Re: 10x Magnifiers for Stereo Viewer

[bercov@xxxxxxxxxx (John Bercovitz)]

Alex writes:
 
> I just came home from a photo-show. I couldn't resist to buy two focusing
> 10x magnifying lenses to build a stereo viewer.
[stuff deleted]
> The lenses have a 23 mm diameter, and the Triplet's magnification is
> approx. 1/3 less than that of the Realist Red Button viewer.
 
Some things don't quite agree here, I think, Alex.  
 
First the magnification doesn't seem right.  The red button has lenses 
maybe 43 or 44 mm in focal length.  The "magnifying power" of a red 
button lens, then, is 250/43 = 5.8X conventional, or 6.8X advertising 
(advertisers get to add 1 for focussing in so that the virtual image is 
at 250 mm instead of at infinity).  A 10X magnifier's focal length would 
be 250 over 10 or 9, say 9, which is 28 mm.  So the 10X would give much 
_more_ magnification than the red button's lens, not less.
 
As for field of view, the Realist format has a 31 mm diagonal so if the 
10X magnifier covers that format, and has a 28 mm focal length, its field 
of view is 2(arctan 31/(2*28)) = 58 degrees.  There are magnifiers with 
that field of view but all the ones I know of have a tremendous amount of 
distortion, readily noticed.
 
These numbers appear conflicted unless there is yet another definition of 
magnification of which I am unaware.
 
John B


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