Even though people spend much more time focusing on frame selection, as a medical device, the lenses of your eyeglasses are the most important part. It is therefore very important that you obtain your lenses (and therefore your glasses) from a reputable source. It is always best to buy eyeglasses through an eye doctor who is able to check that the lenses are made and fitted properly to ensure your best possible vision.
There are a number of variables to consider in selecting lenses.
If you have a high prescription which may require thicker lenses, you may want to ask for aspheric lenses which are thinner than normal lenses.
There are lenses that are made from materials that are more durable and shatter-resistant such as polycarbonate or trivex, which can be useful for children or sports eyewear.
Photochromic lenses can serve as eyeglasses and sunglasses as the lenses darken when exposed to the sunlight to block out the sunlight and UV rays.
Polarized lenses create greater eye comfort by reducing glare specifically from the water or snow and are great for sunglasses for those that spend time outdoors.
There are also a number of coating options that you can add onto lenses to enhance certain characteristics such as anti-reflective coatings, anti-scratch coatings or UV coatings to reduce exposure from the sun. Adding a coating may require special cleaning or treatment so ask your eye doctor or optician about special instructions.