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help with -diopter calculation needed

  • From: P3D E R Swanson <ers@xxxxxxxxxxxxxxxxxxxx>
  • Subject: help with -diopter calculation needed
  • Date: Mon, 14 Jul 1997 21:36:06 -0700 (PDT)

I need some help coming up with the correct minus lens to modify a viewer
into a viewfinder. 

Currently the lens (after bouncing on two front surface mirrors) focuses
at 60mm. What calculation do I use to figure out what amount of -diopter
is needed to have it focus on an image 37mm further away (total 97mm). The
lens is 11mm in diameter. I plan to install the supplemental lens behind
the existing lens.

Thanks, Elliott


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