Visual acuity (VA) is one of the most important measures of our visual performance. Additional aspects are visual field, contrast sensitivity, color vision and motion perception, not covered here.
There exist (too) many measures to quantify visual acuity. Luckily, they can all be converted into each other. Which is best? Most people prefer the one they were brought up with…
They all relate to the visual angle of the smallest perceived (or discriminable) structure. Whichever way we define “smalles perceived structure”, let's call it “MAR” for “minimum angle of resolution”, and let its dimension be minutes of arc. Then:
| Measure | Decimal acuity | Snellen Ratio (ft) | Snellen Ratio (m) | logMAR |
|---|---|---|---|---|
| Shorthand | VAdec | VASnellen | VASnellen | VAlogMAR |
| Formula | 1 / MAR | 20 / (20 · MAR) | 6 / (6 · MAR) | log10(MAR) |
| “normal” | 1.0 | 20/20 | 6/6 | 0.0 |
| “low” | 0.02 | 20/200 | 6/60 | 1.0 |
| conversion | 10^(-VAlogMAR) | VASnellen ≡ VAdec | VASnellen ≡ VAdec | –log(VAdec) |
The “20” resp. “6” assume 20 ft resp. 6 m testing distance (which are rarely used exactly).
For very low vision, categories are used. With the help of the FrACT Vision test, we were able to assign approximate values to FC and HM (Schulze-Bonsel et al. 2006, Bach et al. 2007, Lange et al. 2009). The values for LP and NLP are imputations (Bach et al. 2007).
| Measure | Finger counting (at 30 cm) |
Hand movement | Light perception | No light perception |
|---|---|---|---|---|
| Shorthand | CF | HM | LP | NLP |
| Suggested value | 1.9 logMAR | 2.3 logMAR | (2.7 logMAR) | (3.0 logMAR) |
| VAdec | Snellen (ft) | Snellen (m) | logMAR | Category |
|---|---|---|---|---|
| 2.0 | 20/10 | 6/3.0 | -0.30 | |
| 1.6 | 20/12.5 | 6/3.8 | -0.20 | |
| 1.26 | 20/16 | 6/4.8 | -0.10 | |
| 1.0 | 20/20 | 6/6.0 | 0.00 | |
| 0.8 | 20/25 | 6/7,5 | 0.10 | |
| 0.63 | 20/30 | 6/9,5 | 0.20 | |
| 0.5 | 20/40 | 6/12 | 0.30 | |
| 0.4 | 20/50 | 6/15 | 0.40 | |
| 0.32 | 20/60 | 6/19 | 0.50 | |
| 0.25 | 20/80 | 6/24 | 0.60 | |
| 0.2 | 20/100 | 6/30 | 0.70 | |
| 0.16 | 20/125 | 6/38 | 0.80 | |
| 0.13 | 20/160 | 6/48 | 0.90 | |
| 0.10 | 20/200 | 6/60 | 1.00 | |
| 0.08 | 20/250 | 6/75 | 1.10 | |
| 0.063 | 20/300 | 6/95 | 1.20 | |
| 0.050 | 20/400 | 6/120 | 1.30 | |
| 0.040 | (20/500) | (6/150) | 1.40 | |
| 0.032 | (20/600) | (6/190) | 1.50 | |
| 0.025 | (20/800) | (6/240) | 1.60 | |
| 0.020 | (20/1000 | (6/300) | 1.70 | |
| 0.016 | Values in parentheses above are not defined in ISO 8956 |
1.80 | CF | |
| 0.013 | 1.90 | CF | ||
| 0.010 | 2.00 | CF | ||
| 0.0079 | 2.10 | |||
| 0.0063 | 2.20 | HM | ||
| 0.0050 | 2.30 | HM | ||
| 0.0040 | 2.40 | HM | ||
| 0.0032 | 2.50 | |||
| 0.0025 | 2.60 | |||
| 0.0020 | 2.70 | (LP) | ||
| 0.0016 | 2.80 | |||
| 0.0013 | 2.90 | |||
| 0.0010 | 3.00 | (NLP) | ||
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Created: 2017-10-20