Visual angle
Visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc.It also is called the object's angular size. The diagram on the right shows an observer's eye looking at a frontal extent (the vertical arrow) that has a linear size , located in the distance from point . For present purposes, point can represent the eye's nodal points at about the center of the lens, and also represent the center of the eye's entrance pupil that is only a few millimeters in front of the lens. The three lines from object endpoint heading toward the eye indicate the bundle of light rays that pass through the cornea, pupil and lens to form an optical image of endpoint on the retina at point .The central line of the bundle represents the chief ray. The same holds for object point and its retinal image at . The visual angle is the angle between the chief rays of and . (Wikipedia).
