Field of view (FOV) is the extent of the observable world that is seen at any given moment through someones eyes, on a display screen, or through the viewfinder on a camera. It is the maximum area of the scene that can be seen when looking through an optic. FOV is a fundamental concept in optics and is important for anyone working with optics to understand. In the case of optical instruments or sensors, FOV is a solid angle through which a detector is sensitive to electromagnetic radiation. FOV is also relevant in photography, where it refers to the part of the world that is visible through the camera at a particular position and orientation in space.

FOV is determined by multiple components in the optic, including the eyepiece, thickness of the lenses, position in its assembly, and magnification. The relationships between focal length and FOV are also important in determining the FOV of a camera. Increasing magnification decreases FOV.

In human vision, the FOV is composed of two monocular FOVs, which the brain stitches together to form one binocular FOV. Each individual eye has a horizontal FOV of about 135 degrees and a vertical FOV of just over 180 degrees. Stitching together the monocular FOV yields a binocular FOV of around 114 degrees of view horizontally.

FOV is measured in different units depending on the type of optic. For example, binoculars, rangefinders, and spotting scopes tend to measure FOV in feet at 1000 yards, while rifle scopes provide FOV in feet at 100 yards. In astronomy, FOV is usually expressed as an angular area viewed by the instrument, in square degrees or square arc-minutes.

In summary, FOV is the range of the observable world visible at any given time through the human eye, a camera viewfinder, or on a display screen. It is determined by multiple components in the optic and is measured in different units depending on the type of optic.