Measuring · ABO
Lens clock simulator
Set the lens clock on each surface and turn it to find the principal meridians. Work out the nominal power as read, then correct it for the lens material.
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Why the index matters
A lens clock only measures how curved a surface is. It turns that curvature into dioptres assuming the lens is made of material with an index of 1.53, the old crown-glass standard.
Surface power is (n − 1) ÷ r, so the same curve is stronger in a higher index. On polycarbonate or high-index lenses the clock reads low; on CR-39 it reads a touch high. Multiply each reading by (n − 1) ÷ 0.53 to correct it.
The clock result is also a nominal power: it ignores lens thickness, so check real power on a lensometer.