Archive 2022 · What does Diffraction Correction do, exactly?

AmbientMike
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p.1 #1 · p.1 #1 · What does Diffraction Correction do, exactly?

I leave Diffraction Correction on in lens corrections lately, is this a bit of in-camera USM or something sophisticated you can't really do in pp? What does it do, exactly?

May 02, 2022 at 11:02 PM

jj1804
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p.1 #2 · p.1 #2 · What does Diffraction Correction do, exactly?

I assume you already found this: https://www.canon-europe.com/pro/infobank/in-camera-lens-corrections/

In 2016, the EOS-1D X Mark II introduced Diffraction Correction, which compensates for the loss of sharpness at small apertures. It also compensates for the slight reduction in resolution that can occur as a result of the presence of a low-pass filter in the sensor. The low-pass filter is important for avoiding moiré interference in images with fine repeating patterns.

and below this text there are two images. The text below the left image states:

When you're shooting JPEGs using a camera that offers the Digital Lens Optimizer function, the camera can make image corrections that are otherwise available only in post-processing or in-camera RAW processing.

So, I'd interpret this as it does not do anything for raw files and hence it's useless for most folks on here. What you assumed "a bit of in-camera USM" is also what I would suggest this does to the jpg file depending on used F-stop. I could be totally wrong tough and am interested to read from someone who actually knows what this function does

May 03, 2022 at 02:57 AM

AJSJones
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p.1 #3 · p.1 #3 · What does Diffraction Correction do, exactly?

I expect it is some form of deconvolution - if you know the aperture you can calculate what happens to the image of what shoild be a "point" as it is spread by the diffraction effect into a blurred image of that point. This is known as a point spread function and computer algorithms can "remove" that effect by a process known as deconvolution. It is different in principle from USM and can produce better results, but it's complicated

See, for example, the wikipedia article on Richardson–Lucy deconvolution (FM software messes up a pasted link)

May 03, 2022 at 10:24 AM

gdanmitchell
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p.1 #4 · p.1 #4 · What does Diffraction Correction do, exactly?

AJSJones wrote:
I expect it is some form of deconvolution - if you know the aperture you can calculate what happens to the image of what shoild be a "point" as it is spread by the diffraction effect into a blurred image of that point. This is known as a point spread function and computer algorithms can "remove" that effect by a process known as dconvolution. It is different in principle from USM and can produce better results, but it's complicated

See, for example, the wikipedia article on Richardson–Lucy deconvolution (FM software messes up a pasted link)

I wonder if this is really what is happening, though. If I were Canon and I had actually implemented effective in-camera deconvolution* I would say so. That would be a pretty big deal! (It would also probably be revealed by the occasional presence of some unwelcome artifacts, too, but I digress...)

The description at the above link of what their diffraction correction actually does is quite vague, though. Oddly, the text says that, whatever it is, it also "compensates for the slight reduction in resolution that can occur as a result of the presence of a low-pass filter in the sensor."

My bet is that it simply does a bit of small-radius-large-amount traditional sharpening that varies depending on the aperture information in the file... which is just what we'd do in post to reduce the effect of diffraction blur a bit.

One thing I discovered by testing a lot of files is that while diffraction blur is undeniable a real thing that has an effect of image sharpness as we stop down... the actual effect of finished photographs is often quite small, and once we apply optimized post-production sharpening its effect is even less.

I encourage anyone trying to sort out the real world effects of aperture choice on sharpness to simply take the time to put the camera on a tripod, make some photographs at various apertures, take the files through optimized post-production routines, and then take a look at what happens in the end. Do look at 100% magnification crops to understand what is going on, but also take a look at real world results in the form of typical size electronic files and prints.

BTW, here's another interesting link: https://gregbenzphotography.com/photography-tips/what-is-deconvolution-sharpening-and-how-to-get-increased-detail

Dan

The types of sharpening we usually apply — in-camera or in post — are basically contrast enhancement at edges. As I understand it, deconvolution essentially tries to analyze the blurred image to determine what original image could have produced that blur and then backtrack to the speculative original. That ain't a simple task!

May 03, 2022 at 11:18 AM

AJSJones
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p.1 #5 · p.1 #5 · What does Diffraction Correction do, exactly?

gdanmitchell wrote:
[
The types of sharpening we usually apply — in-camera or in post — are basically contrast enhancement at edges. As I understand it, deconvolution essentially tries to analyze the blurred image to determine what original image could have produced that blur and then backtrack to the speculative original. That ain't a simple task!

Indeed, the full set of (hopefully) convergent deconvolution iterations takes a lot of processor power. There are, however, approximations that claim to produce much of the possible benefit without quite such involved processsing. Some software uses such things (proprietary I suppose) Focus Magic, Topaz InFocus (now part of Sharpen AI, IIRC) and an early form is discussed in this threadRaw Therapee (Apparently Hubble images are processed with it). Whether it's the point spread funcction from the AA filter or diffraction from the aperture (or both), since it's a global effect on all pixels it can, in principle, be "reversed" - in contrast to local softening due to off-axis aberrations and optical flaws, etc.. which are increasingly dealt with by other aspects of "computationl photography"

May 03, 2022 at 12:04 PM

gdanmitchell
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p.1 #6 · p.1 #6 · What does Diffraction Correction do, exactly?

AJSJones wrote:
Indeed, the full set of (hopefully) convergent deconvolution iterations takes a lot of processor power. There are, however, approximations that claim to produce much of the possible benefit without quite such involved processsing. Some software uses such things (proprietary I suppose) Focus Magic, Topaz InFocus (now part of Sharpen AI, IIRC) and an early form is discussed in this threadRaw Therapee (Apparently Hubble images are processed with it). Whether it's the point spread funcction from the AA filter or diffraction from the aperture (or both), since it's a global effect on all pixels it can, in principle, be "reversed" -...Show more →

The article I linked to above reminded me that Adobe reportedly incorporates this into their products, too, via the "details" slider.

Dan

An aside: I'm reminded of something that happened decades ago when I was taking a computer music workshop up at the old Stanford Artificial Intelligence Lab. (I'm not a Stanford grad — just took the workshop there.) This was back when digital (music) synthesis, which I was studying, was not widely known at all. At that point it was all about analog synthesis and tape manipulation and similar.

One morning we were all gathered in the room that housed the multi-track tape (!) recording system used to capture the playback of computer-performed compositions. The faculty member, Dr. Leland Smith, went into a bit of a tangent concerning speculation that the evolving capabilities of computers would eventually allow something like this with music. His example was to take old Louis Armstrong vinyl recordings and to apply everything we could know about the recording process of the era, the acoustics of the space in which the recording took place, the characteristics of the instruments, and so forth... to analyze the recording to create separate tracks of each instrument that could be recombined into a very high quality stereo recording.

Decades later, we can do a few things that are sort of like this, and the power of computers in the production of music is undeniable. But the most sophisticated aspects of it are still beyond us.

May 03, 2022 at 12:33 PM

AJSJones
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p.1 #7 · p.1 #7 · What does Diffraction Correction do, exactly?

As post-processing computers (and those in some cameras, such and the Olympus OM series) improve in power, so these routines become easier to incorporate and implement.

Your aside reminds me of one from the same era: a column in the New Scientist speculated that if we knew the shape of the stylus used by early pottery wheel operators we could take one of their pots and put it on a turntable with that stylus and it would "play back" a record of the sounds happening when the pot was made

May 03, 2022 at 12:52 PM

AmbientMike
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p.1 #8 · p.1 #8 · What does Diffraction Correction do, exactly?

Interesting comments so far here's a couple links hopefully they will go. Richards Lucy deconvolution and stack exchange

https://en.m.wikipedia.org/wiki/Richardson%E2%80%93Lucy_deconvolution

https://photo.stackexchange.com/questions/85304/does-diffraction-correction-correct-the-degraded-resolution-due-to-the-low-pas#:~:text=Canon's%20Diffraction%20Correction%20is%20a,for%20a%20particular%20camera%2Fsensor.

May 08, 2022 at 11:51 AM


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