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Visual acuity

How low you can go on the eyechart, measured on the 20/X scale.


Visual acuity is assessed utilising a Snellen eyechart and the "better 1, better 2" subjective testing method called a refraction; acuity is normally measured in each eye individually and in both eyes together. Visual acuity testing is the most standardised and most frequently performed measurement of visual performance.

Visual acuity does not seek to measure all aspects of vision, but it does measure a very important feature, because in healthy eyes which have not undergone surgery, the normal cause of poor testing in a standard acuity test is refractive error (myopia, hyperopia and/or astigmatism), which can be corrected with glasses or contact lenses.

Visual acuity comes in two flavours: one's "uncorrected visual acuity" (UCVA), which means acuity when you are not wearing glasses; and one's "best corrected visual acuity" (BCVA), which means the best acuity you can achieve with the optimal glasses prescription.

Visual acuity can also separately be assesed for near vision (most relevant to presbyopes, people usually in their 40's or later who lose the ability to focus up close) using a small chart held nearer the patient.

NOTE: Visual acuity is the aspect of vision which laser eye surgery seeks to alter (laser eye surgery seeks to reduce or eliminate refractive error by changing the shape of the cornea), however, it is important to understand that other aspects of vision, specifically vision quality, may also be altered, normally for the worse, through the introduction of aberrations.


Example of a visual acuity score:

20/20 OD (i.e. normal acuity in the right eye)

20/25 OS (i.e. slightly subnormal acuity in the left eye)

20/20 OU (i.e. normal acuity when using both eyes together)


Snellen eyechart






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Last updated 1 Dec 2004.