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[ATM] 35 mm Camera Lens

I finally got around to actually looking at the lenses I
have for my Olympus OM-G.  Both have the opening (near
the flange) of just under 25 mm.  If the film is (at least
that's what I read into R&vV) 24x36 mm and 50 to 55 mm away,
then vignetting takes it down to roughly 20% throughput at
the corners.  Can that be real?

I tried to look up "T mount" and found

    http://www.fli-cam.com/FLIsupport/tmount.htm

and found 1.45", though.  Why is the opening in my lenses so
small?

Now, if I put the inner doublet in the CORRECT order, tweak
the glasses, and cajole the optimizer a bit, here's a 75 mm
f/4 without all that vignetting (of course you're free to
add the stop):

    http://users.rcn.com/rflrs/calcn-88x.atm    ATMOS input
    http://users.rcn.com/rflrs/calcn-88x.len    OSLO input
    http://users.rcn.com/rflrs/calcn-88x.txt    TEXT file
    http://users.rcn.com/rflrs/calcn-88x.zmx    ZEMAX input

It may just pass muster --- or not.  The i-t Strehl is 0.916
(center) to 0.669 (corners).
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