Choose scratch-resistant lenses. Scratched up sunglasses are useless sunglasses. Lenses made from NXT polyurethane are impact-resistant, flexible, lightweight, and have great optical clarity, but they're expensive.

· Glass is heavier, expensive, and will "spider" if broken.

· Polycarbonate is not as scratch-resistant and provides less optical clarity than NXT polyurethane or glass, but it's more affordable.

· Polyamide is a less used material, which provides glass like optical clarity, without the danger of shattering.

· There are significant differences in scratch resistance of polycarbonate lenses depending on the hardcoat finish applied during construction.

· Acrylic is also affordable, but it's the least durable and optically clear. They are also weak when exposed to heat and often deform. The best choice is glass or resin material.

Don't choose frames wider than your face width. Square face: Because this face shape has very straight, pronounced jawline and edges, choose round glasses to balance out these features. Make sure the frames are not too thick. Try to choose wider sunglassesand avoid rectangular glasses with sharp corners.

Tips

· Rounded frames are great with a square face; rectangular frames are great with a heart shaped face, and square frames are great with a round face.

· If you have small eyes, try to choose dark lenses as these can appear to increase eye size.

· Check that the sunglasses fit well or they will fall off your face. Be aware of your surroundings if you are playing sports, they might fly off accidentally.

· Remember to check whether the lens is smooth (no scratches, bubbles or spots) before buying.

· Always put your sunglasses in a hardcover case to protect them when traveling out and about; otherwise you might sit on them and mangle them.

· Make sure they look and feel good on your face. You don't want sunglasses that are too small/big, heavy, or just look good and don't feel good.

· Always check that the lens color is deep enough.

· Sunglasses of a light color, or with white or pink frames and lenses stand out well against dark skin.

· Keep the sunglasses in a safe location whenever you aren't wearing them to avoid getting scratches on the lenses.

Warnings

· Wearing "cosmetic" sunglasses actually promotes eye damage. Dark lenses reduce available light to the eye, which results in dilation of the eye's pupil. Since the sunglasses do not block harmful UVA or UVB rays, these rays are easily passed into the eye by the dilated pupil. Never wear dark lenses unless they block UVA and UVB rays.

· Photochromic lenses (the ones that change depending on light conditions) don't work as well in warm conditions (i.e., they become darker in colder weather than in warmer weather). They also don't work at all in a car, as they darken when exposed to UV rays and car windshields inherently block those rays.

· Polarized lenses reduce glare, but they may also react with the tints in windshields, creating blind spots and diminishing the visibility of LCD readouts.