|
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 |
Image circle versus stopping down?
"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. |
All times are GMT +1. The time now is 03:22 AM. |
|
Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
PhotoBanter.com