# Visual Acuity “Cheat Sheet” – high and low vision

## Decimal acuity, logMAR, Snellen ratio, finger counting, hand movement, light perception…

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…

## Definition and conversion

They all relate to the visual angle of the smallest perceptible structure. However we define “perceptible 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
Formula 1 / MAR20 / (20 · MAR) 6 / (6 · MAR) log10(MAR)
“normal” 1.020/20 6/6 0.0
“low” 0.0220/200 6/60 1.0

The “20” resp. “6” assume 20 ft resp. 6 m testing distance (which are rarely used exactly).

## Low vision categories

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 movementLight perceptionNo light perception
Shorthand CF
HMLPNLP
Suggested value 1.9 logMAR2.3 logMAR(2.7 logMAR)(3.0 logMAR)

## Conversion / equivalence table

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)

## References

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

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

## All my acuity papers :)

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

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

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

Created: 2017-10-20

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Last update 2017-12-11 by Michael Bach (G+)