This post is by Michael Zhang from PetaPixel
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Here’s an enlightening 7-minute video by Fstoppers that explains why “lens compression” is a misconception that’s actually “perspective distortion”.
Basically, the reason a camera can “add 10 pounds” to a person isn’t due to the focal length of the lens used but rather the distance from the camera to the subject. But since that distance is largely dependent on how large the subject needs to appear in the frame, distortions are often explained as “lens compression.”
“These noticeable differences lead most photographers to believe that wide angle lenses are distorting a scene while telephoto lenses are compressing a scene,
they are overlooking what is actually happening: the camera is moving,” photographer Lee Morris says in the video. “In reality, the distance from the camera to the subject is what is creating these distortions.”
To demonstrate this, Morris shoots two portraits of the same subject, one with a 70mm lens and one with a 15mm lens but cropped to have the same field of view as the 70mm. The two photos have virtually identical perspective distortion.
If you bring the 15mm closer to the subject instead of cropping, the face becomes distorted due to the change in distance.