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

by The111   Last Updated June 02, 2020 06:18 AM

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".

Answers 1

Answering my own question, because this post apparently worked as a rubber duck for me.

The relevant concept appears to be focus breathing, which was mentioned later in the same series I was reading.

June 02, 2020 05:32 AM

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