ETDRS charts feature Sloan letters with proportional spacing, 5 letters on each line, and a logarithmic progression of letter sizes with consistent increments of approximately 25% per line (10 lines equal a factor 10x). The ETDRS protocol specifies letter-by-letter scoring, viewing distance, illumination, use of different charts for right and left eye and other presentation parameters.
The full ETDRS protocol requires a wide chart on a light box, and cannot be implemented on the limited screen of a projector (or similar) chart. The logarithmic progression, however, can be implemented on any device. This progression was first proposed by John Green in 1868 and follows the standard "Preferred Numbers, ISO standard 3 (1973) " series and the rounding preferences.
Use of ETDRS charts allows use of letter-by-letter scoring, which is more accurate than the line-by-line scoring used on traditional charts. Each row in the table is equivalent to 1 letter on an ETDRS chart (50 letters for a factor 10x). These steps are smaller than the just discernible difference; steps this small only become significant in statistical studies where a large number of measurements is averaged.
The smaller steps for letter by letter scoring may be expressed in two ways; either by using suffixes to a familiar (sometimes slightly rounded) set of values or by using calculated values. For clinical use suffixes have the advantage of using only familiar acuity notations and reverting to the nearest clinical notation when the suffix is omitted. Calculated values look less familiar; but are sometimes used in statistical studies. Note that suffixes used in the context of an ETDRS chart have a defined value and affect the DICOM storage value, whereas suffixes used in the context of traditional charts do not.
Table RR-2. Reference Table for Use with ETDRS Charts or Equivalent
|
DICOM
|
|
Notations for Research Use with ETDRS Charts or Equivalent
|
|
Scales for statistics and graphical displays
|
|
Decimal Visual Acuity
|
|
Use with suffixes
|
|
Calculated values
|
|
Linear Scales
|
|
|
Decimal
|
US
|
6 m
|
|
Decimal
|
US
|
6 m
|
|
LogMAR
|
VAS
|
|
2.00 E+00
|
|
2.0
|
20/10
|
6/3
|
|
2.00
|
20/10
|
6/3.0
|
|
-0.30
|
115
|
|
1.91 E+00
|
|
-
|
-
|
-
|
|
1.91
|
20/10.5
|
6/3.2
|
|
-0.28
|
114
|
|
1.82 E+00
|
|
- -
|
- -
|
- -
|
|
1.82
|
20/11
|
6/3.3
|
|
-0.26
|
113
|
|
1.74 E+00
|
|
+ +
|
+ +
|
+ +
|
|
1.74
|
20/11.5
|
6/3.5
|
|
-0.24
|
112
|
|
1.66 E+00
|
|
+
|
+
|
+
|
|
1.66
|
20/12
|
6/3.6
|
|
-0.22
|
111
|
|
1.60 E+00
|
|
1.6
|
20/12.5
|
6/3.8
|
|
1.58
|
20/12.5
|
6/3.8
|
|
-0.20
|
110
|
|
1.50 E+00
|
|
-
|
-
|
-
|
|
1.51
|
20/13
|
6/4.0
|
|
-0.18
|
109
|
|
1.45 E+00
|
|
- -
|
- -
|
- -
|
|
1.45
|
20/14
|
6/4.2
|
|
-0.16
|
108
|
|
1.38 E+00
|
|
+ +
|
+ +
|
+ +
|
|
1.38
|
20/14.5
|
6/4.4
|
|
-0.14
|
107
|
|
1.30 E+00
|
|
+
|
+
|
+
|
|
1.32
|
20/15
|
6/4.6
|
|
-0.12
|
106
|
|
1.25 E+00
|
|
1.25
|
20/16
|
6/4.8
|
|
1.26
|
20/16
|
6/4.8
|
|
-0.10
|
105
|
|
1.20 E+00
|
|
-
|
-
|
-
|
|
1.20
|
20/17
|
6/5.0
|
|
-0.08
|
104
|
|
1.15 E+00
|
|
- -
|
- -
|
- -
|
|
1.15
|
20/17.5
|
6/5.2
|
|
-0.06
|
103
|
|
1.10 E+00
|
|
+ +
|
+ +
|
+ +
|
|
1.10
|
20/18
|
6/5.5
|
|
-0.04
|
102
|
|
1.05 E+00
|
|
+
|
+
|
+
|
|
1.05
|
20/19
|
6/5.8
|
|
-0.02
|
101
|
|
1.00 E+00
|
|
1.0
|
20/20
|
6/6
|
|
1.00
|
20/20
|
6/6.0
|
|
0
|
100
|
|
9.55 E-01
|
|
-
|
-
|
-
|
|
0.95
|
20/21
|
6/6.3
|
|
0.02
|
99
|
|
9.00 E-01
|
|
- -
|
- -
|
- -
|
|
0.91
|
20/22
|
6/6.6
|
|
0.04
|
98
|
|
8.70 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.87
|
20/23
|
6/6.9
|
|
0.06
|
97
|
|
8.30 E-01
|
|
+
|
+
|
+
|
|
0.83
|
20/24
|
6/7.2
|
|
0.08
|
96
|
|
8.00 E-01
|
|
0.8
|
20/25
|
6/7.5
|
|
0.79
|
20/25
|
6/7.5
|
|
0.10
|
95
|
|
7.50 E-01
|
|
-
|
-
|
-
|
|
0.76
|
20/26
|
6/7.9
|
|
0.12
|
94
|
|
7.20 E-01
|
|
- -
|
- -
|
- -
|
|
0.72
|
20/28
|
6/8.3
|
|
0.14
|
93
|
|
7.00 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.69
|
20/29
|
6/8.7
|
|
0.16
|
92
|
|
6.60 E-01
|
|
+
|
+
|
+
|
|
0.66
|
20/30
|
6/9.1
|
|
0.18
|
91
|
|
6.30 E-01
|
|
0.63
|
20/32
|
6/9.5
|
|
0.63
|
20/32
|
6/9.5
|
|
0.20
|
90
|
|
6.00 E-01
|
|
-
|
-
|
-
|
|
0.60
|
20/33
|
6/10.0
|
|
0.22
|
89
|
|
5.75 E-01
|
|
- -
|
- -
|
- -
|
|
0.58
|
20/35
|
6/10.5
|
|
0.24
|
88
|
|
5.50 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.55
|
20/36
|
6/11.0
|
|
0.26
|
87
|
|
5.25 E-01
|
|
+
|
+
|
+
|
|
0.52
|
20/38
|
6/11.5
|
|
0.28
|
86
|
|
5.00 E-01
|
|
0.5
|
20/40
|
6/12
|
|
0.50
|
20/40
|
6/12.0
|
|
0.30
|
85
|
|
4.80 E-01
|
|
-
|
-
|
-
|
|
0.48
|
20/42
|
6/12.5
|
|
0.32
|
84
|
|
4.57 E-01
|
|
- -
|
- -
|
- -
|
|
0.46
|
20/44
|
6/13.2
|
|
0.34
|
83
|
|
4.37 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.24
|
20/46
|
6/13.8
|
|
0.36
|
82
|
|
4.17 E-01
|
|
+
|
+
|
+
|
|
0.42
|
20/48
|
6/14.5
|
|
0.38
|
81
|
|
4.00 E-01
|
|
0.4
|
20/50
|
6/15
|
|
0.40
|
20/50
|
6/15.1
|
|
0.40
|
80
|
|
3.80 E-01
|
|
-
|
-
|
-
|
|
0.38
|
20/52
|
6/15.8
|
|
0.42
|
79
|
|
3.60 E-01
|
|
- -
|
- -
|
- -
|
|
0.36
|
20/55
|
6/16.6
|
|
0.44
|
78
|
|
3.50 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.35
|
20/58
|
6/17.4
|
|
0.46
|
77
|
|
3.33 E-01
|
|
+
|
+
|
+
|
|
0.33
|
20/60
|
6/18.2
|
|
0.48
|
76
|
|
3.20 E-01
|
|
0.32
|
20/63
|
6/19
|
|
0.32
|
20/63
|
6/19.1
|
|
0.50
|
75
|
|
3.00 E-01
|
|
-
|
-
|
-
|
|
0.30
|
20/66
|
6/20.
|
|
0.52
|
74
|
|
2.90 E-01
|
|
- -
|
- -
|
- -
|
|
0.29
|
20/69
|
6/21
|
|
0.54
|
73
|
|
2.75 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.28
|
20/72
|
6/22
|
|
0.56
|
72
|
|
2.63 E-01
|
|
+
|
+
|
+
|
|
0.26
|
20/76
|
6/23
|
|
0.58
|
71
|
|
2.50 E-01
|
|
0.25
|
20/80
|
6/24
|
|
0.25
|
20/79
|
6/24
|
|
0.60
|
70
|
|
2.40 E-01
|
|
-
|
-
|
-
|
|
0.24
|
20/83
|
6/25
|
|
0.62
|
69
|
|
2.30 E-01
|
|
- -
|
- -
|
- -
|
|
0.23
|
20/87
|
6/26
|
|
0.64
|
68
|
|
2.20 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.22
|
20/91
|
6/28
|
|
0.66
|
67
|
|
2.10 E-01
|
|
+
|
+
|
+
|
|
0.21
|
20/95
|
6/29
|
|
0.68
|
66
|
|
2.00 E-01
|
|
0.2
|
20/100
|
6/30
|
|
0.20
|
20/100
|
6/30
|
|
0.70
|
65
|
|
1.90 E-01
|
|
-
|
-
|
-
|
|
0.191
|
20/105
|
6/32
|
|
0.72
|
64
|
|
1.82 E-01
|
|
- -
|
- -
|
- -
|
|
0.182
|
20/110
|
6/33
|
|
0.74
|
63
|
|
1.74 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.174
|
20/115
|
6/35
|
|
0.76
|
62
|
|
1.66 E-01
|
|
+
|
+
|
+
|
|
0.166
|
20/120
|
6/36
|
|
0.78
|
61
|
|
1.60 E-01
|
|
0.16
|
20/125
|
6/38
|
|
0.158
|
20/126
|
6/38
|
|
0.80
|
60
|
|
1.50 E-01
|
|
-
|
-
|
-
|
|
0.151
|
20/132
|
6/40
|
|
0.82
|
59
|
|
1.45 E-01
|
|
- -
|
- -
|
- -
|
|
0.145
|
20/138
|
6/42
|
|
0.84
|
58
|
|
1.38 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.138
|
20/145
|
6/44
|
|
0.86
|
57
|
|
1.30 E-01
|
|
+
|
+
|
+
|
|
0.132
|
20/151
|
6/46
|
|
0.88
|
56
|
|
1.25 E-01
|
|
0.125
|
20/160
|
6/48
|
|
0.126
|
20/158
|
6/48
|
|
0.90
|
55
|
|
1.20 E-01
|
|
-
|
-
|
-
|
|
0.120
|
20/166
|
6/50
|
|
0.92
|
54
|
|
1.15 E-01
|
|
- -
|
- -
|
- -
|
|
0.115
|
20/174
|
6/52
|
|
0.94
|
53
|
|
1.10 E-01
|
|
+ +
|
+ +
|
+ +
|
|
0.110
|
20/182
|
6/55
|
|
0.96
|
52
|
|
1.05 E-01
|
|
+
|
+
|
+
|
|
0.105
|
20/191
|
6/58
|
|
0.98
|
51
|
|
1.00 E-01
|
|
0.1
|
20/200
|
6/60
|
|
0.100
|
20/200
|
6/60
|
|
1.00
|
50
|
|
9.55 E-02
|
|
-
|
-
|
-
|
|
0.095
|
20/210
|
6/63
|
|
1.02
|
49
|
|
9.00 E-02
|
|
- -
|
- -
|
- -
|
|
0.091
|
20/220
|
6/66
|
|
1.04
|
48
|
|
8.70 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.087
|
20/230
|
6/69
|
|
1.06
|
47
|
|
8.30 E-02
|
|
+
|
+
|
+
|
|
0.083
|
20/240
|
6/72
|
|
1.08
|
46
|
|
8.00 E-02
|
|
0.08
|
20/250
|
6/75
|
|
0.079
|
20/250
|
6/76
|
|
1.10
|
45
|
|
7.50 E-02
|
|
-
|
-
|
-
|
|
0.076
|
20/260
|
6/79
|
|
1.12
|
44
|
|
7.20 E-02
|
|
- -
|
- -
|
- -
|
|
0.072
|
20/280
|
6/83
|
|
1.14
|
43
|
|
7.00 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.069
|
20/290
|
6/87
|
|
1.16
|
42
|
|
6.60 E-02
|
|
+
|
+
|
+
|
|
0.066
|
20/300
|
6/91
|
|
1.18
|
41
|
|
6.30 E-02
|
|
0.063
|
20/320
|
6/95
|
|
0.063
|
20/315
|
6/95
|
|
1.20
|
40
|
|
6.00 E-02
|
|
-
|
-
|
-
|
|
0.060
|
20/330
|
6/100
|
|
1.22
|
39
|
|
5.75 E-02
|
|
- -
|
- -
|
- -
|
|
0.058
|
20/350
|
6/105
|
|
1.24
|
38
|
|
5.50 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.055
|
20/360
|
6/110
|
|
1.26
|
37
|
|
5.25 E-02
|
|
+
|
+
|
+
|
|
0.052
|
20/380
|
6/115
|
|
1.28
|
36
|
|
5.00 E-02
|
|
0.05
|
20/400
|
6/120
|
|
0.050
|
20/400
|
6/120
|
|
1.30
|
35
|
|
4.80 E-02
|
|
-
|
-
|
-
|
|
0.048
|
20/420
|
6/126
|
|
1.32
|
34
|
|
4.60 E-02
|
|
- -
|
- -
|
- -
|
|
0.046
|
20/440
|
6/132
|
|
1.34
|
33
|
|
4.40 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.044
|
20/460
|
6/138
|
|
1.36
|
32
|
|
4.20 E-02
|
|
+
|
+
|
+
|
|
0.042
|
20/480
|
6/145
|
|
1.38
|
31
|
|
4.00 E-02
|
|
0.04
|
20/500
|
6/150
|
|
0.040
|
20/500
|
6/151
|
|
1.40
|
30
|
|
3.80 E-02
|
|
-
|
-
|
-
|
|
0.038
|
20/520
|
6/158
|
|
1.42
|
29
|
|
3.60 E-02
|
|
- -
|
- -
|
- -
|
|
0.036
|
20/550
|
6/166
|
|
1.44
|
28
|
|
3.50 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.035
|
20/575
|
6/174
|
|
1.46
|
27
|
|
3.33 E-02
|
|
+
|
+
|
+
|
|
0.033
|
20/600
|
6/182
|
|
1.48
|
26
|
|
3.20 E-02
|
|
0.032
|
20/630
|
6/190
|
|
0.032
|
20/630
|
6/191
|
|
1.50
|
25
|
|
3.02 E-02
|
|
-
|
-
|
-
|
|
0.030
|
20/660
|
6/200
|
|
1.52
|
24
|
|
2.90 E-02
|
|
- -
|
- -
|
- -
|
|
0.029
|
20/690
|
6/210
|
|
1.54
|
23
|
|
2.75 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.028
|
20/720
|
6/220
|
|
1.56
|
22
|
|
2.63 E-02
|
|
+
|
+
|
+
|
|
0.026
|
20/760
|
6/230
|
|
1.58
|
21
|
|
2.50 E-02
|
|
0.025
|
20/800
|
6/240
|
|
0.025
|
20/800
|
6/240
|
|
1.60
|
20
|
|
2.40 E-02
|
|
-
|
-
|
-
|
|
0.024
|
20/830
|
6/250
|
|
1.62
|
19
|
|
2.30 E-02
|
|
- -
|
- -
|
- -
|
|
0.023
|
20/870
|
6/260
|
|
1.64
|
18
|
|
2.20 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.022
|
20/910
|
6/280
|
|
1.66
|
17
|
|
2.10 E-02
|
|
+
|
+
|
+
|
|
0.021
|
20/950
|
6/290
|
|
1.68
|
16
|
|
2.00 E-02
|
|
0.020
|
20/1000
|
6/300
|
|
0.0200
|
20/1000
|
6/300
|
|
1.70
|
15
|
|
1.90 E-02
|
|
-
|
-
|
-
|
|
0.0191
|
20/1050
|
6/315
|
|
1.72
|
14
|
|
1.82 E-02
|
|
- -
|
- -
|
- -
|
|
0.0182
|
20/1100
|
6/330
|
|
1.74
|
13
|
|
1.74 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.0174
|
20/1150
|
6/350
|
|
1.76
|
12
|
|
1.66 E-02
|
|
+
|
+
|
+
|
|
0.0166
|
20/1200
|
6/363
|
|
1.78
|
11
|
|
1.60 E-02
|
|
0.016
|
20/1250
|
6/380
|
|
0.0158
|
20/1250
|
6/380
|
|
1.80
|
10
|
|
1.50 E-02
|
|
-
|
-
|
-
|
|
0.0151
|
20/1300
|
6/400
|
|
1.82
|
9
|
|
1.45 E-02
|
|
- -
|
- -
|
- -
|
|
0.0145
|
20/1380
|
6/420
|
|
1.84
|
8
|
|
1.38 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.0138
|
20/1450
|
6/440
|
|
1.86
|
7
|
|
1.30 E-02
|
|
+
|
+
|
+
|
|
0.0132
|
20/1500
|
6/460
|
|
1.88
|
6
|
|
1.25 E-02
|
|
0.0125
|
20/1600
|
6/480
|
|
0.0126
|
20/1600
|
6/480
|
|
1.90
|
5
|
|
1.20 E-02
|
|
-
|
-
|
-
|
|
0.0120
|
20/1660
|
6/500
|
|
1.92
|
4
|
|
1.15 E-02
|
|
- -
|
- -
|
- -
|
|
0.0115
|
20/1740
|
6/520
|
|
1.94
|
3
|
|
1.10 E-02
|
|
+ +
|
+ +
|
+ +
|
|
0.0110
|
20/1820
|
6/550
|
|
1.96
|
2
|
|
1.05 E-02
|
|
+
|
+
|
+
|
|
0.0105
|
20/1910
|
6/575
|
|
1.98
|
1
|
|
1.00 E-02
|
|
0.010
|
20/2000
|
6/600
|
|
0.0100
|
20/2000
|
6/600
|
|
2.00
|
0
|