Your Feet are not a Zoom Lens

Your Feet are not a Zoom Lens

It's often said that the best zoom lens is the "foot zoom," the idea being that you are better off sticking with a fixed focal length lens and moving closer with your feet to zoom in. Baloney.

First, let's review the arguments often made for zooming with your feet. Over the years, I've heard quite a few people advocate this idea, and no doubt you have too. But the typical reasons boil down to just a few: Prime lenses are lighter weight. They are generally less expensive. Zooming with a variable focal length lens takes both hands and can distract you from your subject matter. Relying on a zoom lens can make you lazy, apt to accept whatever view of your subject is most convenient rather than most compelling.

The truth is, I really can't disagree with any of these arguments. But I still say that zooming with your feet is baloney. When you get right down to it, walking closer or moving further away from your subject is simply not the same thing, photographically, as is zooming with a lens from a constant location. It's a matter of geometry and perspective.

Mount a zoom lens on your camera and point it toward a subject some distance away. Now, without changing your position, zoom the lens in to a longer focal length. Your subject, naturally, will increase in apparent size. If it filled half the frame height at 100mm it will span the entire frame height at 200mm focal length. Doubling the focal length will double the apparent height of your subject. Zoom out from 100mm to 50mm, thus cutting your focal length in half, and your subject will appear to shrink to half of its original height in the frame.

If instead of changing the focal length, you alter your shooting distance, a similar effect can be observed. Walk closer to cut your distance from the subject in half and that subject will grow to twice its original height in the fame. In the same way, doubling your distance from your subject by stepping back and that subject will shrink to half its original height in the frame.

So, what's the difference? The effect is not the same if you look at everything in the frame not at the same distance from the camera as the subject.

Let's assume the frame is filled with just two objects, with the main subject being ten feet away, and the other object behind it at twenty feet from the camera. Zooming in as above from 100mm to 200mm focal length will increase the apparent size of both objects equally. Zooming is an optical effect that alters the size if everything you seen in an equal manner. But if you instead keep the focal length constant and get closer with your feet, objects at different distances will be effected differently. Cut your distance from the main subject from ten feet away down to only five feet and that subject will grow to appear twice the size. But when you do so, that other object that started at twenty feet away will end up being fifteen feet away. Twenty feet minus the five feet you walked closer gives us a final distance of fifteen feet. So if cutting the distance in half doubles the size, what happens to that other object? We didn't cut our distance to it by half. It changed only by fifteen divided by twenty feet equals three quarters, not half. As such, rather than doubling in size, that second object will grow only by four thirds its original size, the inverse of three fourths.

Objects at different differences grown and shrink in apparent size when zooming with a variable focal length lens, but the change in apparent size independently as determined by their relative distances when zooming with your feet. The two methods of "zooming" are not the same thing. Your feet are not a zoom lens.

This whole "foot zoom" thing is a fallacy born out of a focus (literally and figuratively) on the main subject at the expense of the rest of the frame. It works only when you are willing to ignore everything else in the frame.

It's hard to say how this bad advice ever caught on. Zoom lenses are heavier and often more expensive that equivalent prime lenses, and there once was a time when they tended to of a lesser optical quality. But all this is irrelevant if "foot zoom" doesn't even do the same thing. The one simply isn't a substitute for the other.

And as for zoom lenses making you a lazy shooter, this tells only part of the story. It is true that if you are willing to stand next to your car and use a zoom lens to frame everything you shoot, you can fill the frame without breaking a sweat. In that sense, a zoom lens can seem to promote laziness. But in reality, the laziness in this example came first, and the zoom lens was merely an enabler rather than a cause. Lazy photographers (or lazy people in general) will look for ways to remain being lazy. The fault does not lie with the zoom lens.

And it strikes me as curious that zoom lenses get scapegoated in this way. Zooms have been around for quite some time now, but back when they were new, there were several other changes taking place in the world of photography. Auto focus and programmed exposure modes can also enable bad habits, but zoom lenses seem to get more than their fair share of bad press.

If you want to get the best images you can, my advice would be to position yourself to achieve the perspective you want, and then zoom your lens to achieve the framing you are after. Rather than substituting for a zoom lens, your feet augment the lens. It takes both for you to adequately control both perspective and framing.

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