Hello,
I read in this forum a reference to Zeiss insiders by Peter K Peter K in a post from 2010.

>>>>>Reckmeyer, as I know a Zeiss staff member, coined the notion "critical f-stop", the f-stop a lens performes at it's optimum. The off-axis-abberations like spherical abberations are minimized and diffraction doesn't reduces the resolution in the center of the image circle. For a Tessar f/4.5 180mm he has measured the "critical f-stop" is f/12. In any case the resolution decreases in the center if a smaller f-stop is used but the resolution in the outer areas increases a little bit, the resolution in the whole image looks "smoother".<<<<

I could not find more details anywhere else.
I am curious to find similar theoretical information regarding other focal length.

Cheers,

Luc