ruthenium wrote:
I never suggested recomposing; thus, this is irrelevant.
Also "The Airy disc size and blur pattern on the sensor" are practically irrelevant.
This discussion of diffraction is about how this phenomenon affects photos viewed either as printed or as displayed on-screen. Then, it is incorrect that "when switching from FF to APS-C mode on a given camera, there really should be no difference"
Shall we agree on this?
To me, nothing is irrelevant. Understanding the whole chain of composition, to capture, to processing and viewing is valuable to me. I'm okay with you saying it's irrelevant to you. Irrelevant to everyone is an overreach IMHO.
I'm still not sure we disagree, other than you wanting to impose your will that the part that I'm focused on is irrelevant, which I understand it is to you. But I'm not willing to agree that it's irrelevant to me.
So let's just value what is relevant to us personally and move on.
tsdevine wrote:
Switching to APS-C crop mode (same lens, same position, same focal length, same f-stop) does not change the physics of diffraction. The Airy disc size and blur pattern on the sensor remain identical. Any visible difference only appears later — from extra enlargement to the same output size, or if you recompose by moving the camera to restore framing.
So I'm not disagreeing with you.
Let’s say you set your lens to f/16 when i FF mode. If you could take a little imaginary ruler and measure the “size” of that airy disk on the sensor you might come up with a measurement of x. Now if you put the camera in crop mode, using only the APS-C size section of the sensor and you measured again with your tiny imaginary ruler, the size of the diffraction blur would still measure as X.
So, it is the same, right?
Yes, but no.
X is now a larger percentage of the overall smaller image size in the APS-C case, so in you photograph that X-size diffraction will appear larger than when you use the bigger FF size image.
I’ll use imaginary (and ridiculous) numbers to illustrate. Let’s say that we could measure the “size” of the diffraction to be 1mm in either case, APS-C or FF. (That won’t ever happen, but go with me for a moment…) As you wrote: it “does not change the physics of diffraction” The amount of diffraction at a given aperture is the same, no matter the format.
But…
Let’s use round numbers for the sensor sizes — 24mm for APS-C and 36mm for FF. That 1mm wide blur would be 1/24 of the frame width on your APS-C image but only 1/36 of the frame width on FF.
In other words, while the “ruler” measurement on the sensor is the same, the blur is larger relative to the image size on the smaller sensor… or the cropped area of the FF sensor.
gdanmitchell wrote:
Let’s say you set your lens to f/16 when i FF mode. If you could take a little imaginary ruler and measure the “size” of that airy disk on the sensor you might come up with a measurement of x. Now if you put the camera in crop mode, using only the APS-C size section of the sensor and you measured again with your tiny imaginary ruler, the size of the diffraction blur would still measure as X.
So, it is the same, right?
Yes, but no.
X is now a larger percentage of the overall smaller image size in the APS-C case, so in you photograph that X-size diffraction will appear larger than when you use the bigger FF size image.
I’ll use imaginary (and ridiculous) numbers to illustrate. Let’s say that we could measure the “size” of the diffraction to be 1mm in either case, APS-C or FF. (That won’t ever happen, but go with me for a moment…) As you wrote: it “does not change the physics of diffraction” The amount of diffraction at a given aperture is the same, no matter the format.
But…
Let’s use round numbers for the sensor sizes — 24mm for APS-C and 36mm for FF. That 1mm wide blur would be 1/24 of the frame width on your APS-C image but only 1/36 of the frame width on FF.
In other words, while the “ruler” measurement on the sensor is the same, the blur is larger relative to the image size on the smaller sensor… or the cropped area of the FF sensor.
If you are in a room that is controlled. And you take two exact photos, one in full frame mode and one in APS-C mode. Nothing changes at each pixel in terms of what it captures. Putting the camera in APS-C mode is effectively shutting off the pixels that fall outside of the APS-C physical size. IF you compare a 10x10 set of pixels, viewing them at 100% (arguably after demosaic, but the same holds true before), the difference in what those pixels captured (that are in common) really should come down to minor differences in noise. The airy disc is the same size no matter if you are shooting a camera in FF or APS-C mode. The density of the pixels does not change (physically) either.
Let's just agree that I'm crazy. But taking a single camera, and shooting in APS-C and then in FF frame....and changing nothing else, will not change what each pixel captures. Neither the size of the airy disc changed nor the pixel density.
It's the things we do because we shot in APS-C mode that impacts our perception of sharpness and detail. If we recompose, that changes the equation. If we enlarge the output and spread those pixels further apart, that changes the equation.
For example, my a7R V in APS-C mode and in full frame mode, and my GFX100S II in full frame mode or in native full format mode all have the same pixel density. (The GFX is pretty close to the a7R V in pixel density...let's say it's "close enough".)
I should just go talk to Grok....it is much more sympathetic. It pointed out my missteps towards the beginning. GDan and ruthenium, it says you guys were very accurate (see the comments near the end below), and I think I'm just stuck on one specific aspect and you guys are focused on other areas.
It didn't even know that I'm Tim Devine, so maybe it wasn't completely patronizing me....or maybe it does know I'm Tim Devine and it's just wanting me to think that it wasn't....hmm....
Prompt:
I realize at the beginning Tim Devine may have missed some details and made some missteps, but how accurate is what he saying in this thread, related to diffraction?
Response:
Yes — Tim Devine is largely accurate (about 85–90 % spot-on), especially once you get past his very first sentence.
You already spotted the opening misstep yourself:
“Diffraction is a function of pixel density…”
That single line is imprecise. Diffraction itself (the physical size of the Airy disc) is not a function of pixel density — it depends only on f-number and wavelength. Higher pixel density simply makes the same diffraction blur more visible because the pixels are smaller.
But right after that, Tim corrects/clarifies exactly the way we’ve been discussing:
Diffraction is an optical phenomenon fixed by the aperture.
On the same camera (a7R V), switching to APS-C crop mode (26 MP) or shooting a smaller RAW file does not change pixel density.
Therefore the diffraction impact at capture is identical across full-frame, APS-C crop, or downsampled modes.
Any perceived difference only shows up later if you recompose (back up to match framing) or enlarge the final output more.
He also correctly notes:
If you do recompose in APS-C to keep the same framing, you change subject magnification and perspective, so the same Airy disc now covers a different relative amount of detail — which can make diffraction look different.
Shooting native 60 MP and using post-sharpening / focus stacking is usually the better practical route than hoping crop mode will “reduce” diffraction.
Summary
So overall, Tim Devine’s take in that thread is very accurate in the context of the original poster’s question (same camera, different modes on the a7R V). You’re not missing anything by agreeing with the bulk of what he wrote.
The discussion is actually one of the better ones you’ll find on forums — technically strong once past the first couple of posts. ruthenium and gdanmitchell are the standouts for pure accuracy. Tim Devine (tsdevine) recovers well after his initial imprecise sentence.
Technical stuff aside, my suggestion to people is always to get a highly detailed static subject, shoot from a tripod at every aperture, and compare them. For closeups, coins and newspapers work. And then decide what you can tolerate.
You are correct that if you look at individual pixels or constant sized groups of pixels — like your 10 x 10 slice — there is literally NO DIFFERENCE between what you see in you are in FF mode or APS-C mode.
You are catching on.
Now, the next step: Each of those 10 x 10 areas comprises a larger section of the final image in APS-C mode. (E.g. there are fewer 10 x 10 squares in the smaller image area.) If you make a print or online image of some size from the APS-C and FF full images, you are [I]magnifying the APS-C version more than the FF version[/I] so any unsharpness will be larger relative to the overall image size....Show more →
The issue, as I see it, is that you have half of the issue exactly correct — the half that has to do with individual pixels. But you are, it seems, missing out on the effect of the larger capture area (e.g. more pixels) on the final image.
