Shorter focal lengths cause distortion

(or longer focal lengths compress features)

This is a classic example of correlation being mistaken for cause and effect.   That is, 2 things being caused by a 3rd thing rather than one causing the other.  When we shoot portraits, you’ll often hear people saying “don’t shoot below 50mm or you’ll get distortion of the subject’s features”.  Or “shoot a longer lens to compress the background”.  It isn’t the focal length that is causing these effects, it’s the distance between you at camera and your subject.  The closer you get, the more distortion of features you get, as the nose (for example) is now proportionally a lot closer than the ears to your camera than if you shot further away.  The shorter focal length is also caused by you being closer – to get a wider view to fit the whole face in.

Try this – take the same shot at 200mm and 50mm with the camera and subject in the same position and then crop the wider shot so the subject is the same size – they will look the same.  In the same way the background is compressed when you move further away (ie the distance between subject and background is proportionally smaller to the distance between you and the background).  A longer focal length is caused by the increase in distance, to keep the subject filling the frame.

So, rather than focal length causing the distortion,  the focal length and the distortion are both caused by the close working distance.

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