Breathing refers to when a lens'

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 optics change the apparent focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 slightly when shifting the mechanical focus

Focus (optics)

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

. Some (often more expensive) lenses are designed to lessen the degree this effect. Lens breathing does not prevent one from racking focus

Racking focus

Racking focus in filmmaking and television production is the practice of shifting the attention of a audience of a film or video by changing the focus of the lens from a subject in the foreground to a subject in the background, or vice versa...

 or following focus

Follow focus

A follow focus is a focus control mechanism used in filmmaking with film cameras and in television production with professional video cameras. It is ergonomic rather than strictly necessary; in other words it does not contribute to the basic functionality of a camera but instead allows the operator...

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

Not to be confused with the suction and expulsion of air from within the lens as its internal volume changes.