Image circle versus stopping down?

Stacey wrote in message ...
Richard Knoppow wrote:

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. A non-orthograpic lens, like a fish-eye
lens, can have better illumination because the light is more
concentrated near the margins.


What about retro focus wide angle lenses? While not common on LF cameras,
how does that design compare to "normal" wide angle lenses as far as light
fall off? I have noticed almost no light fall off on the 30mm medformat
fisheye and even the regular wide angle SLR lenses don't seem to be as
affected by fall off as some of the wide LF lenses I've used.

Extreme examples of reversed telephotos can have zero falloff *and*
zero distortion. The problem is that they are really really big. Way
too big for large format. See the simple two-element example I posted
above. Systems like this are actually used for high performance DMD
and LCD projections systems which require perfect telecentricity,
extremely large working distance, and zero distortion. Zero falloff
means cos^0. Actually, its possible for the corner illumination to be
slightly *greater* than the center illumination.

Brian
www.caldwellphotographic.com

"brian" wrote in message
m...
"jjs" wrote in message
...

Funny, but I don't find the 38mm Biogon that bad in real life.

Richard K. is correct about Biogon types: they are better than cos^4,
but don't expect any better than cos^3. In fact, you'll be lucky if
you're getting as good as cos^3. This amount of falloff can be
surprisingly subtle as long as its gradual. [...]

Thanks to all for the excellent information. I may entertain the idea of
making a graduated filter for the big 76mm lens, but that would be a
separate thread.

To close on the 38mm Biogon - with all of it's other great characteristics
it seems that a center filter would be the finishing touch.