Regarding the loss of quality caused by diffraction when using very small f stops:
Is there a specific diaphragm diameter (in mm) that is common to all lens focal lengths at which diffraction degrades the optics? Typically 50 mm lenses are best around f/8 and longer lenses at smaller f stops. I would think that somewhere in the range of 5 or 6 mm diameter any lens would achieve the best performance, whatever that f stop is for any particular lens. Somewhere I read that most lenses are best at 2 or 3 stops from the maximum aperture. Another factor might be ths shape of the iris. Most older leses have a perfect circle and some newer lenses have a definite geometric opening.

gma