Airy disc size and modern DSLR cameras

hollis_f wrote in post #12692644
However, there's one additional point to bear in mind. Diffraction will not make the image on a sensor with more, smaller, pixels worse than the image on a sensor of the same size with fewer, larger pixels. The image will, in theory, be better or the same.

The nature of digital makes that a bit uncertain.

The reason is that with a higher resolution, some pixels are going to be capturing the edges of the airy disc whereas with a lower resolution, none will (assuming the same aperture setting on both). When you downsize the larger resolution to the smaller, the pixels that captured the edges will be merged in with others that captured something closer to the center, but that result is still worse than one where none of the pixels captured the edge.

With a high enough resolution, the number of pixels that captured the center portion will tend to dominate enough that there should be little difference, but up until that point it's unclear to me how much worse the result will be.

Bang Bang Boy wrote in post #12692701
Thanks, had a rough night yesterday. This is really bad news for landscape photographers.

Not really. From what I've seen, the effects of hitting diffraction limits aren't as bad as you'd think until you really stop down hard. You have to look hard to see the effects, and sharpening can counteract the effect itself (you won't recover the detail but the result will at least look good).

I meant to comment on this earlier.

We know that the resolution degrades beyond f/8, so presumably diffraction becomes an issue. I'm curious whether anyone has tested how their lenses stack up against the diffraction limit - particularly when the lens is between wide open and (say) f/8.

Of course, it's easy to look at the MTF plot and say it has 50% contrast at 40 lp/mm (or whatever) but that only translates to an Airy disc of 1/40 mm (or is it 1/80 mm... it's late.) In either case, the lens would be a long way from the theoretical f/2 resolution of 50% contrast and 1.32 micron spot size.

If not, I have an adapter that allows me to mount my EOS lenses on my CCD camera (5.4 micron pixels, no AA filter.) IF I can find a sufficiently good light source it may be an interesting project for the winter nights. I'm guessing that none of my lenses will come close until they're stopped down to f/5.6 or maybe more...

Logicus wrote in post #12691648
Gotcha... you know, I was recently noticing that the dof seemed almost "exaggerated" when moving lenses over to the 5dmkii- especially on my 10-24mm which is an asp-c lens, but works down to 13mm, which is pretty wide, full frame. It's an f/3.5-4.5 lens, but the shots look more like f/2 or so when actually at f/3.5. Makes sense.

This is a function of distance from the subject. f/3.5 remains f/3.5. How you choose to frame the photo in the VF (thus altering the physical distance between your camera and the subject) will result in the change of DoF.

jwcdds wrote in post #12695982
This is a function of distance from the subject. f/3.5 remains f/3.5. How you choose to frame the photo in the VF (thus altering the physical distance between your camera and the subject) will result in the change of DoF.

Quite right, I should have said "in relation to the 40D - or 1.6x crop cameras."

I encounter this depth of field / resolution trade-off when photographing circuit board assemblies. Varying component height, along with the fact that corners are further than the center, create a need for some depth of field. When using a 60mm macro lens at 1-3 feet, even F/9 doesn't always give enough depth of field. Maintaining the best resolution possible allows zooming the photo to any individual component. The lower F stops sometimes give noticeably better results that higher F stops, but only on components that are perfectly focused.