Originally Posted by
tonyl
Technically you measure the bifocal add by flipping the frame to measure front surface powers. Measure the distance front surface power and then the near to work out the add. Try it out and you will find that when you get a high plus lens, measuring the back surface will get a higher add than measuring the front.
Bifocal lenses are made with the add put into the front surface before the lens power is ground into the back. Therefore the add is only measured through the front during manufacture.
However, the patient does not see either back surface power or front surface power
. They see equivalent power. This means that for example in a high plus BF lens, if
the add is +2.25, the patient will see with it as if the add was +2.50. This is
something prescribers may not think of.
So to check accuracy of manufacture measure the front surface. However, the back surface measurement may be closer to the equivalent power and give a closer idea of what the patient actually sees. The equivalent power cannot be measured with a vertometer.
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