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From: josephs@ttidcb.UUCP (Bill Josephs)
Newsgroups: net.rec.photo
Subject: view area vs. film area
Message-ID: <336@ttidcb.UUCP>
Date: Mon, 22-Apr-85 13:48:38 EST
Article-I.D.: ttidcb.336
Posted: Mon Apr 22 13:48:38 1985
Date-Received: Wed, 24-Apr-85 04:05:01 EST
Organization: TTI, Santa Monica, CA.
Lines: 21


     There's another much more important explanation of why cameras
have different view areas than film areas:  different focal length
lens have different light paths in the film chamber and the rapid
return mirror consequently intercepts this light path differently.  It
is difficult to describe, but consider the cross section of the film
chamber with the lens on the right, the film plane on the left and the
camera as a box in front of the film plane.  The easiest mirror to
design is then a diagonal from the upper left towards the lower right
with a minimum length equal to the size of the film (since when the
mirror is flipped up, it must at least cover the ground glass).  The
longer the mirror, the deeper must be the camer body.  The shorter the
mirror, the more likely some of the rays from long focal length lens
will "escape" below the edge of the mirror.  Also, the longer the mir-
ror, the more force it takes to flip it up and the harder it becomes
to absorb all that torque and stop it smoothly and quietly.

     I hope this makes sense....

								Bill Josephs
								Citicorp/TTI