APPENDIX A

Measuring Visual Acuity

The optician’s test chart was devised by the Dutch ophthalmologist Hermann Snellen (1835–1908). In its usual form it consists of a number of lines of letters of decreasing size. Each line is assigned a distance at which it should be legible to the normal eye; the eyesight is considered normal if the six-metre line can indeed be read at six metres. This standard is expressed as a fraction, or a ratio, so that normal vision is described as 6/6, or 6:6. Someone with only half this acuity would be described as having 3:6 vision; someone with only a third 2:6, and so on. At one time the distance was measured in feet, and 20:20 is the same standard as 6:6.

The distance at which the chart is read is that at which the height of the letter subtends an angle of five minutes of arc (see Figure 12). This value has been chosen quite arbitrarily and in fact represents only a rather mediocre standard of acuity. A standard of 12:6 is regularly attained by people with good eyesight.

Figure 12 (not to scale)

ACUITY TABLES FOR USE WITH TEST CHARTS 

1 Metric units

Chart C at	25	37.5	50	62.5	75	87.5	100	112.5	125	cm
Chart B at	50	75	100	125	150	175	200	225	250	cm
Chart A at	2	3	4	5	6	7	8	9	10	m
C (60 m)	3	5	7	8	10	12	13	15	17	%
E (36 m)	6	9	11	14	17	19	22	25	28
O (24 m)	8	13	17	21	25	29	33	38	42
D (18 m)	11	17	22	28	33	39	44	50	56
U (12 m)	17	25	33	42	50	58	67	75	83
F (9m)	22	33	44	56	67	78	89	100	111
S (6 m)	33	50	67	83	100	117	133	150	167
R (5 m)	40	60	80	100	120	140	160	180	200
B (4 m)	50	75	100	125	150	175	200	(225)	(250)
H (3 m)	67	100	133	167	200	(233)	(267)	(300)	(333)

2 Imperial units

Chart C at	9	13.5	18	22.5	27	31.5	36	40.5	45	in
Chart B at	18	27	36	45	54	63	72	81	90	in
Chart A at	6	9	12	15	18	21	24	27	30	ft
C	3	5	6	8	9	11	12	14	15	%
E	5	8	10	13	15	18	20	23	25
O	8	11	15	19	23	27	30	34	38
D	10	15	20	25	30	36	41	46	51
U	15	23	30	38	45	53	61	69	76
F	20	31	41	51	61	71	81	91	102
S	30	46	61	76	91	107	122	137	152
R	37	55	73	91	110	128	146	165	183
B	46	69	91	114	137	160	183	(206)	(229)
H	61	91	122	152	183	(213)	(244)	(274)	(305)

To measure your own visual acuity, fix the large test chart (Chart A) at a distance of four to eight metres (four to nine yards) and in a good strong light. The letters have been chosen with the Bates method rather than the standard specification in mind, but generally the chart conforms to the standard and will give an accurate result when used with care.

With either eye singly, and then with both together, record the lowest line on which you can read all the letters. Then, knowing how far away the chart is, use the Acuity Table to find your acuity. In the table acuity has been expressed as a percentage rather than a ratio. Thus 6:6 is expressed as 100 per cent, 9:6 as 150 per cent, etc. As an example, if on Chart A you can read the whole of the 9-metre line (the one beginning with the letter F) at a maximum distance of 7 metres, your acuity works out at 78 per cent of normal.

The table may also be used to find your acuity at reading distance. If at 50 centimetres you can read the whole of the “H” line on Chart C, your acuity is 133 per cent of normal. For convenience the table is also given in imperial units.

As mentioned above, the Snellen standard has been chosen quite arbitrarily. A measure of absolute acuity, finding the smallest object that can be perceived, is in some ways preferable. Each cone in the centre of the foveola covers an area of the visual field which subtends an angle of less than 20 seconds of arc. (There are 60 seconds in one minute of arc, and 3600 seconds in one degree.) With the foveola stationary (that is, not taking the scanning action into account), one should in theory be able, at a range of 1.6 kilometres (1 mile), to perceive an object 15 centimetres (6 inches) across. After Bates training, resolution of 10 seconds of arc or better, representing, at 1.6 kilometres (1 mile), an object some 7.5 centimetres (3 inches) across, is readily achievable. The final limit depends on the refinement and quality of the individual’s visual system.

Absolute acuity, measured in seconds of arc (”), may be calculated according to the formula:

assuming that both measurements are in the same units. If range is measured in metres and width in millimetres, divide by 1000, so that the formula becomes:

If range is measured in yards and width in inches, the formula is:

Thus if you can discern a power line 100 millimetres in diameter at a range of 3 kilometres (3000 metres), your absolute acuity works out at , or 7” (7 seconds of arc).

Acuity is of course improved in conditions of good lighting, contrast, and clarity of air. Any object at any range may be used for this test, but, due to the scanning action of the eye, single objects (flagpoles, golfballs, etc.) are not so rapidly perceived as multiple ones. A grid of lines makes a more suitable datum for an acuity test. The pointing in brickwork, particularly if it contrasts well with the colour of the bricks, is ideal. For example, if the average width of the pointing on a wall is 10 millimetres, and you can perceive the pointing at a maximum range of 125 metres, your absolute acuity is , or 16”.