There are almost as many coatings as there are lenses.
Anti-reflective. It can help with glare, reflections, halos around light, and make for a nicer look.
Scratch-resistant and ultraviolet protection. Most lenses today have these built in.
Tinted lenses. Sometimes, a light or dark hint of color on the lens can help you see better. A yellow tint may increase contrast. A gray tint to your sunglasses won’t change the colors of things. A light tint can hide signs of aging around your eyes.
Mirror coatings. This is purely for looks, but it does hide your eyes from view. You can find them in a range of colors like silver, gold, and blue.
