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11-25-2021, 07:42 PM   #1
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Let's talk lens diffraction

Obviously at smaller apertures diffraction increases and it will vary lens to lens. At what point does it become a problem? Specifically, here, this is a 200% crop. Is there anything noticeable? In my example, I see noise and pixelization but can't see any noticeable diffraction, looking for soft focus on hard lines. this was shot at f/22 on a 50mp medium format camera. If I understand correctly, and please set me straight if I have this wrong, but aren't higher resolution cameras a little more forgiving than lower resolution ones? I'm trying to wrap my head around MTF curves and equations. I posted this in another forum and someone mentioned that the image suffers from diffraction due to being shot at f/22. I cant see anything noticeable other than diffraction spikes from the light beacon. I do plan on going out with this set up and do some testing with different f stops to see if I can tell any difference. Attached is the snip. Full image is here (bridge tower closest to the camera, still several hundred feet away). Post your medium format photos! - Page 1176 - PentaxForums.com

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11-25-2021, 10:24 PM   #2
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I think you need two shots or perhaps several shots so you can see the image lose definition. But there are times the extra softness isnít distracting and you should use whatever works well for you.
11-25-2021, 10:47 PM   #3
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Here's a web calculator to help you understand how diffraction from an aperture will affect an image: Circular Aperture Diffraction

A higher resolution sensor (i.e. more and smaller pixels) is of no help.

And I agree with Uncle Vanya's assessment above.
11-26-2021, 02:03 AM - 1 Like   #4
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Diffraction starts to become noticeable when f-stop number equals pixel pitch in micron. It becomes visible when f-stop number reaches 2x pixel pitch in micron.
Part one can be deduced from the formulas, part two is experience. Whether the effect is significant is up to the user. Even in film f/22 resulted in noticeable diffraction.

11-26-2021, 02:34 AM   #5
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QuoteOriginally posted by cdd29 Quote
If I understand correctly, and please set me straight if I have this wrong, but aren't higher resolution cameras a little more forgiving than lower resolution ones?
Higher resolution sensors are more unforgiving the further away from the optimal aperture you get, @ f/22 everything will be basically mush on a 400Mp Multi-shot digital back. The advantage of higher resolution sensors is that for a given print size diffraction can be masked when the image is down-scaled and sharpened for the relevant output media.


QuoteOriginally posted by cdd29 Quote
I posted this in another forum and someone mentioned that the image suffers from diffraction due to being shot at f/22. I cant see anything noticeable other than diffraction spikes from the light beacon. I do plan on going out with this set up and do some testing with different f stops to see if I can tell any difference.
Good, do these tests for yourself and identify the optimal aperture of your lenses, F/8~f/11 is a common range where a vast majority of medium format lenses perform well. I'd also recommend doing tests at different focus distances as some lenses particularly telephoto lenses can sometimes suffer from increasing aberrations as they approach MFD.
11-26-2021, 08:56 AM - 1 Like   #6
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That shot of the light beacon says it all. You actually have light spikes like this on each and every other point in the shot but they are much fainter (and thus smaller) so you don't notice them. However, that's what's responsible for decreasing the sharpness of the photo. If you want to get an idea when diffraction sets in, shoot a shot like this and examine the bright points in it. As you close down the lens (compensating with shutter speed or ISO), you'll get to a place where the star shaped diffraction spikes "bloom" and start to become much more pronounced. Keep in mind, this is happening everywhere there is light in the photo but at a lesser scale due to the brightness of those points.

The actual result is subtle, and may not be that objectionable until it's well pronounced. It's much like shooting with the lens wide open where some sharpness is sacrificed to get the shot. Here you're making an IQ sacrifice to get better depth of field or just to overcome too much light, but it is worthwhile to get a "feel" for where the effect sets in for your most used lenses, and yes, it can be used to an advantage to produce "star like" highlights in a shot if your lens has that much diffraction closed down.

Last edited by Bob 256; 11-26-2021 at 11:26 AM.
11-26-2021, 05:34 PM   #7
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Personally I would go beyond F13 on any lens. The image in my eyes starts to become dull and flat.

11-26-2021, 07:51 PM   #8
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well, the more I look at the image the more I find wrong with it. Color balance is way of (different types of light sources aren't resolving right). Photoshop screwed up the stacking (several artifacts, double imaging). I reran the whole thing and the image is much cleaner. Looking at individual frames, you can't see any fringing and it's much sharper. What's worse is I'm color blind so color correcting is a real challenge. will repost and see what the collective thinks.
11-26-2021, 07:59 PM   #9
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You mentioned medium format and 50MP so lets assume you are shooting with a 645Z since you are on this forum. At f/22 the airy disc from a point source of light would have a diameter of 29.3um and the 645Z looks to have a pixel pitch (good stand in for pixel diameter) of 5.32um. So this tells us that a point source of light from an ideal lens would basically cover almost a 6x6 grid. It isn't higher resolution it is larger pixels that is more forgiving, so a more pixel dense camera would have more pixels covered byt the airy disc from a point light source than a less pixel dense camera. Since there is a Bayer matrix in the mix there is interpolation of getting full color data for each pixel so the apparent detail loss isn't that great but still should be visible in the images even with some light sharpening that was either performed in camera or by the raw developer (RawTherapee lets you turn this off if desired which is nice for exceptionally sharp lenses as it avoids sharpening noise). In the images you posted there is some slight softness and what would appear to be evidence of diffraction existing. Basically diffraction eats fine detail but depending on how much contrast was in that find detail some of it can be brought back with some sharpening. Hard high contrast edges come back pretty easy, texture on a surface not so much. You see this a lot on cellphone shots that contain leaves in them which makes it easy to tell it was shot on a cellphone as the leaves lack any fine detail other than their edge.

This is one of those topics where I don't worry about it unless the really fine detail is absolutely needed and even then there are probably better ways to get that detail and resolution if I really need it (stitched panorama). So I mostly view it as am I shooting to get the depth I need and the shutter speed I need with an acceptable amount of noise. Those are all higher concerns to me over diffraction becoming an issue.
11-26-2021, 08:57 PM   #10
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Here's hopefully what will be the last version of this. Color still could use some tweaking but I'm satisfied with this. No artifacts or double images, color seems to better balanced.
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Last edited by cdd29; 11-27-2021 at 06:58 AM.
11-27-2021, 12:12 AM   #11
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Back when I had the appropriate test equipment f/13 was the knife edge. Some lenses didn't like to go past f/11. Almost always f/16 was obviously worse.
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