Field of View (FOV)
The Field Of View (FOV) is the actual observable area of an optical system like the human eye or a camera.  This area is often expressed as the maximum angular size of an optical system as seen from the entrance pupil. Because the entrance pupil is the reference position for the FOV, the defining ray becomes the chief ray of the system.
If the object being viewed is in close proximity to the lens and circular, the FOV can be expressed as the diameter of the viewable object also known as the full field of view (FFOV); however, it is common practice to mention the half field of view (HFOV) which is the radius of the viewable object.
When the viewable object is very distant from the entrance pupil, the FOV is instead determined by the maximum object size, the detector size, or by the observable field over which the system performs to standards.