I was trying to identify several pairs of spectacle lenses and came across an unusual one. The supposed prescription of the lens is 1.25-1.75@175 with a 1.5 FT add. I don't know if the front curve was spherical or aspheric, so I clocked it and what I found was odd. The front base curve at 180 degrees was 6. I checked the segment and it clocked at 7.5. So far so good. So I went on to clock the back surface. At 180, it clocked at 4.75. 6 - 4.75 = 1.25 like the prescription. Then I clocked the back surface at 90 and found a curve of 8. That would mean the power at 90 would be -2 rather than the expected -.50 (1.25 - 1.75). So I clocked the front curve again and indeed got 6 at 180, but then I clocked the front at different angles and actually got around 7 to 7.5 at 90 degrees. So, 7.5 - 8 = -.50 >>> this is indeed the power expected. So the prescription is correct, but why oh why would the front surface be so toric? Even a flatter aspheric lens should at least be equal curvature all around, and not so toric. I can't imagine the lens (CR39) would have warped that bad. Any ideas here? Thanks.
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