A focal length is just an angle of view. Swap lenses from 14 to 400 mm, walk the camera along its track, and see what really changes the picture.
A lens projects a small image of the world onto the sensor. The focal length sets how big that image is, and so how much of the world fits on the 36 × 24 mm sensor: the angle of view.
From one spot, a longer lens simply enlarges the middle of the picture. Everything grows by the same factor, so near and far things keep their size relationship. Compare the Same spot lens strip: every shot is a crop of the 14 mm one.
How large something is drawn depends on its distance from the camera. An object twice as far away is drawn half as big, whatever the lens. Walk closer and near things grow much faster than far ones; step back and they even out. That, not the lens, is perspective.
Lock framing keeps the subject the same size: zoom in and the camera steps back to match. From far away subject and background are almost equally distant, so the background looms large behind the subject: telephoto compression. Close in with a wide lens and the background falls away while anything near the lens balloons: wide-angle distortion. It's why portraits are shot at 85–135 mm and phone selfies flatter no one.
Move the camera and zoom at the same time, so the subject holds perfectly still while the world behind it stretches or swells. It's the Vertigo effect, first used by Hitchcock in 1958.
The camera focuses on whatever it frames. Long lenses can't focus as close: this one needs about 1.1 m at 400 mm. Get closer and the viewfinder tells you so. At the same framing and ƒ-number the sharp zone stays about the same depth, but a long lens blurs the background into bigger discs, because it draws the background larger.
The camera hangs from a two-axis gimbal under a telescoping jib, on a dolly that runs along a floor track. The dolly carries the camera back across the studio for long-lens shots; to get closer than the table allows, the jib reaches out over the scene. Drag the dolly or the zoom ring, or use the sliders.
Scene scale: 1 unit = 2 cm · 14–400 mm zoom · 36 × 24 mm sensor · ideal lens with a fixed entrance pupil · keys: ←/→ focal length · ↑/↓ distance · L lock · G dolly zoom · 1–3 subjects · K lens fan · V view · T tour · S panel · F full screen