Vision Science Research Group

How is Visual acuity recorded?

    How is Visual Acuity recorded

    When using a standard letter chart the ’Snellen fraction’ (numerator/denominator) describes the smallest size of letter the patient can identify correctly.

    The fraction compares the patient’s result (denominator) to the result expected from the ’normal’ visual system (numerator).

    Example:

    - 6/6 means that at six meters test distance the patient could correctly identify a letter that a ’normal’ sighted person should see at six meters i.e. ’normal’ vision.

    - 6/60 means that the patient could only see at six meters what a ’normal’ sighted person should see at 60 meters. In this case the patients sight is approximatly ten times poorer than ’normal’ or requires detail to be brought ten times closer before it is seen.

    - 6/15 means that the patient could only see at six meters what a ’normal’ sighted person should see at 15 meters.


    Sometimes visual acuity is measured at a distance other than six meters (twenty feet), usually three meters or less. Usually this is when the patient is young and finds it difficult to concentrate at longer distances or when vision is poor. If visual acuity is recorded as e.g. 3/60 the patient sees at three meters what a ’normally’ sighted person should see at 60 meters.

    A visual acuity recorded in this way can be converted to a six meter measure in the following way:

    - Multiply the denominator by the amount by which the numerator must be multiplied to achieve six.

    E.g. acuity recorded at three meters = 3/60

    • - to achieve 6 on the top, the top number (3) must be multiplied by 2 (=6)
    • - then multiply the bottom number (60) by 2 also (=120)
    • - this gives 6/120
    • - 3/60 = 6/120

    More examples:

    • - acuity recorded at one meter = 1/15
    • -> acuity in six meter format = 6/90
    • - acuity recorded at two meters = 2/36
    • -> acuity in six meter format = 6/108