I've recently fielded some questions regarding what type of sunglasses I use in my daily fishing. Instead writing an answer, I'll repost what I published five years ago, as it is still applicable today. I'll add any updates in bold/italics. The following information was published on February 2, 2016:
Most all fly anglers know that one essential stream side tool is polarized sun glasses. But to us tenkara anglers sun glasses are critical. Not only do they increase your ability to see into the stream, to detect fish and to see the bottom better (if you are wading), but they also allow you to see the line better.In level line tenkara all of the line is off the water's surface. This is what makes tenkara so effective. Without this fundamental concept, tenkara is just fly fishing without a reel. Keeping the line off the water allows the tenkara angler to direct the fly around obstacles, adjust for stream currents, and detect subtle subsurface takes with more efficiency than traditional "line on the water" fly fishing. Essentially, keeping the line off the water allows the tenkara angler to achieve that "holy grail" of fly fishing, the drag free drift.
But to keep the line off the water you must have a light line. Monofilament nylon is (generally) too light to cast, unless it's a tapered line like a Midi, Soft Tenkara, or Fujiino Tenkara line. Because of this, fluorocarbon level lines have been the tenkara anglers line of choice. But to effectively keep the line off the water, especially of you fish upstream, like I do, you must use a lighter gauge line like #2.5 or #3.
Light lines work great at keeping the line off the water, but the smaller the line diameter the more difficult it can be to see the line -- even if the line if blazing orange! A #2.5 level line can be exponentially harder to see than a #3.5 line.
To help my tired old eyes see the takes better, I use a sighter, a 6-8 inch section (not 3 feet as mentioned in Jason's article) of contrasting colored nylon (Amnesia). This gives me a focus point and, at least I'm convinced, helps me miss far fewer fish. But even a #2.5-3 orange level line with green sighter is difficult for me to see, particularly if the line is longer than 9 feet. (I use a #3 level line almost exclusively now. I don't use a #2.5 line anymore)
Enter polarized sunglasses. I have found that wearing polarized sunglasses increases my ability to see the sighter. But more important than sunglasses alone are sunglasses with side shields. Light entering between my eye and the lens causes unwanted glare, and thus greatly reduces contrast. Adding side shields dramatically improves contrast and lets you see the line better -- just like wearing a hat does. Personally, since I wear corrective lenses I wear optical grade fit-over sunglasses. These surround my eyes and eliminate any stray light from entering at odd angles.
Another thing that helps me is the tint of the lens. Many sunglasses are grey or neutralizing. These are OK in full sun, but on overcast, foggy or rainy days they may actually make seeing your line more difficult. So, if its overcast, raining or the like, I have found that amber or yellow polarized lenses bring my line out of the background much better than grey scale lenses. The line I use is orange and the sighter is green; amber or yellow lenses makes the orange line color "pop". They also help accentuate the green sighter.
So, here's what helps me see my line:
1) always wear a hat (I wear a long bill baseball-type hat with dark under brim)
2) side shields (or wrap around frames) help improve contrast and reduce lens glare
3) polarized lenses are a must
4) amber or yellow tint help you to see orange fluorocarbon level lines.