
Visual acuity
When an optometrist measures your visual acuity, he or
she may use the Snellen chart, represented in Figure 1.8
in the margin. The results of this test depend upon
viewing distance. The test is standardized for a viewing
distance of 20 feet. At that distance, the strokes of the
letters in the 20/20 row subtend one sixtieth of
a degree 1 o
(/6o, one minute of arc). This is roughly the
limit of angular discrimination of normal vision.
Visual angles can be estimated using the astronomers'
rule of thumb depicted in Figure 1.9 in the margin:
When held at arm's