boxing match

[foxop1]

Perhaps someone can help clarify the exact definition of effective diameter and how to measure it. I was taught that it is the maximum distance measured across a given lens shape, often but not always equal to the A measurement. Another optician defines it differently and says that it is possible to have an ED smaller than the A measurement. I say he's wrong. Please help clarify.
Mike

[renee1111]

"The effective diameter of a lens is found by doubling the distance from the geometrical center of the lens to the apex of the lens bevel, farthest from it. This mesurement helps determine the smallest lens blank from which the lens can be cut." Knowing this can help with lab bills, due to the fact that most labs will charge more for larger or thicker lens blanks.
See page 15 of Professional Dispensing for Opticianry. Hope this helps!

[mullo]

You are correct......the ED can equal the "A" but not possibly be less/smaller than it.

I've attached a diagram that I hope helps with understanding.

[HarryChiling]

Great explanation and it is impossible for the A size to be larger than the ED

[ShuString]

Good Job Mullo