Field of view, also sometimes called angle of view, is one of the most basic concepts in photography. It describes everything you can see in the frame through the lens. However, it can be difficult to understand the physics behind it or some of the practical applications when it comes to photography. To fully understand it, one must be familiar with the basic physics of a lens.
How Lenses Work
A lens serves to collect light and focus it onto a smaller plane. Most lenses used in photography are positive, or converging, lenses, although larger, more complex lenses may use several different kinds of lenses inside their barrels to achieve the full effect. This means that the light they collect is focused onto a specific point, called the focal point. Every object in your frame has a focal point.
The distance between the lens and its focal point is called the focal length. All lenses are marked with a focal length or range of focal lengths depending on the purpose of the lens. For example, a zoom lens would be marked "75 - 200mm." This is the distance from the lens can move from the camera's sensor. The farther away the lens is, the more distant the objects that you can focus on.
Field of View
However, the farther away the lens is from the camera, the "closer" you are to what you are shooting. This is because the angle of the light collected by the lens is more extreme, and less light overall is collected by the lens. In short, the more zoom power on your lens, the narrower the field of view as you go up the scale. A 200mm lens will seem much closer to the subject than an 18mm lens because you are collecting less light overall, and the angle between the light and the focal plane is much more extreme.
Therefore, as we magnify an image, the field of view becomes narrower, and as we pull back from an image in the lens, the field of view becomes broader. It is more difficult to focus at higher magnifications because of the extreme angle and distant focal point; this is called "shallow depth of field." It also may be more difficult to get a steady shot with a hand-held camera, as image magnification will exaggerate subtle movements of the hands and arms. However, wider lenses, especially 15-20mm, may distort perspective in your image.
The shallow depth of field in a zoom lens can be used to "pop" your subject out of the image by ensuring it is the only object in the picture that is in focus. You can set the focus by zooming all the way in on your subject, moving it out of focus, and then carefully focusing it until you can see the greatest detail possible. Zoom lenses also will allow you to set the frame of your image rapidly, and allow you to reframe "on the fly" as needed.