[rollei_list] Re: 6 x 9 search
----- Original Message ----- From: "Eric Goldstein" <egoldste@xxxxxxxxx> 
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, August 30, 2007 4:32 PM
Subject: [rollei_list] Re: 6 x 9 search
You're way off relative to a 60ish mm lens shot on this format... the light fall off (not vignetting) is not anywhere close to an f/stop... 

Eric Goldstein
The half angle of a 60mm lens on 3x4 is around 40degrees. The fall off of an ordinary lens is around cos^4 theta where theta is the half angle. Lenses with tilting entrance pupils have less than this, closer to cos^3 theta. For a cos^4 lens its about 0.34 of the light at the center, for a cos^3 lens its about 0.44. One stop is 0.5, i.e. half the light, so either type will be down somewhat more than a stop at the corners. A Super Angulon is has a tilting entrance pupil. Older WA lenses, like the WA Dagor or Series V Protar are around cos^4, some older very wide angle lenses like the B&L Metrogon have even more than cos^4 theta fall off. For B&W a 60mm on 3x4 or 90mm for 4x5 is acceptable but one would probably want to use a center filter for color work, especially transparencies. 

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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