Originally posted by RioRico I know that other members here, like falconeye and Ben Edict, have written knowledgibly on diffraction. I'm sorta appealing to them for clarification. Help!
Do I see my name here ?
Well, for easier reference by others researching the issue, I'll refer to my post first, where I tried to sum it all up.
It studied 2 different comparisons:
STUDY:
We compare two cameras, with same pixel count but different sensor size, and the camera with the smaller sensor has a focal length smaller by the same factor, too. Where:
(A) Both cameras use a lens of exactly the same physical diameter in mm.
(B) Both cameras use a lens of exactly the same f-stop number.
Read it all here:
https://www.pentaxforums.com/forums/459034-post21.html
With respect to your original question, I noticed that you already confused circle of confusion with pixel size.
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To sum up my answer above with respect to diffraction:
Take situation (A). Here you see that the diffraction effect (as far as a printed photo would be concerned) is independent from the sensor size. But because smaller sensor cameras tend to go with smaller diameter lenses they tend to hit the diffraction limit sooner (in real photography practical terms).
As for the resolution limit:
The limit is where two Airy disks overlap at distance of radius r (formula above r = 1.22 lambda N), i.e., where a line pair has width r and a line (pixel) has width r/2. But at that limit, you are left with marginal contrast only and you would need a lot of sharpening.
For practical purposes, you double this and require r <= feature size.
Where feature size is pixel size (pixel peeping) or circle of confusion CoC, whatever is your requirement.
K20D: Pixel = 5 µm, CoC = 20 µm.
So, the most obscene requirement translates to:
N <= 5 µm / (1.22 lambda) ~= 8
but the naked eye won't see diffraction on a print on the wall if
N <= ~ 32 (i.e., never).
Nevertheless, the best one or two 35mm SLR lenses are so good that diffraction makes them resolve best at f/2.8! And only quite bad lenses resolve better at f/8 than at f/5.6.
If you have to trade softness from DoF against diffraction, the 100 y.o. term "Förderliche Blende" comes to mind ... (critical / usable aperture)