My prescription is OD -8, -0.50, OS -8.25, -1.00. I have two pairs of glasses that are both large (size 61) and made of CR39 standard plastic. However, one is much thicker and shows more concentric rings than the other, while the thinner pair is much more reflective (the entire lens frequently seems to "light up" when I look in a mirror by a window, and the reflected image seems distorted like an old funhouse mirror).

My optometrist is unable to tell me why these supposedly identical pair of glasses look so different. I have been told the base curve is about the same (one reflective pair had no base curve and one had as much as a 1.5, while one of the thicker pairs had 0 and one had .75, if they did an accurate reading).

Do you have any idea why they could be so different, and how I might be able to request that the lenses be made like one pair and not the other? - Peter

Cara

There are only a couple of possibilities that I can think of.

One is whether or not the center thickness of the lens is truly CR39 "standard" 2.2mm thick at the center. The other is whether the base curve readings were truly accurate. Generally the lab should keep both lenses with approximately the same base curve.

Thirdly, and possibly the most likely explanation, is that the two pairs of lenses are edged differently. One pair may have a standard 1/3 - 2/3 bevel. Which means that the bevel comes to a point where the lens is inserted into the frame. 1/3 of the lens sits in the front of the frame, whereas 2/3 sits in the back.

Another edge possibility is what's called a hide-a-bevel, generally used for high minus lenses, where the insertion point is immediately in the front of the lens, with the bulk sticking straight out the back. The first edge treatment I described would generally produce more rings from the observers standpoint.

Glasseslover

My best guess (and it's only a guess) is that they came from two different labs and while they are both CR-39 it is not always of the same quality. Also, the slight difference in base curve could be a factor, but as these were "grinders" which is to say they weren't finished lenses but had to be ground to the prescription how they were finished could also figure in.