Straight facets meet at sharp corners instead of curving, trading a round or square outline for angles.
also: angular sunglasses
The basics
What to know about sunglasses
A filter on a bracket
A pair of sunglasses is a light filter mounted on a face-shaped bracket. The bracket is a frame front carrying two lenses, a bridge over the nose, two temples running back over the ears, and a hinge at each corner. The filter does three separate jobs, and buyers merge them constantly. Blocking ultraviolet light, the invisible part that does the damage, is the first. Reducing how much visible light reaches you is the second, and that is what you feel as comfort. Glare bouncing off a flat surface gets handled by something else again, a polarizing filter. Any lens can do one without the other two, so check each separately.
What the lens is made of
Material sets both the optics and the safety of the lens. CR-39, an optical plastic, gives clean vision at low weight but scratches unless it carries a hard coat. Polycarbonate survives a hard impact without shattering. Sport and children's lenses use it, though it splits color slightly more at the edge of your view. Glass resists scratching best and weighs roughly twice what plastic does. Protection from ultraviolet is not the tint: an absorber mixed into the lens or applied as a coating does that work. A clear lens can block as much as a black one. The marking to look for is UV400, naming the wavelength in nanometers below which nothing gets through. Tint darkness runs on its own scale from category 0 to category 4, where category 3 passes 8 to 18 percent of visible light and covers ordinary bright sun. Category 4 passes 3 to 8 percent and is not legal to drive in across much of Europe.
Where a frame gives out
Frames fail in predictable places, and the build tells you where. Hinges carry every open and close, so count the interlocking barrels at the joint and look for a spring hinge, a small coil hidden in the temple that lets the arm flex outward past parallel and pull back. Screws work loose slowly, and one loose screw is usually why your lenses stopped sitting level. Metals differ against skin: monel, a nickel and copper alloy, is the common inexpensive choice and releases enough nickel to irritate some people at the pads and behind the ears, while titanium and 316 stainless release little or none. Coatings live on the surface and wear from it. Anti-reflective film on the back stops sun from behind bouncing into your eye. Mirror film on the front cuts brightness while scratching sooner than the lens beneath.
Coverage matters more than shape
Match a shape to where you actually use them, because coverage rather than outline carries the consequences. Water, snow and long roads throw light in from below and from the side: Aviator answers the first by carrying the lens past the cheekbone, while Sport and Shield answer the second by wrapping toward the temple. City sun and short exposures forgive a lot, so Round and Cat-Eye trade coverage for outline at no real cost. Square and Wayfarer sit between, with a level brow that shades from directly overhead, the direction most people forget. Oversized buys coverage with raw lens area instead of curvature. Each shape has its own guide below, with the faces it flatters and the way each one lets you down.
What extra coverage weighs
Every gain here is paid for somewhere. A bigger lens shades more and weighs more, and that weight lands on two small pads on your nose. Wrap tighter and you seal out side light, but you also trap warm air against the lens. It fogs the second you stop moving. Polarization strips reflected glare and strips the image off some phone screens, car displays and aircraft instruments, and it can hide the shine that warns you a road is icy. Darker is not safer, and past category 3 it becomes a hazard at dusk. Prescription wearers pay twice, since a strong curve and a strong correction fight each other and most labs stop glazing past roughly base 6, a number describing how hard the lens bends.
Find the standards marking
Judging a pair takes about a minute. Find the standards marking on the temple or swing tag: EN ISO 12312-1 in Europe, ANSI Z80.3 in the United States, AS/NZS 1067 in Australia, plus a filter category number. Any optical shop will put a pair on a UV meter for free, which settles the question on a cheap pair. Fold the temples and set the frame on a flat counter. It should rest without rocking, and both arms should meet the front with equal resistance. To confirm polarization, hold the pair up to a phone screen and rotate it a quarter turn, since the screen goes black when the two filters cross. Check the lens edge inside the rim too: thickness that varies around the circle means the lens was cut off center, so your pupil misses the optical center.
Heat and grit
Keeping a pair alive comes down to heat and grit. Opticians reshape acetate with hot air at around 175 F, and a dashboard in summer sun climbs close enough to warp a frame and lift a coating off a lens, which ruins more sunglasses than dropping them does. Dust sitting on a dry lens is an abrasive, and a cloth pushes it across the surface in one long scratch. Wet the lens before anything touches it, and use the case instead of a pocket.
Buyer checklist
Before you buy sunglasses
Small accessories are easy to buy by appearance, but measurements, materials and hardware decide comfort and durability.
01
Check the actual measurements
Belt length, hat circumference, glove size and lens width are more reliable than one-size labels.
02
Confirm material and lining
Fiber content affects warmth, breathability, stretch, skin feel and cleaning.
03
Inspect moving parts
Buckles, hinges, clasps, elastic and stitched joins usually show wear before the main material does.
04
Plan for storage
Structured hats, leather goods and sunglasses last longer when their shape and surfaces are protected.
Measure across your face temple to temple, then work out the frame front from the three numbers printed inside the arm: double the lens width, add the bridge width, add about 6mm for the rims. Most adult faces land between 130mm and 150mm, and a front more than 4mm wider than your face will not brace and will slide.
Your pupils should sit in the horizontal center of each lens and a shade above the vertical center. Pupils sitting toward the nose mean the bridge is too wide for you. The frame creeps down all day no matter how it is adjusted.
Blink with them on. Lashes brushing the lens smear it within minutes, and the fix is more pantoscopic tilt, the forward lean of the frame, or a deeper bridge that pushes the lens away. About 3mm of clearance is enough.
Temples should run straight back and only begin to curve after they pass the top of your ear. A bend that starts before the ear presses into the side of your skull and gives you a headache in an hour or two.
Take them off after 20 minutes and look at your nose. Deep red dents mean the entire weight is landing on the bridge, so you need silicone pads, a wider bridge or less lens area.
Low or flat bridges do better with adjustable metal pads, or with a frame built on a raised deeper bridge and sold as an alternative or low bridge fit, which lifts the lens clear of the cheek.
Care
Caring for sunglasses
Never leave them on a car dashboard. Opticians bend acetate with hot air near 175 F. A closed car in summer sun gets close enough to that to warp a frame and lift a coating off the lens.
Rinse under lukewarm water before anything touches a lens. Dust is an abrasive, and a dry cloth turns one grain into a scratch that runs the full width of the glass.
Wash with a drop of plain dish soap and dry with clean microfiber. Skip glass cleaner and paper towel, since ammonia dissolves anti-reflective coatings and paper is wood fiber. This is harder than the coating it is rubbing.
Remove them with two hands. Pulling on one temple twists the frame front, and that is the usual reason lenses end up sitting at different heights on your face.
Tighten the hinge screws every few months with a jeweler's screwdriver. Screws back out from ordinary opening and closing, and a loose one lets the whole frame go crooked before it ever falls out.
Stop parking them on your head. Your crown is wider than your face, so the temples splay a little further each time until they no longer grip at all.
Questions
Common questions
Does a darker lens block more UV?
No, and that assumption is really risky. Tint controls visible light, while ultraviolet protection comes from an absorber in the lens or a coating on it, and the two are set independently. A very dark lens with no UV filter opens your pupil wider and lets more ultraviolet reach the eye than bare sight would. Confirm UV400 or one hundred percent UVA and UVB.
How can I tell if cheap sunglasses really block UV?
Looking at them proves nothing, and the seller usually cannot tell either. Most optical shops keep a UV meter on the counter and will test a pair in seconds at no charge. Short of that, buy from a seller who prints a standards reference such as EN ISO 12312-1 or ANSI Z80.3 rather than a vague sentence about protection.
What is the difference between polarized and mirrored lenses?
They solve different problems. A polarizing filter blocks light waves vibrating in one plane, which is what reflected glare off water, wet road and glass is made of. A mirror coating is a thin reflective layer on the front that bounces back a percentage of all light, cutting brightness without touching glare. A lens can have either, both or neither.
Why does my car display go black through my sunglasses?
Your lenses are polarized and the screen puts out polarized light. Cross two filters near ninety degrees and almost nothing passes. Laminated windshields add their own stress pattern on top, showing as blotches across the glass. If you depend on a dashboard screen or you fly a plane, choose a non-polarized lens instead.
Which lens color should I choose?
Gray keeps colors looking natural and suits bright everyday sun. Brown and amber dim blue light, which lifts contrast in flat or patchy conditions such as snow and tree cover. Green sits between the two. Rose and yellow raise contrast further in dim light but shift color badly, so they belong to specific sports rather than general wear.
Can any frame be made into prescription sunglasses?
No. The lab needs enough depth in the rim to hold an edged lens, and a strongly curved wrap fights a strong correction because the optics shift as the lens bends around your face. Most labs stop glazing somewhere past base 6. Rimless designs and single-lens shields are hardest, and a prescription insert mounted behind the lens is often the workaround.
Why do my sunglasses keep sliding down my nose?
Three usual causes. The frame front is wider than your face, so nothing braces at the temples. The bridge is too wide for your nose, leaving the full weight on skin instead of seated in the groove. Or the temples have splayed from being pushed up onto your head. Only the last is free to fix, and an optician does it in minutes.
Do children need real sunglasses or is it a gimmick?
Real ones matter more for them than for you. A child's crystalline lens, the focusing lens inside the eye, is clearer than an adult's and passes more ultraviolet through to the retina, and children spend far more hours outside. Pick polycarbonate for impact resistance, a wrapped shape to cut side light, and a strap so the pair lasts the week.
