The Measurement of Vision and Visual Acuity. On these charts, the letter size is specified not by its height, but by the distance at which the complete letter subtends 5 mins of arc. (If the letter subtends 5 mins arc, then the detail subtends 1 min arc.) 6 metres 12 metres The traditional way to measure vision and visual acuity is to use an Optometrist’s test chart, which contains letters of different sizes.

The Measurement of Vision and Visual Acuity 6 6 Vision - the ability to read the 6 metre letter at a distance of 6 metres (USA - 20 feet) hence minute of arc Minimum angle of resolution (MAR) here is 1 minute of arc logarithm A recent development is to use as a metric the logarithm of this angle - logMAR scale (Bailey and Lovie)

H VF ETP NUFCOK UTFEVNP Problem? The scale is not linear (i.e. not equal interval)

The Measurement of Vision and Visual Acuity 1 minute of arc Minimum angle of resolution (MAR) here is 1 minute of arc logarithm A recent development is to use as a metric the logarithm of this angle - logMAR scale (Bailey and Lovie) 6/6 = 1 minute arc => log 1 = 0 6/12 = 2 minutes arc => log 2 = 0.3 6/60 = 10 minutes arc => log 10 = 1 The Bailey-Lovie chart has the same number (5) of letters on each line, and it does provide an equal-interval scale.

The Measurement of Vision and Visual Acuity - The use of different targets