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Glass or plastic item that goes in front of the camera or enlarger lens to either absorb and remove selected wavelengths of light or add special effects such as soft focus, starbursts or multiple images. There are different ways of attaching filters, for instance screwing a round filter in front of the lens, or sliding a square or rectangular filter in a filter holder which is attached to the lens with an adapter ring.
Indicates the amount the unfiltered exposure should be increased when an optical filter is attached.