The field of view is the size of the largest object that can be seen through an optical system by a detector that is located at a fixed position. The size of the field can be expressed in units of length (such as meters), although it is more convenient to use an angular measures (e.g., degrees) in some situations.
For example, if you are looking at your own image in a plane mirror, then both the object (your shoes, for example) and the detector (your eye) are the same distance from the mirror. A ray of light can travel from your shoes to your eye by striking a mirror that is considerably smaller than your own height. In fact, in order to see your entire body at once, the mirror need be only one-half as tall as you are.