[BasilG]

I am open to learning something new here, but so far, I have not seen an explanation for the phenomenon at hand - zero contrast transfer rings and contrast inversion - that is better than mine (with support from Wikipedia, for what it's worth ). So, my conclusion is - the transfer function drops below zero due to defocus and in fact oscillates back and forth across zero. The zero crossings are seen as grey rings lacking stripes, and where the transfer function is below zero, contrast inversion occurs, giving rise to the alternating zones of inverted contrast in my image (and the image in the original post).

 

As I said, photons are not my favorite discipline, but if you scroll down to the Figure 8 in the following link, you will see that at least these guys agree with me (in addition to the guys at Wikipedia):

 

http://micro.magnet.fsu.edu/primer/anato...intro.html