bottom, top and the 45° positions in between) and the task of the tested person is to decide on which side the gap is. The size of the C and its gap are reduced until the subject makes a specified rate of errors. The minimum perceivable angle of the gap is taken as measure of the visual acuity. It is generally practised in the laboratory.

The stroke width is of the diameter, and the gap width is the same. This is identical to the letter C from a Snellen chart. The Landolt C is the standard optotype