Breathing originally referred to any geometric change in field-of-view when changing focus distance. Even if the angle-of-view is constant distortion changes will cause visible breathing. It has been recently used by photographers for changes of

focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...

of a

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

when changing the

focus Focus, or its plural form foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film *''Focus'', a 1962 TV film starring James Whitmore * ''Focus'' (2001 film), a 2001 film based ...

. Breathing is sometimes used for the suction and expulsion of air from within the lens as its internal volume changes. A lens with a constant focal length will exhibit narrowing of the angle of view at closer focus, and conversely, maintaining a constant angle of view requires precise reduction of focal length as focus is decreased, which some (often higher quality) lenses are designed to do. Lens breathing does not prevent one from

racking focus A focus puller or first assistant camera (1st AC) is a member of a film crew's camera department whose primary responsibility is to maintain the camera lens's optical focus on whatever subject or action is being filmed. "Pulling focus" refers t ...

or following focus with this lens, but it lessens the desirability of any type of focus adjustment, since it noticeably changes the composition of the shot.

References

Film and video terminology Photographic lenses {{film-term-stub