format image comparisons

[bercov@xxxxxxxxxxx (John Bercovitz)]

I think I'll go with Dr. T's theory of image size and quality being
the determinants.  Image size is really image field of view and that 
goes roughly as the diagonal of the image divided by the focal length 
of the viewer lens.  Before you condemn the smaller formats for lower
field of view, remember that it is very very tough to design a viewer 
lens for wide coverage if it is made short to match a small format.  
So a compromise is made and the lens is made a little too long and 
the coverage is reduced.  Here are some rough values.  (They're rough
because I can't remember the exact figures for V-M.)

      format          angle of view,  % ortho*  diagonal    focal
                         degrees                            length
ordinary V-M viewer        17           50         15?        50
better V-M viewer          22           64         15?        39? 
Realist                    40           81         31         43 
medium format              48          100         72         80

I gave the input values I used after the % ortho.  That way if I've got 
some wrong, someone can correct them.  Because ordinary-design lenses have 
wide coverage if they're only long enough, the best viewer is the medium 
format viewer I made using Edmund achromats.  When I use it with Sputnik
pairs, there is no more resolution than Realist pairs, just more 
coverage.  However, if I do slide bar work with the Mamiya, it
definitely puts the red button to shame.  Note that the medium format
is the only ortho viewer in the lot.  Makes a difference with closeups
of known objects.  So it has the widest coverage and it's ortho.

After Dr. T sold me a better quality V-M viewer, I found the V-M reels
a whole lot more interesting.  Not only are the viewer's lenses shorter, 
they're better quality (lower aberration) lenses.  The V-M has a very
strong advantage over the other formats - it has compact multi-slide 
capability.  Medium format has a strong advantage in view quality.
The 35 mm format is a nice compromise.

*focal length of camera lens divided by focal length of viewer lens.
Inverting this ratio gives you percent stretch.

John B


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