nobody wrote:

Again, I'm very interested to have others try my Photoshop experiment,
to see if my results are duplicated. This experiment eliminates all
other factors - it is solely the eye looking at a flat image field
simulating the image field at focus.

Since the effect can be photographed, it isn't an "optical illusion".

A more germane PhotoSlop experiment: make a large white area on a
black background. Put some small black areas on this white piece, at
various distances from the edge.

Engage the gaussian blur tool and watch as "black drop effects" appear
and disappear as you wiggle the radius of the blur around.

www.google.com: "point spread function"