If a lens is of poor quality, it can strain your eyes. It’s also important to make sure the lenses are 100 percent UV blocking. Some lower-end glasses may claim they are “UV blocking” but use triacetate, which will only filter out about 40 percent of the harmful rays. The darkness of the lens is no indicator either, because some clear lenses can be 100 percent UV blockers. It’s also been claimed that cheap sunglasses can do more harm than good, since the dark lenses mean the pupils will dilate due to less light while still allowing harmful UV rays to pass through.