An optical filter is a physical accessory, typically made from optical glass or high quality plastic resin, that is placed in front of a camera lens, enlarger lens, or other optical device to modify the light passing through it before it reaches the film or image sensor. Filters are most commonly circular in design and attach to the front of a lens via a screw thread, though square and rectangular filters used with dedicated holder systems are also widely available and offer greater versatility across different lens sizes.
Optical filters serve two broad purposes. The first is corrective or technical, where the filter is used to absorb or block selected wavelengths of light in order to modify the tonal or colour characteristics of an image. In black and white photography, coloured filters such as red, orange, yellow, and green are used to alter the way different tones are rendered, for example darkening a blue sky to increase contrast or lightening foliage for a more dramatic effect. In colour photography, filters such as UV filters, polarising filters, and neutral density filters are used to reduce haze, control reflections, manage exposure in bright conditions, or achieve longer shutter speeds for creative effects such as silky water or motion blur.
The second purpose is creative or special effects, where filters are used to add distinctive visual qualities to an image in-camera. Soft focus filters introduce a gentle diffusion or glow that flatters portrait subjects and creates a romantic or dreamy atmosphere. Starburst filters use an etched or cross-screen pattern to transform bright point light sources into radiating star shapes. Multiple image or prism filters split and repeat the subject across the frame for surreal or abstract compositions.
Optical filters are also used in the darkroom, where they are placed beneath the enlarger lens to control contrast when printing on variable contrast photographic paper. While many filter effects can now be replicated digitally in post-processing, optical filters continue to be valued by photographers for their ability to produce results in-camera that interact directly with the light at the point of capture, producing effects that are often difficult or impossible to fully replicate in software.