Zoom lens "breathing"

[aholdmeyer]

Is it true that zoom lenses will "breath" while performing a rack focus? And if so what is "breathing" and how can one compensate for it? I've read a lot of blogs and it is all still a bit fuzzy.

 

[David Mullen ASC]

Because of the moving elements when you rack focus, the lens actually zooms a few millimeters, which is why the image looks like it is breathing.

Some lenses are better or worse than others. Lenses with a big zoom ratio, like 10:1 or 20:1, tend to breathe worse than one with a short ratio, like a 5:1. For example, the Panavision Primo 4:1 hardly breathes but the Primo 11:1 breathes quite a bit. Physically small zooms tend to breathe more than large ones of the same zoom ratio, because it is easier to mechanically correct for the breathing when you have a larger lens with bigger elements. Closer-focusing lenses tend to breathe more than lenses that don't focus as closely. There are lots of factors involved.