View Single Post
  #3  
Old July 1st 04, 03:24 PM
jjs
external usenet poster
 
Posts: n/a
Default Image circle versus stopping down?


"Richard Knoppow" wrote in message
om...

[...]
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?