Re: calculating -diopter correction

[P3D E R Swanson <ers@xxxxxxxxxxxxxxxxxxxx>]

Sorry to bore the optical experts, but I kept looking at the numbers I was
coming up with, which looked about right via guesstimate, but I wasn't
clear on the math/optical functions. According to Kingslake, which I just
looked up, diopter is the reciprocal of 1 meter. So, a 100mm lens is 10
diopters of lens power. 

I remeasured the item I'm working on and refined the numbers (it involves
front surface mirrors and my initial center point was off). The focal
length I need is 90mm. The focal length I have is 55mm. Therefore if the
diopter value of the 90mm is 11.11, and the diopter of the 55mm is 18.18,
my correction factor is -7.07, which I can (via adjustment) probably get
by using a plain -7 (e.g. an off the shelf item vs. a specialty lens). 

So, if I finally have this correctly figured, I need an optician to cut
down a -7 diopter lens blank to 11mm diameter to make the correction
(actually two, since were dealing with stereo... :) 

If I screwed this up, please tell me! This is new ground for me...

Elliott


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