How To Determine the Number of Degrees that a Given Image Subtends

"Cycles per degree" is a unit of measurement frequently employed in psychophysics and vision research, as a way of characterizing the spatial frequency of visual images. For those, like myself, who have forgotten most of their trigonometry:

Length of onscreen object = TAN ( Degrees / 2 ) * Distance from Monitor * 2

Degrees = ArcTan ( (Length of onscreen object / 2) / Distance from Monitor) * 2