The definitive guide to focal length, perspective, and why “zooming with your feet” is nonsense

January 2, 2018 by John Aldred 20 Comments

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.

About John Aldred

John Aldred is based in Scotland and photographs people in the wild and animals in the studio.

You can find out more about John on his website and follow his adventures on YouTube and Facebook.

« Use strobes to freeze subject movement and action

This video was shot on a DIY 8×10 large format video camera »

Вергунов Сергей

“Zoom with their feet” is a stupid phrase. Focal lengths are not about that. Yet personally I would prefer primes. Because of different considerations…
Linus Reuer

yeah but to get a zoom and dont move at all (and a lot of people do this) is also not really advisable because of the same problem 🤔
- John Aldred
  
  It’s not a problem, it just doesn’t work the way most people think it does.
  
  You position yourself where you need to be to get the perspective that you want. Then you choose the focal length you need to crop the composition as you want it.
  
  “Zoom with your feet” is like saying “Change your perspective with a zoom”. Neither is correct. :)
  - Doug Sundseth
    
    Exactly correct. You compose with your feet and crop with your focal length (either by zooming or switching lenses).
    
    If you want a background (presumably something physically large, like a mountain or building) larger relative to your subject, you need to move backward until the ratio of apparent sizes is “correct”, then gather as much data as you can by using a lens that captures only the part of the entire sphere about you as necessary.
Motti Bembaron

Great video, very well done and informative.

I prefer primes for portrait because they are light, fast and generally sharper. However, when doing an even I use zoom the whole time. The seconds it would take to move closer or further means lost shots. Not to mention that in a typical wedding day we walk like…A LOT! Why make it more difficult?
Christopher Pang

Lili Belle
Ian Brace

That thing with the cup is because our eyes process a left and a right image, to form the whole picture.
- John Aldred
  
  That thing with the cup… The camera filming it and illustrating the point only has 1 lens, 1 sensor. :)
- Ian Brace
  
  In a way I guess it comes under perspective.
- John Aldred
  
  Yup, it’s entirely perspective. Not parallax. :)
Tj Ó Seamállaigh

zooming by moving is not nonsense, it is all about the perspective and look you want to achieve, beside the technical aspect you are into working with.
Michele M. Ferrario

I sense zoomer butthurt
- John Aldred
  
  Not at all. The majority of lenses I own are primes. I have about 30 of them for three different camera systems. The only zoom I use regularly is a Nikon 70-200/2.8VR. The rules of moving changing perspective don’t change whether you’re using a single zoom or many primes.
  
  Swapping out a 50mm prime for a 105mm prime and not moving has the same effect as zooming a lens from 50mm to 105mm without moving. Just as sticking with a 50mm prime and moving has the same effect as keeping that zoom lens at 50mm and moving. :)
  - Sandy Waller
    
    Wrong. You seem to not understand compression. Using a 105mm, then a 50mm with the walked to the same crop and the background is not going to look the same.
JustChristoph

This is a contrived argument. Nobody uses their feet to ‘zoom’ from a virtual 16mm field of view to a virtual 200mm field of view because, guess what, there will be evident distortion. However, as the video points out, the brain is designed to compensate for distance related visual distortion. Putting two images side by side, they may seem comparatively distorted. But viewed individually, your brain does the arithmetic and compensates so that they feel identical. The exception to this is at the extremes, particularly wide-angle where the brain is not evolved to compensate for this level of distance distortion.

Zooms are for when it is not practical to be at a more convenient shooting position. But prime lenses are quicker, faster, lighter. Oh and cheaper. If you are doing fine art photography a zoom lens is an impediment.
Albin

“Zooming with your feet” today, means “zooming with your drone.” That’s where these optics (and control of their interaction on the same device) would seem to be increasingly important.
Donald Giannatti

So if I put a 200 mm lens on my camera, I must stand in the same place to make the shot? I am no longer able to walk toward or away from my subject? What? What what?

That is what I mean (every time) when I say zoom with your feet. It means instead of changing focal lengths one choose the correct focal length for what they want the image to look like, and then frame it by moving forward or backward INSTEAD of standing in one fkn location zooming in and out… which results in different focal lengths at every micro spot.
Sandy Waller

This is an ignorant and doesn’t seem to actually understand distortion and the dolly zoom. Sad.
Yako

“ Zooming is not the same as changing the focal length.” …Except it literally is. The definition of “zooming in” is to increase the focal length. The definition of “zooming out” is to lower the focal length.
Michael Clark

The phrase is not nonsense. It just doesn’t mean what you are trying to force it to mean.

There’s no such thing as true equivalency in photography. To get the exact same shot in the exact same light one must use the exact same focal length from the exact same position and record it with the exact same size sensor or film using the exact same aperture, exposure time, and ISO. If we change any one of these factors then we must change at least one of the others to compensate. If we insist on having the same perspective, angle of view, and depth of field with a different focal length and sensor size, then we have to change the exposure time to compensate for the change in aperture to maintain the same depth of field. If we use the same exposure time with a different aperture, then we must change the ISO, but that will also affect how our image looks by affecting the signal to noise ratio.

So while it is true that “zooming with your feet” won’t give you the same perspective as zooming by changing the focal length from the same shooting position, if all you’ve got is a single prime lens and you wish to make your subject twice as large in the frame, your only choice is to move to half the original subject distance. That is, you need to zoom with your feet to increase the magnification of your subject while accepting that the perspective is going to change.