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 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 S {displaystyle S} , located in the distance D {displaystyle D} from point O {displaystyle O} . For present purposes, point O {displaystyle 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 {displaystyle 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 {displaystyle A} on the retina at point a {displaystyle a} .The central line of the bundle represents the chief ray. The same holds for object point B {displaystyle B} and its retinal image at b {displaystyle b} . The visual angle V {displaystyle V} is the angle between the chief rays of A {displaystyle A} and B {displaystyle B} . The visual angle V {displaystyle V} can be measured directly using a theodolite placed at point O {displaystyle O} . Or, it can be calculated, using the formula, V = 2 arctan ⁡ ( S 2 D ) {displaystyle V=2arctan left({frac {S}{2D}} ight)} .