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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: net.rec.photo
Subject: Re: re: Camera Vibration
Message-ID: <3468@alice.UUCP>
Date: Thu, 14-Mar-85 17:56:05 EST
Article-I.D.: alice.3468
Posted: Thu Mar 14 17:56:05 1985
Date-Received: Fri, 15-Mar-85 04:47:19 EST
References: <1253@drusd.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 16

Phil Lavette says:

> Similarly, there is a two-fold increase in brilliance achieved when a 60mm
> main lens is substituted for one 50mm in diameter.  A lot of the advantages of
> a larger aperature are lost when a compound lens refracting telephoto is used
> due to internal reflections between the glass surfaces. Cassegrian and catidiop-
> tric lenses perform better in this respect due to their simplicity.

I do not see how I can parse this as a true statement.
The obvious meaning -- that a 60mm lens puts twice as much
light on the film as a 50mm lens -- is false.  How much light
reaches the film depends on only two things: the amount of light
on the subject and the f-number of the lens.  Focal length
has nothing to do with it.

Phil, what did you mean?