Nathan Myhrvold's addition
to the discussion started by Charles Johnson is a useful additional perspective: not really digital specific, but just about what the aperture limits are if one wants to get the full resolution that one's film or sensor is capable of.
I have a disagreement on the actual numbers though, at least for the case of sensors using Bayer CFA's and interpolation, because that process lowers the resolution of the sensor output beyond "green pixel diagonal size", and thus relaxes the diffraction spot size limit a bit.
For example observations of several users of the Nikon D2X, with 5.5 micron pixel spacing, say that difffraction starts to limit resolution at somewhere between f/8 and f/11. Thom Hogan is the author of the f/11 figure, which gives it some credibility. Myhrvold's calculation instead gives about f/5.8.
So I propose a rule of thumb (dependent on how Bayer interpolation is done, details of anti-aliasing filters and such) that diffraction starts to cut into the resolution that a Bayer CFA sensor is capable of somewhere around twice the pixel spacing in microns, or as much as one stop under. That is, I would modify "Myhrvold's formula" to
max f-stop = P x 1.4 to P x 2 (instead of P X 1.054).
That makes quite a difference in "effective useful pixel counts" at a given high f-stop, by a factor of two to four.
For example, I suggest that full sensor resolution limits one to maximum f-stops of about
- f/8-f/11 for the D2X or the 400D and similar for other new 10MP SLR's
- f/11-f/14 for the 7.2 microns pixel spacing of the 1DsMkII, and for the 6.8 microns of the E-1 (for which Olympus recommends an f/11 limit)
- f/7-f/9 for the current smallest DSLR pixels, 4.75 microns in the Olympus E-400
- f/2.8-f/4 for the current digicam sensors with pixel pitch about 2 microns or a bit under.
Has anyone have sharpness tests at various f-stops on the 1DsMkII, G7 or other cameras, to allow a test of this idea and give us a better estimate of the best constant in Myhrvold's formula?