First and foremost you want to have lenses that protect against UVA and UVB light. The cumulative effect of that light will cause damage to the lens and retina inside the eye and can contribute to dryness and chronic redness on the front of the eye. We’re much more exposed to this in the Southeast. Interestingly you can’t always trust because it’s a brand name that they’re going to protect against UV light. You can always have your glasses tested on a UV analyzer, which we can do here.
Secondly, the quality of the optics really varies in no-power sunglasses. You should be able to put a good pair on and it should not distort the world as you look straight ahead and also as you move the glasses up and down in front of your eyes. Do that trick to make sure the world doesn’t move because there is some power ground into the lenses. That can strain your eyes.
We buy sunglasses for fashion and function. Are they for the beach? Driving? Golfing? Biking? Racing? All of that affects which type works better. Some can be made with a person’s prescription. As with any product you want to know who is standing behind it. Are they made well? You’re paying good money, what are you covered for? The quality of the scratch-resistant surface can vary widely. Some are automatically replaced. The color and degree of tint should be consistent with how you use the sunglasses. Gray or green will provide a more natural view. Brown will change the color perception. In general we don’t recommend blue tint because those are actually allowing higher levels of blue light, which is scattered more, to be transmitted.