point and its retinal image at .

The visual angle is the angle between the chief rays of and .

Measuring and computing

The visual angle can be measured directly using a theodolite placed at point .

Or, it can be calculated (in radians) using the formula, .

However, for visual angles smaller than about 10 degrees, this simpler formula provides very close approximations:

The retinal image and visual angle

As the above sketch shows, a real image