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

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Subject: [Leica] Lens Design Question
From: red735i at (Frank Filippone)
Date: Wed Oct 10 07:45:15 2007

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