Lens Compression and Lens Distortion

(Originally published 2016-03-16 as /archives/10930)

This lens-compression-thing: we all agree on it correct?

Lens Compression

The definition of lens compression is this: if you use a longer focal length, then the background will appear to be closer to the subject. There are numerous examples of images that “prove” this out there (including the GIF below), but guess what? Lens compression doesn’t actually exist.

Here’s how the prove-it examples work:

  1. First the photographer makes a photo of a subject, standing in front of a background object, with a short focal length lens (wide angle, less magnification).
  2. Then the photographer takes the same picture, of the same subject, at the same distance from the camera, but with a longer focal length lens (telephoto, more magnification).
  3. Then we compare the two images, and note that the background object appears to be closer to the subject in image #2.Here’s the kicker:
  4. If we then crop image #1, so that the subject takes up the same amount of space in each image, we will note that the subject now looks to be the same exact distance from the background object as in image #2.

In other words: lens compression is just an optical illusion.

Lens distortion

Barrel Distortion, Pincushioning, Bokeh, etc., may be different with each of the two lenses. Generally the wider angle lens (shorter focal length, less magnification) will distort the image more and cause more foreshortening relative to the telephoto lens (longer focal length, more magnification), but not always. A lot depends on lens design, and post processing software is really good at removing distortion these days.

The following GIF was shared at “reddit /r/educationalgifs How different lenses affect portraits“. It shows the foreshortening issue and the so-called-lens-compression-issue at various focal lengths of a telephoto super zoom lens. For many people this will be proof that lens compression is real. It still isn’t real, and not all wide angle lenses will cause such drastic foreshortening.

OK: so if the focal length is changing, then why is the subject’s head mostly the same size? Because the photographer is moving physically closer to the subject, for the wider angle shots, and farther away for the telephoto shots.

Here’s the GIF:
XBIOEvZ - Imgur

Back to the Lens Compression Example

OK, so Paul must be wrong about lens compression. I mean look at how that tree in the background of that GIF moves closer to the subject: right?

In the following video, Dieter Schneider does the steps (from above) to prove that lens compression does not exist.

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