No, it’s not pat of some meditation exercise for relaxation. Focus breathing, or lens breathing as it’s also called, is really the term used in reference to the change in focal length (magnification etc.) as your focal distance changes. A good way to think about it, if you don’t have a lens handy (I’m sure you do), is to manipulate the focus ring of the lens and look into the viewfinder and notice if/how the field of view and magnification changes. That change is focus breathing.
One of the reasons it can be frustrating for some people is that camera and lens manufacturers don’t ever put it in their marketing material, and so it can catch you off your guard, and the problem is not isolated to either zoom or prime lenses – it’s all fair game. Fair to say, too, that many people don’t even notice it happens, so don’t feel bad if this is the first you’re really hearing about it.
It certainly will pop out to you now that you know about it and there’s no getting away from that now. Typically, it won’t matter to the casual shooter or even most avid and professional shooters, but anyone can be frustrated with it. It’s more of a trouble for macro photographers. To illustrate a scenario where this would be frustrating, consider that a 105mm prime, will get higher magnification at 105mm, especially at closest focus, than say, some 70-200mm lenses when they are set to 105mm.
In my personal experience, I haven’t seen a particular difference in the rate at which zooms or primes breathe. I’ve seen primes, especially some 85mm primes that really change your frame when focusing, but again, it doesn’t much bother me if I have to reframe a little. In the video below, Matt Granger does a pretty good job at illustrating the problem visually, and I dare say for some of you it will be an ‘a-ha’ moment.
Source: Matt Granger YouTube