I've been shooting my IQ180 at f8.0 (pixel pitch 5.2um) with very sharp results (Rodenstock HR lenses)
I figured the D800 at a pixel pitch of 4.8um should be the same, perhaps f7.1 would be better?
Hi Marc,
My rule of thumb is that when the diameter of the diffraction blur pattern exceeds 1.5x the sensel pitch, the effects of diffraction blur will start to be come visible at the pixel level. The microcontrast will be visibly reduced. That should not be a surprise, because starting at that diameter there will be 8 surrounding pixels that add a part of their infuence as well.
Using that criterion, it becomes easy to calculate the aperture where diffraction will start to take it's toll. For green light, which contributes most of the luminance signal, just multiply the sensel pitch in microns by 1.108 and the f-number is the result. So for a 5.2 micron sensel pitch that means f/5.7, and 4.8 micron means f/5.3 will be the onset of visible diffraction limitation. A lens that is free of most resolution limiting aberrations at that aperture is assumed. Many lenses will benefit from stopping down further to improve corner resolution, at the expense of center resolution.
Mind you, that doesn't mean that all resolution is lost at narrower apertures, but it does mean that low contrast microdetail becomes harder to reconstruct. It also means that aliasing artifacts will start being suppressed a bit. Restoration with an accurate Point Spread Function (PSF, model of the blur pattern for the lens at hand) can be quite effective if deconvolution sharpening is used. The issue then becomes finding the optimal PSF to use.
Cheers,
Bart