Visual Acuity

• Visual acuity is quantified in terms of visual angle, or the minimum angle of resolution.

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• Any object of regard, including letters or optotypes and their subcomponents (e.g. such as the bars of the E, or the dot over the letter i), will subtend a visual angle on the retina based on its actual size and the distance at which it is viewed. 

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• For any such object, the further away it is viewed the smaller its visual angle on the retina, and the harder it is to see. 

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• Acuity charts are therefore designed and calibrated to be viewed at very specific viewing distances.

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• A 20/20 E is defined as subtending a visual angle of 5 minutes on the retina, with each bar subtending an angle of 1 minute (1 minute of angle is equal to 1/60 of a degree). 

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• Someone with 20/200 vision needs to be 20 feet away from a letter that a person with 20/20 vision could see from 200 feet away.

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• Conversely, vision can be better than 20/20. Some people have vision of 20/15, or even 20/10, though that is the uppermost limit physically. They can accurately see things that are half the size of the smallest object a person with 20/20 vision can see.

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Remember:

• The macula is the only area of the retina with sufficient density of photoreceptors and associated relay cells to allow for high grade visual acuity (better than 20/200). This is the location of central vision. 

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• It is also important to note that visual acuity with both eyes working together is generally a bit better than when each eye is measured monocularly, because of binocular summation.

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• Preferably a quantitative measurement of visual acuity should be obtained from each eye separately, using a standard optotype chart. The results should ideally be reported in a standard format (i.e. Snellen equivalent or logMAR).

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• LogMAR is shorthand for Logarithm, base 10, of the Minimum Angle of Resolution. LogMAR notation actually provides a more concrete measure of acuity than Snellen notation.

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• In 20/20 vision, each bar subtends a visual angle of 1 minute. So the logMAR equivalent is 0, since the log101 = 0. And for a 20/200 E, where each bar subtends a visual angle of 10 minutes, the logMAR equivalent is 1, since the log1010 = 1.