Portrait advice says wide lenses distort faces and long lenses flatter them, but I have also read that focal length has nothing to do with perspective and only changes the field of view.
Both claims are repeated confidently. Which is right?
Accepted Answer
Both, and the reconciliation is that focal length changes where you stand.
Perspective, meaning the relative size of near and far objects, is determined by camera position alone. Shoot the same scene from one spot at 24mm and at 200mm, then crop the wide shot to match: the perspective is identical.
What actually happens is that nobody does that. To fill the frame with a face at 24mm you must come very close, and at close range the nose is meaningfully nearer the lens than the ears, so it renders larger. At 85mm you stand back, the distance ratio between nose and ears shrinks, and the face flattens.
So the lens is not distorting anything. It is changing the distance you shoot from, and distance is what does it.
One place the clean version breaks down slightly, since someone always raises it.
The equivalence holds for a flat subject and a rectilinear lens. Faces are not flat, and very wide rectilinear lenses stretch objects near the frame edge noticeably, which is why heads at the corner of a group shot look wrong even at a sensible distance.
So "focal length does not affect perspective" is the right principle, with the caveat that at extreme widths the projection itself starts to matter, independently of where you stood.
This is why the effect is often called perspective distortion rather than lens distortion. Lens distortion is a separate defect, barrel or pincushion, and it bends straight lines rather than changing relative sizes.
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