at twigs too, so as to distinguish between pinhole and spherical aberration effects?

[Saorsa]

Yep, here is an example of what I've seen in shady forest with sunlight through leaves.

[TonyW]

Sorry, Tony, I should have made it clearer that it was a snap for an email. So I'm not looking to get a more pleasing image; I was just interested in why the bokeh had that particular appearance for that lens at that distance. I'll give it try, though, might tell me something, eh?

Would I keep the same aperture?

Yes, I would use the smallest aperture available at the larger focal length. I don't understand bokeh but it is probably to do with how much out of focus the diaphragm plane is, which could vary with focal length on a zoom lens.

[xpatUSA]

Yes, I would use the smallest aperture available at the larger focal length. I don't understand bokeh but it is probably to do with how much out of focus the diaphragm plane is, which could vary with focal length on a zoom lens.

Here we are, about twice the linear distance and 200mm, f/6.3 instead of 120mm, f/5.6:



Please click on image. Still plenty of "frogspawn"!

Looks like there was a mixture of back-lit gaps and specular reflections, sun was almost overhead. Yesterday's shot was more like 2PM local solar time, so not too different, IMHO.

[Shadowman]

Here we are, about twice the linear distance and 200mm, f/6.3 instead of 120mm, f/5.6:



Please click on image. Still plenty of "frogspawn"!

Looks like there was a mixture of back-lit gaps and specular reflections, sun was almost overhead. Yesterday's shot was more like 2PM local solar time, so not too different, IMHO.

Not too different but enough that it (bokeh patterns) are less noticeable to the viewer, I had to pixel peep to make out the doughnut shaped bokeh.

[xpatUSA]

[twice the distance looks] not too different but enough that it (bokeh patterns) are less noticeable to the viewer, I had to pixel peep to make out the doughnut shaped bokeh.

It looks like the ratio of background distance to subject distance has a bearing on the apparent bokeh (image) size and it's "sharpness" - all other things being equal (which they rarely are).

Once again, standard advice (in this case, max aperture for best bokeh) fails to cover all situations

[Max von MeiselMaus]

A lens never works at its best when stretched, so at its maximum aperture. It might well be the case that a lens prone to aberrations will have those exaggerated when pushed. So, try stopping down a little. If you get closer to your subject, thereby decreasing your depth of field, you should still get a good amount of out-of-focus background.

[xpatUSA]

A lens never works at its best when stretched, so at its maximum aperture. It might well be the case that a lens prone to aberrations will have those exaggerated when pushed. So, try stopping down a little. If you get closer to your subject, thereby decreasing your depth of field, you should still get a good amount of out-of-focus background.

Thank you, Max, for the information and the advice. Now I know that, at maximum aperture, a lens is not at it's best.

Going to be difficult to get closer to the subject, though: