Perspective distortion seems to be one of the most confusing topics in photography. There are all kinds of erroneous “facts” about it. Like focal length being that which distorts your subject, and not the distance to the subject. Well, this video from This Place puts that myth well and truly to rest and also illustrates why “zooming with your feet” is the dumbest phrase in photography.
Yes, there are optical distortions in lenses, but those have nothing to do with the focal length. You could take 50mm lenses from Nikon, Canon, Sigma, and every other manufacturer out there, put them side by side and they’d all show different optical distortion. But that’s not the same thing as perspective.
You can see from this animation that with a theoretically perfect lens, focal length changes absolutely nothing about the perspective of the subject. It’s simply cropping into a tighter field of view or opening up to a wider one.
This next animation demonstrates the effect of actually moving while keeping the focal length the same. The image on the left shows the full field of view of the lens. The image on the right shows a fixed size crop around the subject scaled to fit, and show the effect of perspective from moving in more detail.
You’ll note that the screen on the right kind of looks like the Hitchcock Zoom made popular in movies such as Vertigo. This is because changing the focal length is essentially only changing the crop of the scene. The image on the right is basically just the same crop scaled to fit the size of the screen. Changing focal length does nothing to perspective whatsoever. It’s the moving that changes the perspective. Changing the focal length simply changes the crop of the scene before us.
And this is why “zooming with your feet” is such an asinine phrase. Zooming is not the same as changing the focal length. Zooming is cropping. When you move, you’re not cropping. You’re completely changing the perspective of your shot. Zooming simply being a crop is why this Dolly Zoom in post effect works, too.
There are plenty more examples in the video for those who care to watch it.
If you actually want to know what you’re talking about when you tell people that a 50mm lens is too short for photographing people, or you’ve been telling people to buy primes and “zoom with their feet”, then you should probably watch it, too.