UV filter
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UV filters are used for general photography to reduce haziness or fogginess created by ultraviolet light.
A UV filter is transparent to visible light, and so can be left on the lens for nearly all shots. As UV filters are usually inexpensive (about US$20, depending on size), many people use them as protection for their lenses, although this is subject to some controversy. For this purpose they are preferred over other kinds of filters which are more intrusive, such as neutral density filters.
The UV filter absorbs ultraviolet rays without changing the exposure. With most images, most people will not see a difference when a UV filter is used.