The retinoscope is a hand held instrument which allows the optometrist to focus a spot of light onto your pupil, so that it passes through it and is projected onto the retina, lining the back your eye. The optometrist twists the retinoscope from side to side, which causes the light patch reflected by your retina to move side to side. The optometrist can estimate from the direction and speed the light appears to move across the retina, whether you are short or long sighted and by how much. The optometrist will then places different lenses in front of your eye until the reflected light appears still even when the retinoscope is moved.

The prescription required for each eye can then be determined by a simple calculation.