What actually matters when buying sunglasses?

The single specification that matters is ultraviolet protection, labeled as 100 percent UVA and UVB blocking or UV400, meaning it blocks wavelengths up to 400 nanometers. Cumulative UV exposure to the eyes is associated with cataracts, macular degeneration, pterygium, and eyelid cancers, and the protection is achieved through a coating or lens material rather than through tint. This is why price is a poor proxy: inexpensive sunglasses meeting the standard protect as well as expensive ones, and expensive fashion frames without the specification protect no better.

Dark lenses without UV protection are actively worse than wearing nothing. Tint reduces visible light, which causes your pupils to dilate, and dilated pupils admit more ultraviolet radiation to the retina than they would in bright light without glasses. That makes the cheap unlabeled dark pair a genuine hazard rather than a neutral choice. Tint darkness and UV protection are independent properties, so a clear lens can block 100 percent of UV and a very dark one can block almost none.

Other features and what they actually do. Polarization reduces glare from horizontal reflective surfaces such as water, snow, and road surfaces, which improves comfort and visibility meaningfully for driving, fishing, and skiing, and it is unrelated to UV protection. Larger frames and wraparound styles reduce light entering from the sides, which is why they provide more real-world protection than the lens specification alone suggests. Lens color affects contrast perception, with amber and brown enhancing contrast and gray preserving color accuracy, and it is preference rather than safety. Children should wear UV-blocking sunglasses too, since their lenses transmit more UV than adult eyes and a substantial share of lifetime exposure occurs before adulthood.