Originally Posted by
ml43
Sorry for the confusion, when you said franklin bofocal, I assumed you meant executive bifocal(old habits die hard).
you don't need to surface anything to prove the math.
Easiest way is to take two FSV lenses with opposing powers(i.e. +1.00 ds and -1.00 ds)
Put them back to back, total power of plano.
If you moved them together in a lensmeter there should be no prism induced.
But if you move one separately from another, it will behave like a +/- 1.00 DS in terms of prism
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