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For use in photography, an ultraviolet filter is typically a clear filter attachment that screws onto the end of a camera lens. They're used to filter out ultraviolet rays which can cause outdoor photography to look hazy and bluish. Most any removable camera lens can accept a UV filter, including video, digital and film cameras. These filters are generally not for use on point-and-shoot cameras with fixed lenses.
Though ultraviolet light can't be seen with the human eye, some bees and birds can see it — and so can a camera. In indoor situations this isn't so much of a problem, but in outdoor photography UV light can result in an image that isn't crisp and has a bit of an indigo hue. To avoid this, a ultraviolet filter can be used to help absorb ultraviolet rays, adding clarity and reducing extraneous hues.
Not only is ultraviolet light more of a problem in outdoor settings than indoor ones, it affects film photographs more than its digital counterparts. Still, ultraviolet filters can be useful for both film and digital cameras. In digital photography, the ultraviolet filter also helps curb a color problem known as purple fringing — a purple ghost-like line around a photographic subject.
The typical ultraviolet filter is clear and filters out shorter wavelength UV-B and UV-C rays, while being almost completely transparent to visible light. Therefore, these filters can be left on for almost any shot. Ultraviolet filters offer an added benefit of lens protection as well; if the lens is dropped, the filter will absorb the damage rather than the lens. It also helps to keep the lens clean and free of scratches from frequent cleaning; it generally is much cheaper to replace a scratched ultraviolet filter than a scratched camera lens.
Some stronger ultraviolet filters that filter heavier haze or higher ultraviolet levels can cut off some visible light in the violet part of the color spectrum, causing a yellowing effect on photos. Still, they do an even better job at preventing purple fringing. As a result, using this strong ultraviolet filter might require some color compensation when shooting a photo or during photo processing.
There is some controversy regarding the use of ultraviolet filters. On the plus side, it helps filter the ultraviolet rays that cause haziness and bluish hues, and it offers added lens protection. Opponents argue that added filters can cause flare which degrades photo quality. Another potential downside is that using a UV filter reduces the use of light-blocking lens hoods. This is because not all filters have threads to screw other accessories into, and adding a lens hood to a filter can result in brightness and color saturation problems.