For photographers: "lens compression" explained

[William Seger]

Last edited Sun Jan 21, 2018, 05:43 PM - Edit history (2)

(Note: A friend asked me to explain this effect in an email, and after taking quite a bit of time to really understand it myself and write it up in a way that is hopefully clear, I thought I'd share it here. )

I first noticed this effect as a kid watching baseball on TV: When there was a telephoto shot of the pitcher taken from behind home plate, it would look like the second baseman had suddenly moved up to be right behind the pitcher's mound. I later noticed similar "depth-altering" effects when using binoculars and telescopes.

Photographers call it lens compression, depth compression, perspective distortion or some such term, and for a long time I assumed that it's because telephoto lenses somehow distort relative sizes depending on distance. Wide-angle lenses have an opposite effect, exaggerating apparent depth instead of compressing it, and photographers know that if they use a wide-angle lens for a portrait, it will make the nose look bulbous and protruding.

But I've since learned that these effects aren't because the lens is distorting anything in the image. It's because different lenses can fool our brains into making incorrect estimates of depth (i.e. relative distances between objects) in a two-dimensional image, because we often base those estimates on relative angular sizes.

If you were to stand at home plate, your eye would see a pitcher 60 feet away as having about twice the angular size of the second baseman who is about twice as far away. And since your brain knows that the two are really about the same height, with no other information it can use their relative apparent sizes to estimate that the second baseman is twice as far away.

If you were to take a picture from home plate, the image of the pitcher will also be about twice the size of the second baseman's image, regardless of what kind of lens you use -- wide-angle, "normal" or telephoto. That is to say, if you were to crop any wide-angle shot down to that part of the scene that a telephoto lens captured and then enlarge it, they would look the same. That's because light travels in straight lines, so relative angular sizes are determined only by the geometry of the scene -- i.e. the camera's position, not its lens.

However, what a telephoto lens does is make it look like the camera was closer to the scene than it really was -- that's why we use them. So, if a telephoto lens makes it look like the camera was maybe 10 feet away from the pitcher, then with no other depth cues in the two-dimensional image, our brains will deduce that the second baseman is only another 10 feet farther behind. If the camera is behind the backstop, 120 feet from the pitcher, the effect is even more exaggerated: The second baseman's image would then be 2/3rds the size of the pitcher's, so we might estimate that he's only six or seven feet behind.

We generally use wide-angle lenses to take in more of the scene, but they have the side effect of making it look like the camera was farther away from the subject than it really was. If an image taken with a wide-angle lens at home plate looks like the camera was over 100 feet away from the pitcher, then we would judge the second baseman to be over 200 feet away. And likewise, a portrait taken with a wide-angle lens will make the tip of the nose look farther away from the rest of the face (and therefore relatively larger) than it really is.

But it's really our brains that are doing all the "distorting" by guessing wrong about where the camera was.

[Ferrets are Cool]

1. I may be misunderstanding your post, but I

must disagree with your assessment that lens compression is a "fooling of our brains".
It is nothing more than a lesson in physics.

This link describes compression/distortion very well.

https://www.edmundoptics.com/resources/application-notes/imaging/distortion/

"distortion is determined by the optical design of the lens. Lenses with larger fields of view will generally exhibit greater amounts of distortion because of its cubic field dependence. Distortion is a third-order aberration that, for simple lenses, increases with the third power of the field height; this means that larger fields of view (a result of low magnification or short focal length) are more susceptible to distortion than smaller fields of view (high magnification or long focal length). The wide fields of view achieved by short focal length lenses should be weighed against aberrations introduced in the system (such as distortion). On the other hand, telecentric lenses typically have very little distortion: a consequence of the way that they function. It is also important to note that when designing a lens to have minimal distortion, the maximum achievable resolution can be decreased. In order to minimize distortion while maintaining high resolution, the complexity of the system must be increased by adding elements to the design or by utilizing more complex optical glasses."

If I misunderstood your post, apologies.

[flotsam]

2. The OP is discussing not distortion but rather perception

your link is a discussion of pin cushion or barrel optical distortion which is a real (and measurable) phenomenon of wide and extreme wide and particularly fisheye lenses...

[William Seger]

3. That's talking about actual distortion

... such as barrel (outward bowing toward the edges) and pincushion (inward bowing). Those are true distortions that warp the image: straight edges in the scene don't appear straight in the image. Without correction, wide-angle lenses typically show barrel distortion (fish-eye) and telephoto lenses show pincushion, but there are lens designs that can compensate for that and make the lines look straight. (However, you can't really "map" a spherical view onto a flat surface without some kind of distortion. A corrected wide-angle lens, for example, makes the lines look straight, but the image looks slightly stretched toward the corners to compensate for the barrel distortion.)

(ETA: Oops, replied before I saw the reply above.)