Diffraction is not linearly consistent with "f/stop" ratios. It is a function of aperture diameter and distance-to-film plane, but not in the *same* mathematical proportion.

I've reviewed one of my old textbooks - "What is Light", by A.C.S. van Heel and C.H.F. Velzel (translated from the Dutch). Here they offer one of the simplest explanations of the diffraction phenomenon I've ever found. At that - `WAY too involved to be coherently condensed enough to be posted here.

An extract:
"We now notice that a light beam cannot indefinitely be made thinner and thinner. The following experiment will show that phenomenon of Airy's disc is indeed caused by the bounding of the light beams. We allow a wide parallel beam of light to fall on a screen with a small hole ( figure 34). A good ways off we put a second screen on which we observe Airy's disc, the diameter of which is seen to be inversely proportional to the diameter of the hole (and proportional to the wavelength of the light used), ...
... We will be able to explain the so-called diffraction phenomenon when we have studied the effects which appear as a result of the interference between several waves - ... "

The next ten or twelve pages are fascinating... but require a *great* deal of focused attention.

The net result, at least in my humble interpretation - applied to camera lenses - is, "Don't worry - be happy".
The lens designers are taking care of you.

Then again - there is the Holga...