A prime lens is a lens in which the field of view (focal length) cannot be adjusted. The only way to take in more or less of the scene is to walk closer to or further from the subject. (sometimes called “zoom with your feet,” though this is technically inaccurate since changing camera position is not the same thing as adjusting the focal length) Or you have to carry a selection of lenses of different focal lengths with you and swap them out as required.

A zoom lens is a lens in which the field of view can be adjusted. If you can’t fit in all your friends in the picture, for example, you could just rotate the zoom ring on the lens until they’re all in there. Or if that bird is too far away you could rotate it the other way to zoom in closer.

We’ve all had the experience before. You’re trying to take a photograph of some friends, but you just can’t fit everybody into the picture. You step backwards further, but there’s a wall or cliff or something, and it just isn’t going to work, so you tell your friends to squeeze in closer. Or you see a bird flying by in the distance, you grab your camera, and you end up with a big photograph of sky with a disappointingly tiny little dot in the middle of the frame.

In each case the field of view provided by the lens isn’t appropriate for your subject matter. In the first instance your lens is not “wide” enough to take in the whole scene, and in the second your lens isn’t “long” enough. There are three rough categories of lenses when it comes to how much of a scene they can take in, and the field of view of each type is defined by an optical property known as the focal length of the lens (a property explained later in this document).