Diffraction, aberrations, & fill-factor in the object field

This is a continuation of a report on new ways to look at depth of field. The series starts here:

A new way to look at depth of field

Yesterday, we looked at object field MTF50 behavior with the lens focused at infinity, and found that with our simulated top drawer 55mm lens, a simulated Sony a7RII, the middle-distance curves were not flat, as object-field theory says they should be. The gross reason is not a mystery: lens aberrations, diffraction, and fill-factor considerations all conspire to provide other sources of blur than just geometrical optics.

But which one is the most important?

I ran a series of curves at f/4 at pixel pitches varying from 3 micrometers (um) to 8.5 um, with each pitch being 1.414 times the previous one.

First in the image plane:

And then in the object field:

The blue line is for the 3um pitch. You can see that decreasing the pitch makes a big difference.

Zooming in:

OK, now let’s back out the lens aberrations:

That helps. The 3 um line is now pretty flat until 40 or 50 meters.

Now I’m going to take out diffusion. You can’t do this in real life, but you can with a sim.

Diffraction doesn’t make all that big a difference; that’s because we’re at f/4, and because we took it out last. Diffraction is a slightly larger effect than the lens aberration measure that we’re using.

{The material below was added later.]

I’ve been asked to show what the above vurves would look like at f/8, Here goes, first in the image plane: