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 right shows an observer's eye looking at a frontal extent (the vertical arrow) that has a linear size ${\displaystyle S}$, located in the distance ${\displaystyle D}$ from point ${\displaystyle O}$.

For present purposes, point ${\displaystyle O}$ 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.

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