Parallax

Things close to the camera slide quickly and things far away barely move, and the gap between those two rates is how the eye measures distance. Put it in a shot and the scene becomes solid.

It is also the clearest way to tell a camera that moved from a camera that did not. No amount of light or lens work fakes it, because it only happens when the viewpoint travels.

The camera travels across a scene that has things at several distances. The nearest object crosses the frame fastest, the middle ground more slowly, and the horizon hardly at all.

A near object is not optional. Something within a metre or two of the lens is what gives the far distance a rate to be compared against.

A wider lens exaggerates the separation, because it takes in more of the near object and more of the far background at once. A long lens compresses the difference and can hide the effect entirely.

Use it to establish depth in an opening shot, to make a flat location feel three dimensional, and any time a generated scene needs to read as a place rather than an image.

Skip it when the frame is deliberately flat. A head on shot of a wall has no depth to reveal, and forcing the move produces a slide rather than parallax.

A zoom changes the focal length and nothing travels, so the relationship between near and far stays exactly as it was. Parallax needs the camera to move through space.

The test is a foreground edge against the background. In a zoom they stay locked. In a move with parallax they separate, and the near one wins.

The model returns a zoom, where everything scales together and nothing shears. Say the camera travels through the space and ban zoom by name.

The other failure is a move with nothing near the lens. Name a foreground object and say it passes close to the camera, so there is something to measure the background against.