Visual acuity is the ability of the eye to perceive shapes, colours and details. Generally speaking, the better the eye is adapted to the level of brightness present, the better the visual acuity. Other factors are the capacity of the retina to resolve fine detail and the quality of the optical image.

Causes of inadequate visual acuity include:

• eye defects, such as short- or far-sightedness,
• insufficient contrast in the field of vision
• insufficient luminance.
  
  

Visual performance depends crucially on distribution of luminance – the brightness of a luminous or illuminated surface as perceived by the eye – and on visual acuity. Other factors include:

• the state of adaptation of the eye (light/dark adaptation)
• the contrast of the visual object in relation to its surroundings
• the size of the visual object
• the speed of perception (time needed to process the visual impression).
  
  

For optimum vision, the adaptation process needs to have been completed, contrast must be adequate and the visual object needs to be of a sufficiently large size. Apart from that, the slower the speed of perception, the better the visual performance. A scene glimpsed from a passing car is perceived in less detail than by a passer-by on foot. Visual performance also decreases with age.