Contrast sensitivity refers to our ability to perceive small variations in lightness in what we see. Subtle variations in lightness, or low contrasts, abound in the world around us, and often play an important role in many everyday visual tasks including reading, driving and recognizing people and things around us. Good contrast sensitivity enables us to identify shapes from shading, recognize faces (which always have shading) and identify other objects whose structure is delineated by shadows and light. Look at some of the images below and ask yourself how easy it would be to fully appreciate or discriminate among visual objects if you weren’t able to distinguish subtle differences in lightness, i.e. low contrasts, in your environment. Note that contrast refers to lightness differences between adjacent image regions, and that identifying darker (and colored) objects may also require good contrast perception, for good eyesight.
Most young people whose vision tests at least as good as 20/20 (or 6/6 in countries where metric units are used) and without diagnosed vision problems, have normal contrast sensitivity as well as good detail vision (also known as visual acuity). But contrast sensitivity deficits accompany many eye problems, especially age-related ones, including cataracts, glaucoma, and macular disease, and to a degree, normal aging as well. In some cases, contrast sensitivity loss occurs without reductions in visual acuity. Some contrast sensitivity losses can be corrected, by interventions including cataract extraction and properly prescribed eyeglasses. But many are more difficult to correct, especially those that involve disorders of the retina, optic nerve and visual areas of the brain.