Sunglasses Lens Categories, Explained

Somewhere on a good pair of sunglasses — printed inside a temple, on the swing tag, or in the listing — there is a number from 0 to 4. That is the lens’s filter category, and it is one of the few standardized things a sunglasses label can tell you. It measures exactly one thing: how much visible light the lens lets through. It does not measure UV protection, polarization, lens quality or anything else, and most of the confusion about it comes from assuming it does. Here is what each category means, which one you almost certainly want, and where the number matters enough to check.

The five categories

The categories come from ISO 12312-1, the international standard for general-use sunglasses (in Europe, EN ISO 12312-1). Each category is a band of luminous transmittance — the share of visible light that gets through the lens to your eye. Lower number, lighter lens; higher number, darker lens.

Two things are worth noticing in that table. The bands are wide — a category-3 lens can let through anywhere from 8 to 18 percent, so two category-3 lenses can look noticeably different — and the bands overlap only at their edges, so the category is a coarse description of darkness rather than a precise one. That is fine for its purpose, which is to tell you whether a lens is roughly right for the conditions you will wear it in.

Which category you want

For most people, in most places, category 3. It is dark enough for a bright summer day, a beach and a boat, and it is what the majority of sunglasses on the market are — including, in practice, most of the frames we recommend on this site, even where the listing does not say so. If you buy a pair of ordinary dark sunglasses from a reputable maker and the category is not stated, category 3 is the overwhelmingly likely answer. Our roundup of the best acetate sunglasses is, functionally, a list of category-3 frames.

Category 2 is the one worth considering if you wear sunglasses in variable light: overcast mornings that turn bright, city streets shaded by buildings, long drives that pass through tunnels and tree cover. Serengeti labels its “Drivers” photochromic lenses Cat 2 to 3 for exactly that reason. Category 1 is a light tint that mostly reduces glare on dull days, and category 0 is a fashion or indoor lens. Our guide to lens tint colors covers the other axis of the choice — a category tells you how dark, the tint tells you what color, and both change what you see.

Category 4: what it is for, and why it is not for the road

Category 4 lets through only 3 to 8 percent of visible light, and it exists for conditions most people never encounter: glaciers, high-altitude snow, open ocean under a high sun, where the glare is so intense that a category-3 lens still leaves you squinting. In those conditions it is the right tool. Everywhere else it is too dark, and that is not a matter of taste — it is a road-safety rule. The standard’s guidance is that filters suitable for road use are categories 0, 1, 2 or 3, which puts category 4 outside them. A lens that dark leaves your eyes unable to recover fast enough in a tunnel, under trees or at dusk. Our driving roundup covers the rest of the lens rules for the road; the short version is that a moderate category-3 polarized lens is the safe, boring, correct answer, and category 4 belongs in a glacier bag.

There is a second driving threshold most guides miss. The standard also treats filters transmitting less than 75 percent of light as unsuitable for twilight or night driving — which rules out most of category 1 and all of 2, 3 and 4. In plain terms: no sunglasses after dark, whatever the category.

Photochromic lenses and “Cat 2 to 3”

A photochromic lens changes darkness with the light, so it cannot sit in a single category. Makers that publish the number write it as a range — “Cat 2 to 3” means the lens rests at category 2 in low light and darkens to category 3 in bright sun. Serengeti, which builds its photochromic lenses in mineral glass, labels its Foyt exactly that way, and it is one of the frames below. A photochromic range is genuinely useful if you want one pair for a whole variable day. What it cannot do is make a lens safe for night driving, because even the light end of “2 to 3” is well under the 75 percent threshold.

What the category does not tell you

UV protection. This is the one that matters. Category measures visible light; ultraviolet is invisible, and UV protection comes from the lens material or a coating, not from darkness. A category-1 lens can block all UV. A category-4 lens can block none — and a very dark lens with no UV filtering is worse than no sunglasses, because it opens your pupils to light it is not blocking. The number that protects your eyes is a stated UV400 or 100% UVA/UVB claim, and our UV400 explainer and guide to checking UV protection cover it. Every frame on this page states both a category and a UV claim, which is the combination to look for.

Polarization. Also separate. A polarized lens filters glare reflected off flat surfaces; it can be any category. Two of the three frames below are polarized and one is not, at the same category-3 darkness, which is the clearest illustration that the two have nothing to do with each other. Our guide to telling if sunglasses are polarized covers the test.

Lens quality. A category-3 lens from a budget brand and a category-3 mineral-glass lens from Serengeti let through a similar share of light and behave completely differently in every other way — clarity, scratch resistance, weight, color fidelity. Category is a darkness setting, not a grade. Our guide to sunglasses lens materials is where the quality question actually lives.

Why the number is so hard to find

In Europe the category is routinely marked on the product under the EN ISO standard, so European buyers see it as a matter of course. On US listings it rarely appears, and the great majority of them — including most of the frames in our own registry — simply do not state it. That is not a red flag on its own; a reputable dark lens is almost certainly category 3. But it does mean that a maker who states the category is giving you something the others are not, and it is one of the reasons Serengeti and OTIS listings read differently from a typical Amazon page. If a listing does state a category, believe it. If it does not, assume 3 for a dark lens and 2 for a lighter one, and put your attention where it belongs: on the UV claim, the stated frame material, and the fit. Our guide to buying sunglasses online covers what else to read a listing for.