What to look for in sport sunglasses:
UV protection - 100% UVA and UVB, aka 400-nanometer protection.
Plenty of wrap - 8-base or 10-base for peripheral protection.
Impact-resistant lenses - That means polycarbonate, SR-91, or NXT. Definitely NOT glass. Even good lenses can pop out of bad frames. Stick with major makers, like the ones reviewed on this site. If the maker states that the shades pass the ANSI Z-97.1 standards for high-mass and high-velocity impact, excellent. That means they’ve survived rigorous testing.
Lens clarity - You and your sport shades are likely to get cozy for some long stints. They better be sharp—for at least two reasons: a.) Crummy shades will strain your eyes, make you feel fatigued and headachey, and you may not even know why. You just won’t have as much fun as you should. b.) Clarity could be critical. Whether you’re skiing, playing golf, mountain biking, or fly-fishing, you need to read details. If the shades don’t impress you at a glance, they’ll be worse later. How else can you judge clarity? It’s tricky. Price is a fairly good indicator, but not always. Read up or ask salespeople about the lenses—do they use “decentered” technology? That means each lens is subtly thicker in the middle and tapered to the outside so that incoming light waves reach your eye at the same time. The result: no eyestrain, and a crystal-sharp image. Makers of cheap shades don’t bother.
Quick Field Test for Lens Clarity Stand outside and stare at a stop sign. Switch between glasses. See how the white is affected by the lens tint. How crisp is the “P”? Is there a blur inside the “O”? Hold the glasses at arm’s length and look at a distant vertical line. Move the glasses up and down and side to side. Does the line stay straight? Most important, though, is how the view looks to you. Try lots of shades. One will stand out, bring the world into crisp focus. That’s the one you want.
Comfort and fit - Sport shades have to remain securely on your face. Move your face around, emulating your sport if possible. The temples need to squeeze your mandibles without causing pain. Nonslip rubber on the temple ends are a worthy bonus. Nonslip rubber on the nosepiece is indispensable. The frame needs to flex readily, with much more give than you’d want in street shades.
The tint you want - There’s no “right” tint for sport shades. I favor brown, copper, or rose for sports such as cycling when I might start out in low light. Those tints are bright and contrasty. But for a long day in bright sunshine, I favor a relaxing gray or green.
The right light - Don’t get lenses too light or too dark for the job. The sweet spot for visible light transmission (VLT) is right around 12% for most sports. Darker than 10% is for intense conditions like glacier travel. Lighter then 25% is for special situations like fly-fishing in mottled shade or mountain biking in the woods. Of course, interchangeable lenses can carry you through any situation, and photochromic lenses can carry you through most.
Interchangeable lenses - Good old-fashioned sport shields with interchangeable lenses give you a lens for every situation. A typical shield comes with three lenses: a gray for bright sun, yellow or copper for low light, and clear for cloudy days or nighttime. Will you really take the time to switch them out? A question worth asking. And how easy are they to swap? Most require a Tab A into Slot B process that can be tough on the fumble-fingered or vision-impaired, and you’ll smear grubby fingerprints all over the lens. Still, it’s the best way to guarantee that you’ll have exactly the right lens when you need it.
Photochromic lenses - Lenses that darken in bright light are gradually overtaking interchangeables. Start out early in the morning and the lens might permit, for example, 25% visible light transmission, and darken to 10% when the sun comes out. Not as much range as interchangeables, but a lot more convenient. Don’t look for miracles from photochromics. They can take several minutes to lighten, meaning you might feel Mr. Magooish when you mountain bike from sun to woods. Their range is limited. Don’t expect them to swing from clear to glacier-glass dark. Typical visible light transmission ranges are 16% at the dark end to 40% at the light end, or 13% to 30% or 10% to 25%—fine for normal circumstances, but too dark for, say, bike riding at dusk. The greater the range the better—but lens reaction time will be slower. And no matter what, you pay a premium for the convenience of photochromics.
Polarized lenses - Glare-cutting polarization is seldom a necessity for sport shades, but why not have it? Its main benefit is subduing stabbing glare off water, but it’s also comforting on a long road-bike ride or a cross-country ski jaunt across a sunlit meadow. It’s becoming more common on less-expensive shades, but be warned: Cheap polarization is a pale imitation of the real deal.