In photography, a bellows is the pleated expandable part of a camera, usually a large or medium format camera, to allow the lens to be moved with respect to the focal plane for focusing.
The bellows provides a flexible dark enclosure (the camera obscura) between the film plate and the lens. In some cameras, the photographer can change the angle of the film plate with respect to the optical axis of the lens, providing alterations of perspective distortion and of the object plane of focus.
There are 2 common kinds of camera bellows:
For large format cameras, “double extension” refers to bellows that extend to a length equal to about twice the focal length of a standard lens, e.g. 300 mm for the 4×5 inch format. “Triple extension” for the same format indicates bellows extension of 450 to 500 mm.
Bellows on a camera can be used to correct distortion in a photograph. For example, when shooting a scene with strong vertical elements which are truly parallel to each other, some camera systems would tend to curve these elements and make them appear in the resulting photograph to NOT be parallel. Use of a bellows-based camera can ensure that parallel elements in a scene remain parallel in the final photograph.