That's a bit of a distinction without a difference.
...And the flaw in your methodology (and your conclusion) is that you used an unsharp lens for your tests. The worse a lens is optically, the more you can stop it down before diffraction becomes a greater problem than the lens aberrations. [a href=\"index.php?act=findpost&pid=69319\"][{POST_SNAPBACK}][/a]
Hi! Jonathan,
Thanks for taking the time off from helping protect America .
I don't believe there is a flaw in my methodology. (After all, I used tripod, remote cord, high shutter speeds, no IS
and even MLU, to be sure to be sure.) I've perhaps just given the thread a misleading title. My purpose was not to demonstrate that all lenses can be sharpest at f22. Of course I know that the better the lens, the sharper it is at larger apertures.
My purpose here was to give a thorough test to the issue of 5D versus 20D. Is it really worth bothering carrying around my 20D just in case I want to shoot a distant subject which is out of the reach of my 5D?
I fully expected the 20D with 400mm and extender to produce a sharper and more detailed image than the 5D, at least to some small degree. But I was very surprised that this advantage was most apparent at f22. There are 2 main reasons for my surprise.
(1) There seems to be widespread concern that increasing pixel count with Canon DSLRs is serving no purpose because Canon lenses are (generally) not up to the job. Yet here, I've used a lens that is so bad it actually appears to be sharpest at f22. In such a situation, one would expect the sensor with the highest pixel density (the 20D) to be at a disadvantage. It doesn't appear to be at a disadvantage.
After cropping the 5D/560mm image to the same FOV as the 20D/560mm image, I'm comparing (in 16 bit) a 15MB image with a 46MB image. That's a huge difference in pixel count and it seems to be paying off, even with a lousy lens.
(2) I've seen lots of arguments on the net, from smart arse pundits quoting mathematical formulae describing the size of Airy Disks, who have tried to make the point that once the size of the Airy Disk at a particular f stop exceeds the pixel pitch of the sensor, then no further purpose is served by greater pixel density.
I find it difficult to believe that the Airy Disk at f22 is not a lot bigger than the 20D pixel pitch.
The bottom line for me, that's emerging from these test shots, is that a future 22mp upgrade to the 5D should serve a worthwhile purpose even with a mediocre lens.