Focus Breathing
A change in a lens's angle of view as it is focused, so the framing appears to zoom slightly during a focus pull.
Measured in
%
In depth
Breathing is a side effect of moving elements to change focus, and it varies from imperceptible to severe. It is largely irrelevant for stills but highly visible in video, where a focus pull that also changes framing looks amateurish. Cine lenses are specifically designed to minimise it, and some cameras now crop dynamically to compensate.
Examples in practice
Pulling focus from background to foreground on a stills lens makes the frame appear to creep wider by several per cent.