The main task of the dark lenses - a decrease in the amount of light entering the eye. Points must absorb at least 70-80% of the sun's rays. When this is important not only the color of the lens, but also the chemical composition of the glass. On good quality lenses always suffered icon with the number indicating the bandwidth.
It happens that the dark glasses distort the natural colors. Some lenses impede the rays of the spectrum, changing color perception. For people in certain occupations it can be dangerous. There are special lenses that change the color of objects with a purpose. For example, lenses for motorists increase the contrast of red, yellow and green lenses for golfers specifically highlighted in green, and sunglasses with amber lenses improve visibility in fog.
There are also photochromic lenses or "chameleon" responsive to ultraviolet radiation. Such glasses darken the light, reducing eye strain during the transition from darkness to light. Among the "chameleon" is a model that can darken almost instantly.
Mirror coating on the windows are intended to reinforce the reflection of the sun. This may be necessary particularly in bright sunlight, for example in the sea or mountains. Covering SLR lenses can be silver, and color. But this is only the outer part of the glass. For the perception of eye glasses are made of traditional shades. But to protect against glare, better to choose a polarizing lens.
If the inside of the points you have noticed the reflection of his own eyes, it could mean that the lens is not applied anti-reflection layer, removing such an effect. This is quite inconvenient for the eyes.
For people with disabilities of producing the same lens, but with diopters. In addition, there are nozzles on the sun, which can be put on a simple corrective glasses.