The spec is called PD, or Pupilary Distance.

1) Measure it with a ruler, it's measured in milimeters. Needs a trusted
friend to hold the ruler, and do the check.
2) Ask your optician for the number
3) Zenni ships rulers to balance on your nose, and take your own measurement

--

Christopher A. Young
Learn more about Jesus
www.lds.org
..

"Percival P. Cassidy" wrote in message
...

I thought of something else. When I get a prescription, whether from the
optometrist or from the ophthalmologist, it specifies only the optical
characteristics. When I take the prescription to get the glasses made
(for the past many years it's been at Costco), they take the frame I
selected (with its clear glass dummy lenses), fit it on me and mark the
position of my pupils on the "lenses," then measure the distance from
that mark to ... (I don't know: perhaps the bottom of the lens). That
measurement is then part of the specification of the lenses that suit
*me* in that frame. Then when I pick them up, they sometimes fiddle with
the nose pads to make sure that the fit is correct and that the center
of the lens is in the correct position. How does all that get taken care
of with mail-order eyeglasses?

Perce