Focal length is a property of the lens that helps determine how strongly it focuses light and, more practically, how wide or narrow the lens’s view is. It is not the same as zoom: focal length is the lens setting itself (like 24mm, 50mm, or 200mm), while a zoom lens is simply a lens whose focal length can change. A prime lens has a fixed focal length.

In use, shorter focal lengths (for example, wide-angle lenses) give a wider field of view, so they’re common for landscapes, interiors, and other situations where you want to fit more into the frame. Longer focal lengths give a narrower field of view and more magnification, which is useful for distant subjects like wildlife.

“Normal” focal lengths are often thought of as giving a natural-looking perspective. Sensor size also matters: a smaller sensor crops the image, so the same lens gives a narrower field of view than it would on full frame. The focal length itself does not change, only the field of view does.

A good way to choose is simple: use shorter focal lengths to include more of the scene, and longer focal lengths to isolate or magnify distant subjects.