Field of view and Focal length are widely used terms in Photography. This post discusses about these terms.
The focal length of a lens is an inherent property of the lens, it is the distance from the center of the lens to the point at which objects at infinity focus. In other words, focal length of a lens is determined when the lens is focused at infinity.
Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens. Focal length of lens tells us the angle of view i.e. how much of the scene will be captured and the magnification. The longer the focal length, the narrower the angle of view and the higher the magnification.
Filed of View
Field of view is simply what your lens together with the camera can see and capture from left to right, to top to bottom. If you are shooting with a DSLR, the field of view is typically what you see inside the viewfinder. Some DSLR cameras, have less than 100% viewfinder coverage, which means that what you see inside the viewfinder is actually less in size than what the final image will be. Angle of view or maximum angle of view define what the lens is capable of seeing in degrees.
The main difference between the angle of view and field of view, is that the former is an attribute of the lens, while the latter is the result of both the lens and the camera. For example, the above angle of view of 84° for the 24mm f/1.4G is only for a full-frame camera. Once mounted on a camera with a cropped/APS-C sensor, the field of view, or what you see through the camera actually gets narrower to 61°.
Below image shows the same scene captured with different filed of view. The top-left 70 mm image looks almost “wide”, while the 105 mm image shows a much greater magnification with a much narrower field of view.