A UV filter is a substance that prevents ultraviolet (UV) light from passing through. In sunscreens, these chemicals and particles are combined in a lotion or spray to block harmful light from reaching the skin and causing cell damage. In photography, a UV filter is a camera accessory placed over the lens to stop UV light from creating a blue hue to photographs. The term UV filter can also refer to a water filtration system which produces, rather than blocks, large amounts of UV light to kill microorganisms, such as viruses or bacteria, in water.
Ultraviolet light is a type of electromagnetic radiation with a wavelength that is shorter than visible light. UV rays cannot be seen by humans and are emitted by the Sun as well as man-made black lights. On their way toward the Earth from the Sun, the rays scatter, reflect and get absorbed by the gases in the atmosphere, so there is less UV light at sea level. At higher altitudes, such as on top of a mountain, however, there is more UV light. The amount of radiation that hits the Earth varies from day to day and is typically higher between ten AM and two PM, regardless of the weather.
In photography, a UV filter is a clear glass accessory that fits over the lens of a camera to block UV rays, while letting in all visible light. This allows the photographer to capture the true colors in a picture without getting a blue hue. These filters are particularly recommended at higher altitudes and during the times of day when there is more UV light coming through the atmosphere. As a cheaper option, the UV filter is often used as a protective cover for the lens. This is especially useful when shooting pictures in rough circumstances, such as in the midst of ocean spray or a sandstorm, where there is high risk of the lens, which is much more expensive than the filter, getting dirty or scratched.
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It is highly debated, however, as to how much the filter protects the lens and whether it does more to benefit or degrade the quality of the shot. Because there is a low amount of UV rays that meet the surface of the Earth, especially at sea level, a UV filter is often not needed unless at a high altitude. Also, newer lenses let less UV light pass through and newer film is less sensitive to UV light, further cutting the need for a filter. New lenses are also more resistant to scratches, so a filter may not offer much more protection than the lens itself, in most situations. Finally, the presence of the filter may decrease the contrast of the photo and create a flare, or a reflection of light, off the glass of the filter.