There are a few different materials that lenses can be made of. This is not the same as different brands of progressive lenses - I think you can get most brands in most materials.
Again, I'm not a professional, so the following is what I believe. It's may not be 100% accurate.
I think you can get the following kinds of lens materials. The list is in order of optics (quality of transmitting light), best to worst:
- Glass - I think some people can still get glass lenses, though you probably have to have a fairly weak prescription (so that they're not too heavy) and probably convince your optician that you won't break them and cut your face open.
- Regular Plastic - These will be pretty much unbreakable, but will be thicker than anything else on this list. I think regular plastic has better optics than any of the high index, but you it's always worth checking.
- High Index - This means extra-thin; the higher the index, the thinner the lenses are. The higher the index, the less good the optics are.
- Polycarbonate - Polycarbonate lenses are thin, unbreakable and inexpensive. I believe that they have the worst optics you can get. Every good optician I've talked to has told me to stay away from them (given my history of problems with glasses), but unless you explicitly say you don't want polycarbonate, you're likely to end up with them.
Lens thickness depends on:
- the lens material
- the strength of your prescription (stronger = thicker)
- the size of your frame (bigger frames = thicker lenses, at least at the edges where it shows)
I personally have not found the difference between how I see with regular plastic and high index to be noticeable enough to put up with the extra thickness, except for my PC glasses, where every tiny bit counts (plus I don't much care how I look at work).
Other Lens Options
Two other lens considerations I know about are:
- Edge Polishing - This is where the optician or the lab polishes the edges of your lenses so that they're clear instead of frosted-looking. Both the optometrist and optician I trust recommend against it (something about light reflection or maybe refraction).
- Glare Coating - Apparently this is a lot better now than it was several years ago, when I tried it and my glasses always looked dirty. It does cut down glare, but from what I've read, even lenses with the best glare coating scratch more easily than those without it.
But there are no doubt others as well.