This central vision corresponds with the macula (particularly the fovea) of the retina. When someone mentions 20/20 vision, visual acuity is being referenced. Despite popular belief, 20/20 vision is not actually “perfect” vision. The numerator describes the distance at which the test was performed. The denominator describes the size of the letter being viewed. Therefore 20/40 vision indicates a person sees a 40 ”foot” sized letter from 20 feet away (“foot” here is a different unit of measure than the English “foot”). It is possible to see better than 20/20.
Though visual acuity is the most common method for testing central vision, professionals agree that this is neither the most useful method of measuring vision nor always the most accurate method for gauging functional vision.
Visual acuity is measured in a high contrast setting – crisp, black letters on a white screen. This test does not accurately simulate real world visual scenarios. Despite achieving “good” vision with glasses, patients may still experience poor vision when driving at night, reading road signs, in the rain or fog, or watching television.
It is common for patients with poor contrast sensitivity, glaucoma, early cataracts, retinitis pigmentosa, and other conditions to achieve a visual acuity of 20/20 despite the patient still regularly experiencing poor vision. Remember, visual acuity is only a test of central vision, not of peripheral vision.
A patient may have lost the majority of their functional and peripheral vision, yet still see “20/20” because their central vision remained unaffected. Visual acuity is also a static method of visual assessment and does not require tracking of targets through eye movements.
Though a best corrected (i.e. with glasses) visual acuity of worse than 20/20 often indicates an ocular abnormality, such as cataracts or macular degeneration, achieving 20/20 vision does not exclude the possibility of an underlying ocular problem.
A comprehensive and functional visual assessment should include more than visual acuity alone. Perimetry, contrast sensitivity, and color vision are other visual assessments used to further evaluate visual function.