optics

Visual angle

The 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 below shows an observer s eye looking at a frontal extent (the vertical arrow) that has a linear size S, located in the distance D from point O.
For present purposes, point O 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 A 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 A on the retina at point a. The central line of the bundle represents the chief ray
Likewise for object point B and its retinal image at b.
The visual angle V is the angle between the chief rays for A and B.

Diagram showing visual angle V.