Archived posting to the Leica Users Group, 1999/03/22

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Subject: Re: [Leica] Rangefinder/Superwide lens + Question
From: c.blaue@bmsg.de
Date: Mon, 22 Mar 1999 13:34:30 +0100

>> Buzz Hausner <Buzz@marianmanor.org> writes:

    >> Can somebody tell me if the coverage specified for the lenses (e.g. 92
    >> for 21 mm) refers to the diagonal or to the coverage from straight left
    >> to straigt right?
    >> 
     > 	I believe diagonal.  However, I don't know how accurate this stated
     > angle is from lens to lens...Erwin?

     > 		Buzz

A more precise value is 91.702105 for a 21mm lens. However this angle depends
on the real focal length and it holds only for the infinity setting of the
lens. If you set it to some other distance (usually less than infinity) the
distance between lens and film becomes bigger and the angle of view becomes
narrower.

The formula for the above value is derived as follows:

1. negative size is 24 x 36
2. The distances from the middle of the neg to the borders (not the corner!)
   are 12 x 18
3. the distance from the middle to the corner is by usage of the Pythagoras
   theorem sqrt(12*12 + 18*18).
4. the tangens of the half angle of view is then this radius divided by the
   focal length.

christoph