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Path: utzoo!watmath!clyde!cbosgd!hplabs!oliveb!tymix!kanner
From: kanner@tymix.UUCP (Herb Kanner)
Newsgroups: net.rec.photo
Subject: Re: Rangefinders and other things
Message-ID: <709@tymix.UUCP>
Date: Sat, 22-Mar-86 13:48:38 EST
Article-I.D.: tymix.709
Posted: Sat Mar 22 13:48:38 1986
Date-Received: Wed, 26-Mar-86 02:44:47 EST
References: <707@tymix.UUCP>
Reply-To: kanner@tymix.UUCP (Herb Kanner)
Organization: Tymnet Inc., Cupertino CA
Lines: 36

In article <707@tymix.UUCP> kanner@tymix.UUCP (Herb Kanner) writes:
>
>With this in mind, let me relate a calculation I did some years ago on the
>fake rangefinder screen that you find in most SLRs.  These are made by
>imbedding two crossed prisms in the glass.  The amount of image shift for a
>given focussing error is determined by the angle between these two prisms.
>The limitation put on this angle in manufacture is set by the requirement
>that it be possible to find a position for the eye at which both halves
>of the area be illuminated.  You all know, of course, that as you stop
>down, particularly with long focus lenses, that half of your split image
>thingy tends to go dark.  There is a certain fixed diameter D, for a given
>split image prism, such that if the lens is stopped down below D, one half
>will always go dark.  The relation D=F/f holds, where F is focal length and
>f is f-number.  So, the manufacturer designs the prism for a D value which
>is reasonable for his line of longer lenses.  I.e., if it is to work with a
>100 mm f3.5 lens, then D must be set at about one inch.  
>
This posting is to correct some errata before I get flamed.  I did the
above derivation that establishes the relation between a SLR split image
and an equivalent rangefinder some ten years ago, showed it to a couple of
interested people, and threw it away;  I have been writing from memory.
"Particularly with long focus lenses" should read "... with short ..."
				   ^^^^
"line of longer lenses" should read "... shorter ..."
		 ^^^^^^
A more precise statement would be that D is the diameter of the exit pupil
of the lens.  The D value for the shortest focal length lens made by the
manufacturer when the lens is wide open determines the maximum angle
between the crossed prisms.

Peace.

-- 
Herb Kanner
McDonnell Douglas (TYMNET)
...!hplabs!oliveb!tymix!kanner