How to buy sunglasses: the only spec that matters is printed in small type
Dark lenses without UV protection are worse than no sunglasses at all. What UV400 means, why polarisation is a different thing entirely, and where price stops buying protection.
Sunglasses are the one piece of eyewear where the health claim is real, and also the one where the price tells you least about whether you are getting it. A £12 pair can protect your eyes completely. A £300 pair can protect them no better, and in one specific scenario a cheap dark pair does active harm.
The only thing you must check
UV protection, stated as UV400 or as “100% UVA and UVB.” UV400 means the lens blocks wavelengths up to 400 nanometres, which covers the entire UVA and UVB range reaching your eyes. This is not marketing language; it is a measurable property of the lens coating or material.
It matters because ultraviolet exposure accumulates across a lifetime and is associated with cataract and with pterygium — tissue growth across the surface of the eye. Unlike most optical claims, this one has a straightforward dose-response story behind it.
Why a dark lens without it is worse than nothing
This is the part worth understanding properly, because the intuition runs backwards. Your pupil opens in response to visible dimness. Put a dark tinted lens in front of your eye and the pupil widens, because the eye reads the scene as darker. If that lens is not blocking ultraviolet, you have now created a larger opening for UV to pass through than you had with bare eyes in bright light, where the pupil would have constricted.
So the tint is not the protection. The tint without the protection is a liability. Squinting in unshaded sunlight is safer than an unmarked dark lens.
Polarisation is a different thing entirely
Constantly conflated with UV protection, and unrelated to it. A polarised lens filters light waves oscillating in one plane, which is what glare off water, wet roads, snow and car bonnets is. It cuts that glare dramatically and it is genuinely excellent for driving, on water and in snow.
It has nothing to do with ultraviolet. A polarised lens may or may not be UV400; you still have to check. And there are situations where polarisation is a nuisance rather than a benefit — it can make LCD screens, including some car dashboards and phone displays, go dark or patchy at certain angles.
What price actually buys
Not protection, past the very cheapest end. What it does buy, in rough order of how much you will notice:
Optical clarity of the lens. Cheap lenses can have slight distortion that you stop noticing consciously but which causes low-level eye fatigue over a long day. This is real and it is the best argument for spending more.
Frame fit and materials. Hinges that survive, acetate that holds its shape, nose pads that do not slide. This is where the durability difference sits, and it is the same argument as everywhere else on this site: the construction is the product.
Lens category for actual conditions. Lenses are graded 0 to 4 for how much visible light they block. Category 3 is the normal bright-sunlight choice; category 4 is very dark and is not road legal for driving in many places. Worth checking if you are buying for mountains or water.
What price does not buy is UV protection, which the cheap marked pair already has.
The practical conclusion
Buy a marked, inexpensive pair to keep in the car and a bag, so there is always one within reach. Spend properly on the pair you actually wear, and spend it on lens quality and frame construction rather than on the logo. Check the marking on both. And treat an unmarked pair from a market stall as an ornament rather than eyewear — it is the one category where the cheap version is not a smaller version of the expensive one, it is a different and worse thing.
If you wear prescription glasses, the same logic applies as in the blue light question: spend on the filter that has evidence behind it, and not on the one that does not.
Questions people ask about this
What does UV400 mean on sunglasses?
That the lens blocks light up to 400 nanometres, which covers the whole UVA and UVB range reaching the eye. It is equivalent to "100% UVA and UVB" and it is the one specification worth checking, because cumulative UV exposure is associated with cataract and pterygium.
Are dark lenses without UV protection dangerous?
Yes, genuinely worse than no sunglasses. Your pupil widens in response to a dark lens because the scene reads as dimmer, so an unprotected tint creates a larger opening for ultraviolet than bare eyes in bright light would. The tint is not the protection.
Is polarisation the same as UV protection?
No, and they are constantly confused. Polarisation filters light oscillating in one plane, which is what glare off water, snow and wet roads is — excellent for driving and on water, and unrelated to ultraviolet. A polarised lens may or may not be UV400, so check separately.
Do expensive sunglasses protect your eyes better?
No. Past the very cheapest end, price buys optical clarity, frame durability and fit, not UV protection — an inexpensive marked pair already blocks UV completely. Lens distortion in cheap lenses is real and causes low-level eye fatigue, which is the best argument for spending more.
What do sunglass lens categories mean?
They grade how much visible light a lens blocks, from 0 to 4. Category 3 is the usual choice for bright sunlight. Category 4 is very dark and is not road legal for driving in many places, so it is worth checking if you are buying for mountains, snow or water.
