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Assessing Visual Acuity

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The universally used standard for assessment of visual acuity is the Snellen's Test Types, as above. The types consist of a series of letters of diminishing size. Each letter is of such a shape that it can be enclosed in a square the size of which is five times the thickness of the lines composing the letter.

The size of these squares, that is, the breadth of the lines, is such that their edges subtend a visual angle of 1 minute when they are a certain specified distance away. Each entire letter therefore subtends an angle of 5 minutes at this distance, but in order to analyse its form completely and see its constituent parts, the eye must be able to resolve them down to the standard limit of 1 minute.

The first line of type is so constructed that this angle is formed at a distance of 60 metres, the second at 36 metres, then 24, 18, 12, 9 and 6 metres. Additional lines may be inserted that subtend the same angle at 5 and 4 metres.

These letters should thus be read by a person with standard vision at these distances away. Consequently if a patient is placed at a convenient distance, which is usually taken as 6 metres, he should be able to read easily down to the line with a theoretical view-point 9 metres away, while the 6 metre line should just be distinct. If he cannot reach this limit, his distant vision is defective; and if he can exceed it, it is above standard. In practice it will be found that the standard is a liberal one for the acuity of the vision of the great majority of people can readily be raised above it.

The results of the test are expressed as a fraction, the numerator of which denotes the distance at which the patient if from the type, and the denominator the line he sees at this distance. Thus if his vision is 'normal' and he sees the line which ought to be read at 6 metres when he is 6 metres distant, his visual acuity is 6/6; if when he is at this distance, he can only see the line which a person with standard vision should see at 24 metres, his visual acuity is 6/24; while if he reads still further and reaches the line constructed to subtend the normal visual angle at 4 metres, his acuity is 6/4.

In the United States of America the metric system is not usually employed but the fraction is written in terms of feet (6 metres = 20 feet): 6/6 therefore is 20/20.

On the continent of Europe Monoyer's scale is used wherein the relative sizes of the test types are 10/10, 10/9... giving a relative visual acuity of 1.0, 0.9, 0.8...0.1 (in arithmetical progression).

The following table gives the approximate equivalence in Snellen's notation of metres and feet and in the decimal system, there are added the corresponding visual angles in minutes and the generally accepted percentages of visual efficiency and percentage loss of vision.

Snellen's-Feet

Snellen's-Metres

Decimal Notation

Visual angle(1')

%Visual efficiency

%Visual loss

6/6

20/20

1.0

1.0

100

0.0

6/9

20/30

0.7

1.5

91.4

8.6

6/12

20/40

0.5

2.0

83.6

16.4

6/18

20/60

0.3

3.0

69.9

30.1

6/24

20/80

0.25

4.0

58.5

41.5

6/60

20/200

0.1

10.0

20.0

80.0

Note: All the text on this page has been taken from "Duke-Elder's Practice of Refraction" Revised by David Abrams, 9th edition/first Indian edition 1986.