“Zoom with your feet” is a common thing that many photographers say. But that really just means that you get closer and not much more. The folks at Vsauce recently put together a video that partially shows off what the difference is between zooming in with a lens and simply moving closer. It has to do with the size of the subject, but there are many other differences that they don’t talk about.
The video on Zooming a Lens, and more of an analysis is after the jump.
Zoom your lens in also does much more than changing the size of the subject. It also greatly changes the depth of field because you’re working with a much different focal length. If you’re shooting with a 70-200mm f2.8 lens, then much less of a subject will be in focus at 200mm than at 70mm. In fact, if you’re shooting a portrait at 135mm, then you’ll for sure want to stop down more than to f2.8.
This also changes distortion quite a bit as with a 24mm to 70mm lens. At 24mm, you’ll get much more linear distortion at a distance than you would at 70mm from the same distance. By moving in closer, you’ll increase the visual distortion–which can’t always be corrected but has been improved on more and more over the years.
This is part of the reason why many photographers recommend that you don’t shoot portraits with a 24mm lens and get up close to the subject–because of distortion. Rather, you can do a full body or upper half image with them.