We’ve all seen the animations showing “how focal length affects your subject“. Whenever one gets posted, the smart ones chime in with “It’s nothing to do with your focal length, it’s all about subject distance”. And, they’re right. The confusion really all comes down to equivalent framing of the subject. If you stay where you are and just change focal length, nothing happens to the distortion in your subject’s face. They just get smaller or larger in the frame.
But, if you want to keep your subject the same size regardless of lens used, you have to move. With a longer lens you go further away. With a shorter one, you have to get closer. To illustrate this, the folks at Fstoppers have put a video together showing how the two work in combination with each other. The correlation between changing focal length and subject distance.