Why are zoom lenses 'in-camera "cropping" with max pixel resolution'? They should zoom physically the same way a prime would, just with a less optimal optic structure, no?

I mean the perspective projection you get from a 70-120 zoom at 90mm should be the same as the one from a 90mm prime

A 70-120 zoom at 90, a 90mm prime, and a 70mm prime cropped in to the same field of view should produce the same perspective projection. Try it sometime – take two photos from the same spot, one at 70 and one at 100 and then crop the 70 to the same field of view as the 100. It won’t be perfectly identical but those differences are due to lens imperfections, not something fundamental.

Not gp, but as an example of what they mean: take a photo at 70mm then at 120mm. You can crop the first photo to be have the same composition as the second, but the resolution will be lower.

Zooming in with the lens will also change the background because by zooming in your are changing the focal length. Also there is the lens distortion to consider as well. A portrait taken at 28mm will look very different than one taken at 150mm no matter what you do.

How does the focal length affect the background? Are you referring to the focal length's relationship to the depth of field? Because otherwise, I guess that the effect you're referring to ("A portrait taken at 28mm will look very different than one taken at 150mm no matter what you do.") is an artifact of the same thing the GP comment talked about, which is the effect of the distance between the camera and the subject.

Wide vs. tele foreground/background lens distortion, not just the field of view (framing). Some call this choice of lens "lensing" and as noted, the distortion of a 28mm wide vs 200mm tele are part of their physical design, optical "laws" being what they are.

GP had it exactly right, perspective distortion ONLY depends on the distance between objects to the camera (entrance pupil). The focal length literally does not matter at all.

(This is an interesting point of contention, because conventional wisdom up until a few years ago was exactly what you said, and relaying what GP said got you tarred and feathered as an imbecile in the usual "photog forums". It's also one of these things which were never in doubt at all, and are very easy to check for yourself.)

Wrong. Zooming optically can have the same effect as digitally cropping a photo. Here's a video explainer: https://www.youtube.com/watch?v=_AmA_Tv6V1k

Totally right. I shoot almost exclusively with a 50mm prime and make heavy use of cropping. I get to carry a very light, small, and FAST lens that works in many different situations without needing a flash. Modern cameras have such high resolution that the output still has good quality even after significant cropping.

There are some situations where "zooming with your feet" is the right answer and others where it is impossible. Try everything.

Try everything...to make the shot your "own." Yep.

The first time I ever went out with a zoom lens after starting with just a 50mm prime, the amount of creative power and freedom I felt really opened my eyes. Without having to worry about focal length you can actually compose with the naked eye and walk around til you get everything in the right position, then zoom in to get the framing right.

Zoom With Your Feet and other photography minimalist cliches seem to me kind of mystifying, and more interested in a kind of romantic vision of photography than in technical excellence

Agreed, but would you think it to be good advice for beginners? One of those "learn the rules to break the rules" kind of rules?

Zooming with your feet and zooming with a lens will give you quite different results regarding "geometric relations between objects". The word you are looking for here is "Parallax" :-)

Cropping in post is identical to digital zoom and will not change parallax and of course reduce resolution (which neither of the above will do)

All three give quite different results and of course are viable methods depending on what your intended outcome is

Every fixed lens and sensor configuration has some "level of zoom" (better terms would be "focal length" or "angle of view"). There's not some "natural zoom," unless the author wants to prescribe his favorite configuration as the privileged one.