[rollei_list] Re: 6 x 9 search
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 30 Aug 2007 18:05:11 -0700
----- 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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