Change of focal length = Change of focus!
When you change the focal length or in good English when you zoom!
The focus changes and is not in the same focus as to when you focus first at a given zoom.
So to cut a long story short, you need to re-focus when you zoom!
In simplified terms, the above explanation is a description of a vari-focus design in a zoom, not focus breathing. What most people consider as a "true zoom" design is when one focuses on a subject at a given distance at the long focal length of the zoom range, the subject remains in focus as you zoom out to a wider focal length without changing the lens to subject distance. Sometimes even the best designed zooms will change focus by a
small amount as one "zooms out".
Again in simplified terms, focus breathing is where for example a 70-200mm zoom focused on a subject at near infinity distance at 200mm will frame the subject much like a fixed focal length 200mm lens would on the same subject & same distance.
Conversely, if the same 70-200mm zoom is set to 200mm and focused on a subject close to or near minimum distance of the zoom lens, the subject will be framed much like a 150mm fixed focal length lens, not a 200mm fixed lens, so the zoom appears to have lost focal length at it's long end.
The 70-200 f2.8 VRII exhibits noticable focus breathing at it's long end. The older 70-200 f2.8 VRI much less (to any siginificant degree).
Again both explanations above are an oversimplification and there do have exceptions.
Dave (D&A)