(MONEY Magazine) – The U.S. eyewear market tops $8 billion a year, not counting $1.8 billion for eye examinations. Yet the family optometrist faces urban extinction, as chains such as Pearle Vision Center (with more than 900 outlets) and Sterling Optical (222), now constituting about 23% of the vis biz, wedge themselves between the frozen-yogurt and video stores in your favorite shopping mall. While the spread of these eyetiques may be holding down prices, the costs of eyeglasses -- and the quality -- defy ordinary logic. As Money discovered recently, you can get top quality at bottom price or bottom quality at top price. We recently commissioned a faculty member at the college of optometry of the State University of New York to conduct a shopping test. Our consultant recruited a 61-year-old woman as the shopper, equipping her with six copies of a bifocal prescription to correct her farsightedness and astigmatism and six identical plastic eyeglass frames. She had lenses made and installed by three chain stores and three independents in Manhattan.

What she got (see the table) were lenses at prices ranging from $65 to $129.95. Furthermore, our optometry college examiner, Professor Alan Innes, found that only one set of lenses was without flaws, the $65 pair from a Pearle Vision Center outlet. The costliest lenses were the worst. Most defects were minor: either the lenses did not line up with the center of the eye or the bifocal dividing line was not even with the lower eyelashes. These deviations can cause headaches and other discomforts. In one pair of glasses, the bottom bifocal segment was a few millimeters smaller than called for in the prescription. That forced the wearer to tilt her head back to look at close-in objects.

The only lenses that could have caused serious discomfort, according to the examiner, were the top-priced ones from Cohen's Fashion Optical, an East Coast chain. The optical centers were found to be off both vertically and horizontally, and the lenses tilted in the frames, causing the wearer to see close objects bit by bit rather than all at once. ''With those flaws,'' says Innes, ''you get an unwanted prism effect, and the patient sees double.'' When we told the opticians about the flaws found in their lenses, all expressed concern and assured us that they guarantee their work and will correct any defects. And indeed, the quality of six pairs of glasses -- bought pretty much at random -- proves nothing about the general reliability of workmanship at a chain or an individual optician. Although spec skeptics consider eyewear chains the fast-food outlets of their profession, it shouldn't matter where you buy your glasses if you walk in prepared as follows: , -- Get a thorough examination from an ophthalmologist (an M.D. specializing in the eyes) or optometrist (a non-M.D. with four years of medical and eye-care training). ''Thorough'' means testing not only for vision but also for eye movement, focusing skills and glaucoma. -- Shop around for the best prices since, as we found, they vary widely. -- Look for an optician who asks about your ways. Do you squint a lot in the sun or while working at a computer screen? If so, you may need tinted lenses (about $15 extra). Do you play much squash or basketball? Impact-resistant lenses (no extra charge) may be best. Are you careless with your glasses? Scratch-resistant plastic ($25) may be worthwhile. -- Wear your new glasses for a few days. If they don't seem right, have them checked by the doctor who prescribed them. There may be an additional fee, but he will detect any defects. -- If your glasses flunk the test, return them to the optician for correction. -- Be skeptical of the hard sell. Says Fair Lawn, N.J. optometrist Leonard Press: ''You can get talked into spending $100 more for add-ons such as ultra- violet filters, which block out the damaging part of the sun's spectrum. UV filters are fine if you live in Arizona but useless in Manhattan.''

CHART: WHAT WE PAID, WHAT WE GOT

The price of lenses bore little relation to their quality when Money asked six New York City optical shops to fill the same prescription and had the results checked by an optometry professor.

Store Type Price Defects

Pearle Vision Center Chain $65.00 None

Sterling Chain 65.95 Optical centers out of line Optical

Ultimate Single store 79.00 Optical centers Spectacle too far apart

Optical Single store 97.95 Bifocal segments Exchange set too low

H.L. Purdy Two stores 116.00 Bifocal segments set too low

Cohen's Chain 129.95 Optical centers Fashion off horizontally Optical and vertically; lenses tilted -

CREDIT: GENE GREIF CAPTION: NO CAPTION DESCRIPTION: See above. Color illustration: Eyeglasses.