Archived posting to the Leica Users Group, 2007/10/11

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Subject: [Leica] Lens Design Question
From: frank.dernie at (FRANK DERNIE)
Date: Thu Oct 11 04:14:54 2007

Hi Frank, particularly with WA lenses the size of the elements strongly 
depends on how retrofocus the lens is and how much vignetting the designer 
has accepted to eliminate the high abberation potential of the marginal 
rays. The super angulon 21s, which are close to symmetric, have smaller 
front elements than the Elmarits which are slightly retrofocus.... If I 
remember correctly (I am in the wrong country to check my own).

----- Original Message ----
From: Frank Filippone <>
To: Leica Users Group <>
Sent: Wednesday, 10 October, 2007 4:45:18 PM
Subject: [Leica] Lens Design Question

I just acquired ( from another Lugger) a 21mm F4 Voigtlander Skopar lens.

What I notice is that this lens uses tiny elements.  Whereas the Leica 21mm
offerings, even though they are 1 stop faster, use much larger elements.
Maybe 3 times the diameter.   I thought that the F stop of the lens is
dependent on the element size, and that the area of the element determined
its speed.  Going from F2.8 to F4 should be a doubling of the lens area...
or around a 1.4 times the diameter of the lens.  But t he actual lenses in
question are substantially larger then this....

Is the 1 stop faster speed increasing the element size by this really large
amount?  What gives?

Frank Filippone

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