Breathe in… Breathe out… No do not panic, we are not really going to discuss some kind of ancient photography meditation ritual with the power to deliver you the perfect photograph; focus breathing is actually a phenomenon displayed in most lenses, especially telephoto zoom lenses.
The term ‘focus breathing’ may be new to you, but it is something you have experienced before; but probably also never noticed before. With the majority of zoom lenses, your focal length will change as your focal distance changes.
At this point, you are probably scratching your head thinking “what?”
It is probably easier to see for yourself, than it is for me to explain it. Grab your camera and stick on some kind of telezoom lens. Stand quite far back from a subject and zoom all the way out to your maximum focal length. Switch to MF and manually set your focal distance to infinity and take a photo. Now, leaving your zoom at the same focal length, manually rotate your focus ring to the minimum and take another photo (it will be blurry but do not worry; that is not what we are looking at here).
Look at the two photos. Notice anything? The second photo taken at the minimum focal distance probably appears slightly more wide-angle, as if it has zoomed back in a little bit. There is nothing wrong with your lens; this is just focus breathing in action. A 70-300mm lens will only be 300mm when focused out at a distance, at or near infinity. As you focus closer on something at or near the minimum distance, your focal length is going to be more like 230-250mm.
The amount of focus breathing experienced will vary from lens to lens, manufacturer to manufacturer. It is not something that is published, so you are really never going to know how much of an issue it will be until you start using the lens.
Is this really that much of an issue though? Probably not to the casual photographer; and quite frankly even the more professional photographers are unlikely to struggle with focus breathing. There may be times when it becomes an issue, but they are probably few and far between, and you can always just reframe a little. I certainly wouldn’t lose sleep over it; however you will now always notice it!
Sorry about that…