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… See the excellent historical review by Colenbrander (2008) or the next section.
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 the “opposite direction” of MAR versus the other acuity measures: think of MAR in terms of “visual loss”.
So which one is best?
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 represent logical extensions but 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) | ||
Schulze-Bonsel K, Feltgen N, Burau H, Hansen L, Bach M (2006) Visual acuities “hand motion” and “counting fingers” can be quantified with the Freiburg visual acuity test. Invest Ophth Vis Sci 47: 1236–1240
Bach M, Schulze-Bonsel K, Feltgen N, Burau H, Hansen LL (2007) Author Response: Numerical Imputation for Low Vision States. Invest Ophthalmol Vis Sci. eLetter, Aug 2007 →PDF
Colenbrander A (2008) The Historical Evolution of Visual Acuity Measurement. Visual Impairment Research 10:57–66. doi: 10.1080/13882350802632401
Lange C, Feltgen N, Junker B, Schulze-Bonsel K, Bach M (2009) Resolving the clinical acuity categories “hand motion” and “counting fingers” using the Freiburg Visual Acuity Test (FrACT). Graefe’s Arch Clin Exp Ophthalmol 247:137–142 →PDF
Hoffmann MB, Brands J, Behrens-Baumann W, Bach M (2017) VEP-based acuity assessment in low vision. Doc Ophthalmol 135(3):209–218
Bach M (2016) Dichoptisches Training bei Amblyopie. Der Ophthalmologe 113(4):304–308
Bach M, Schäfer K (2016) Visual acuity testing: feedback affects neither outcome nor reproducibility, but leaves participants happier. PLOS ONE 11(1):e0147803
Bach M, Reuter M, Lagrèze WA (2016) Vergleich zweier Visustests in der Einschulungsuntersuchung – E-Haken-Einblickgerät versus Freiburger Visustest. Der Ophthalmologe 113:684–689
Bartholomew AJ, Lad EM, Cao D, Bach M, Cirulli ET (2016) Individual differences in scotopic visual acuity and contrast sensitivity: genetic and non-genetic influences. PLOS ONE 11(2):e0148192
Heinrich SP, Bock CM, Bach M (2016) Imitating the effect of amblyopia on VEP-based acuity estimates. Doc Ophthalmol 133:183–187
Heinrich SP, Lüth I, Bach M (2015) Event-related potentials allow for optotype-based objective acuity estimation. IOVS 56:2184–2191
Marhöfer DJ, Bach M, Heinrich SP (2015) Objective acuity assessment with self-face P300 responses. Doc Ophthalmol 131(2):137–148
Tebartz van Elst L, Bach M, Blessing J, Riedel A, Bubl E (2015) Normal visual acuity and electrophysiological contrast gain in adults with high functioning autism spectrum disorder. Front Hum Neurosci 9:460
Koenig S, Tonagel F, Schiefer U, Bach M, Heinrich SP (2014) Assessing visual acuity across five disease types: ETDRS charts are faster with clinical outcome comparable to Landolt Cs. Graefe’s Arch Clin Exp Ophthalmol 252:1093–1099 <http://dx.doi.org/10.1007/s00417-014-2670-y>
Wenner Y, Heinrich SP, Beisse C, Fuchs A, Bach M (2014) Visual evoked potential-based acuity assessment: overestimation in amblyopia. Doc Ophthalmol 128:191–200 <http://dx.doi.org/10.1007/s10633-014-9432-3>
Heinrich SP, Bach M (2013) Resolution Acuity versus Recognition Acuity with Landolt-style Optotypes. Graef Arch Clin Exp 251:2235–2241
Dehnert A, Bach M, Heinrich SP (2011) Subjective visual acuity with simulated defocus. Ophthalmic Physiol Optics 31:625–631 [DOI: 10.1111/j.1475-1313.2011.00857.x]
Heinrich SP, Krüger K, Bach M (2011) The dynamics of practice effects in an optotype acuity task. Graefe’s Arch Clin Exp Ophthalmol 249:1319–1326 [DOI 10.1007/s00417-011-1675-z]
Tavassoli T, Latham K, Bach M, Dakin SC, Baron-Cohen S (2011) Psychophysical Measures of Visual Acuity in Autism Spectrum Conditions. Vision Res 51:1778–1780 [DOI 10.1016/j.visres.2011.06.004]
Heinrich SP, Krüger K, Bach M (2010) The effect of optotype presentation duration on acuity estimates revisited. Graefe’s Arch Clin Exp Ophthalmol 248:389–394 [DOI 10.1007/s00417-009-1268-2]
Heinrich SP, Marhöfer D, Bach M (2010) “Cognitive” visual acuity estimation based on the event-related potential P300 component. Clin Neurophysiol 121:1464–1472 [DOI 10.1016/j.clinph.2010.03.030]
Bach M, Dakin SC (2009) Regarding “Eagle-Eyed Visual Acuity: An Experimental Investigation of Enhanced Perception in Autism” Biol Psychiat 66:e19–e20
Lange C, Feltgen N, Junker B, Schulze-Bonsel K, Bach M (2009) Resolving the clinical acuity categories “hand motion” and “counting fingers” using the Freiburg Visual Acuity Test (FrACT). Graefe’s Arch Clin Exp Ophthalmol 247:137–142 →PDF
Bach M, Maurer JP, Wolf ME (2008) Visual evoked potential-based acuity assessment in normal vision, artificially degraded vision, and in patients. Brit J Ophthalmol 92:396–403
Bach M (2007) The Freiburg Visual Acuity Test – Variability unchanged by post-hoc re-analysis. Graefe’s Arch Clin Exp Ophthalmol 245:965–971
Schulze-Bonsel K, Feltgen N, Burau H, Hansen L, Bach M (2006) Visual acuities “hand motion” and “counting fingers” can be quantified with the Freiburg visual acuity test. Invest Ophth Vis Sci 47: 1236–1240
Jagle H, de Luca E, Serey L, Bach M, Sharpe LT (2006) Visual acuity and X-linked color blindness. Graefe’s Arch Clin Exp Ophthalmol 244:447–453
Bach M, Kommerell G (1998) Sehschärfebestimmung nach Europäischer Norm – wissenschaftliche Grundlagen und Möglichkeiten der automatischen Messung. Klin Mbl Augenheilk 212:190–195 (→HTML)
Bach M (1997) Anti-aliasing and dithering in the “Freiburg Visual Acuity Test’. Spatial Vision 11:85–89
Bach M (1996) The “Freiburg Visual Acuity Test” – Automatic measurement of visual acuity. Optometry and Vision Science 73:49–53