Visual acuity

What is visual acuity?

• smallest spatial detail that can be resolved

  • depends on frequency and contrast

What do optometrists use to characterize visual acuity?

• use distance

• “20/20 vision”

  • when at 20ft, smallest letter readable is the smallest letter average person can read at 20ft

How do visual scientists measure visual acuity?

• measure the size of visual stimuli by how large an image appears on the retina, not by how large the object is

• visual angle of an object is a function of both its actual size and distance from observer

  • corresponds to the size of object on retina

• visual acuity is related to smallest visual angle of a cycle of grating

  • better vision = smaller visual angle required to identify grating cycle

  • if you consider a grating of alternating black/white stripes, a cycle is one black and white stripe

• for most people, visual acuity corresponds to one minute of arc, or 1/60 of one degree of visual angle

What is visual acuity determined by?

• determined by spacing of photoreceptors in the retina