# MFD equals 4x focal length? But published specs contradict that

I spent the better part of this evening perusing the series of equations behind this popular magnification calculator.

Equation F10 states `d ≥ 4f`, where `d` is focus distance and `f` is focal length. I followed the math up to here, and it makes sense to me. I did some testing with one of my all-around lenses, and the results fit within these equations. All of this suggests that MFD is 4x a lens' focal length.

So how then can the Sony FE 90mm F2.8 macro lens have `d = 280mm` and `f = 90mm`? This would appear to violate the previously defined limit, since 280 < 4*90. What am I missing?

EDIT: I found the concept of "focus breathing" and added it below as an answer. But, I have a followup question. Does this mean after the focus breathing, the 90mm truly is focused at a focal length of 70mm? (i.e. 280mm/4) This seems like a non-trivial change; whereas the definition of focus breathing describes these changes as "small".