Image circle versus stopping down?
"Richard Knoppow" wrote in message
One of the inherent properties of lenses which produce
orthographic images is that the illumination falls off with image
angle. For a "normal" design lens this fall off is approximately
proportional to cos^4 theta where theta is the "half angle" of the
image point. There are designs of lenses which have improved
illumination but these are not better than about cos^3 theta, better,
but there is still fall off. [...]
Care to help an innumerate? Taking the 38mm Biogon as an example, what's the
light fall-off in terms of F-stops?