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Originally Posted by Emil Martinec
Thanks for the chart, Bart.
I was spurred by a related DPR thread
http://forums.dpreview.com/forums/re...ssage=26504942
to do a few calculations. I haven't yet done the calculation for a round aperture, as our lenses have (approximately, sue to the aperture blades); the math is much easier for a square one. For a square aperture, MTF as a function of spatial frequency linearly decreases with aperture until the size of the diffraction spot equals the spatial wavelength, at which point MTF is zero thereafter. I expect the result for a round aperture will be similar.

I haven't gone as far as calculating the MTF, my chart is just based on my empirical judgement that visual degradation seems to set in at approx. 1.5x the sensel pitch. This is also often supported by an observed declining MTF score at higher spatial frequencies when using narrower apertures.
An MTF degradation can be used to calculate a lowest limit of usefulness, and certainly a Sinc function is easier to use than an Airy pattern on an assumed square (sensel) aperture. I don't think we have to draw the line at a zero response, because at 10% MTF we're already pretty much at the practical limiting visual resolution (for average contrast subjects). The ISO also mentions a good correlation between 10% MTF and limiting visual resolution.
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So if you want maximum fine detail in the focal plane, even in the "green zone" of your chart, it's best to keep the aperture at the minimum value needed for DOF. I haven't yet done the calculation for the tradeoff between diffraction and DOF for the best MTF near the focal plane. Maybe later.

I could also run a simulation in Mathematica (which I use to calculate a 2D diffraction pattern PSF kernel), but it would still give a theoretical limit, because of the unknown properties of the AAfilter and residual optical aberrations. That's why I use the rule of thumb of 'sensel widths' which is useful enough to predict the visual onset of diffraction degradation. Any narrower aperture will degrade the per pixel resolution, and reduce the chance of successful deconvolution to restore the losses.
Bart