Focal length
The mm number on a lens: how wide a view it takes in, and therefore how far back you stand to fill the frame.
See it
What it is
Focal length is the millimeter number printed on a lens, and it decides how much of the world fits in the frame. On a full-frame camera: 24mm is wide and dramatic, 35mm is the reportage 'in the room with them' look, 50mm is roughly what your eye reads as normal, 85mm is the flattering portrait standard, and 135mm and up crops in hard. Crop-sensor and phone cameras quote a full-frame-equivalent field of view rather than changing the physical lens, which is why a phone main camera is described as a 24 to 28mm equivalent.
Name one whenever you want a specific feeling. 24mm to make a small studio look cavernous and a sneaker look heroic. 35mm for founder and workspace shots that keep context in frame. 85mm for a headshot where the face is the only thing that matters.
Gotcha: focal length controls field of view and nothing else. Distance controls perspective, which means the size relationship between a nose and an ear, or a subject and a mountain. The wide-lens 'big nose' effect happens because a wide field of view tempts you to stand two feet away to fill the frame; stand in the same spot with a long lens and crop the wide shot to match, and the two look identical. Pick the focal length, and you have really picked how far back you stand. Useful side note: focal length is one of the most reliable levers in an image prompt, and asking for '85mm' beats asking for 'cinematic' almost every time.
Ask AI for it
Shoot a head-and-shoulders portrait from about 2.5 meters away using an 85mm full-frame-equivalent lens. Preserve the perspective created by that shooting distance, so facial proportions stay natural with no wide-angle nose exaggeration. Use a narrow field of view, render the background large behind the subject, and avoid any wide-angle stretching toward the frame corners.