Generally, the higher the index of the lens, the greater the lens distortion
(as measured by abbe value - the lower the better).

The only exception to this rule is that polycarbonate (1.59 index) has the
worst (lowest) abbe value (30) of just about any common lens material, even
those with a much higher index.

For an Rx like yours, avoid polycarb like the plague. I would suggest a good
1.60 or 1.67 index lens.

The main advantage of a higher index lens is aesthetics (thinner lens,
although plus lenses are usually not thick at the edges like minus lenses),
and a lighter lens (mostly because the lens is thinner and there is less
material in the lens). Having a lighter lens can help you see better in some
cases if it can help prevent the glasses from slipping down your nose in a
less than optimal position.

Ask for lenses that are fully aspheric, or put an other way, lenses that are
optimized to use flatter curves. This will make the lens look thinner, reduce
weight, and decrease magnification. Rodenstock and Hoya are two companies that
use progressive designs with flatter curves. Signet Armorlite and Sola also have
lined bifocals mid-index lenses that are aspheric.

Frame shape and size will also play a significant role in lens thickness and
weight. Your optician should be able to make further recommendations based on
your visual needs and cosmetic concerns.

Hope this helps,
 

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
Wauwatosa Wi.
~~~~~~~~~~~~~~~~~~
"An expert is a person who has made all the mistakes that can be made in a very narrow field."
 - Niels Bohr