Visual Acuity “Cheat Sheet” – high and low vision

by Michael Bach

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… See the excellent historical review by Colenbrander (2008) or the next section.

Definition and conversion

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:

WhatDecimal acuitySnellen Ratio (ft)Snellen Ratio (m)LogMAR
Formula1 / MAR20 / (20 · MAR)6 / (6 · MAR)log10(MAR)
Conversion10^(-VALogMAR)VASnellen ≡ VAdecVASnellen ≡ 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 LogMAR versus the other acuity measures: think of LogMAR in terms of “visual loss”.

So which one is best?

Averaging acuities

For studies, visual acuity results frequently need to be averaged. For instance if you measure everything twice (my recommentation), and/or for treatment group comparisons. So how to average?

Low vision categories (CF, HF, …)

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)
 No light
Suggested value1.9 LogMAR2.3 LogMAR(2.7 LogMAR)(3.0 LogMAR)

Conversion / equivalence table

VAdecSnellen (ft)Snellen (m)LogMARCategory
Values in parentheses above
represent logical extensions
but are not defined in ISO 8956


All my acuity papers :)

  1. Bach M, Farmer JD (to be published) Evaluation of the “Freiburg Acuity VEP” on commercial equipment. Doc Ophthalmol
  2. Rohrschneider K, Spittler AR, Bach M (in press) Vergleich der Sehschärfenbestimmung mit Landolt-Ringen versus Zahlen. Der Ophthalmologe
  3. Wesemann W, Schiefer U, Heinrich SP, Jägle H, Bach M (in press) Neue DIN- und ISO-Normen zur Sehschärfebestimmung. Der Ophthalmologe
  4. Bach M, Heinrich SP (2019) Acuity VEP: Improved with machine learning. Doc Ophthalmol 139(2):113–122. read→here
  5. Reiniger J, Lobecke A, Sabesan R, Bach M, Verbakel F, Brabander J, Holz F, Berendschot TTJM, Harmening W (2019) Visual hyperacuity and acuity in the presence of ocular aberrations. JOV 1;19(5):11
  6. Hoffmann MB, Brands J, Behrens-Baumann W, Bach M (2017) VEP-based acuity assessment in low vision. Doc Ophthalmol 135(3):209–218 read→here
  7. Bach M (2016) Dichoptisches Training bei Amblyopie. Der Ophthalmologe 113(4):304–308
  8. Bach M, Schäfer K (2016) Visual acuity testing: feedback affects neither outcome nor reproducibility, but leaves participants happier. PLOS ONE 11(1):e0147803
  9. Bach M, Reuter M, Lagrèze WA (2016) Vergleich zweier Visustests in der Einschulungsuntersuchung – E-Haken-Einblickgerät versus Freiburger Visustest. Der Ophthalmologe 113:684–689
  10. 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
  11. Heinrich SP, Bock CM, Bach M (2016) Imitating the effect of amblyopia on VEP-based acuity estimates. Doc Ophthalmol 133:183–187
  12. Heinrich SP, Lüth I, Bach M (2015) Event-related potentials allow for optotype-based objective acuity estimation. IOVS 56:2184–2191
  13. Marhöfer DJ, Bach M, Heinrich SP (2015) Objective acuity assessment with self-face P300 responses. Doc Ophthalmol 131(2):137–148
  14. 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
  15. 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
  16. 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
  17. Heinrich SP, Bach M (2013) Resolution Acuity versus Recognition Acuity with Landolt-style Optotypes. Graef Arch Clin Exp 251:2235–2241
  18. Dehnert A, Bach M, Heinrich SP (2011) Subjective visual acuity with simulated defocus. Ophthalmic Physiol Optics 31:625–631
  19. 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
  20. 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
  21. 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]
  22. 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]
  23. Wesemann W, Schiefer U, Bach M (2010) Neue DIN-Normen zur Sehschärfebestimmung. Der Ophthalmologe 107:821–826 [→Visustafeln zum Ausdrucken ]
  24. Bach M, Dakin SC (2009) Regarding “Eagle-Eyed Visual Acuity: An Experimental Investigation of Enhanced Perception in Autism” Biol Psychiat 66:e19–e20
  25. 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
  26. 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
  27. Bach M (2007) The Freiburg Visual Acuity Test – Variability unchanged by post-hoc re-analysis. Graefe’s Arch Clin Exp Ophthalmol 245:965–971
  28. 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
  29. 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
  30. Bach M, Kommerell G (1998) Sehschärfebestimmung nach Europäischer Norm – wissenschaftliche Grundlagen und Möglichkeiten der automatischen Messung. Klin Mbl Augenheilk 212:190–195 (→HTML)
  31. Bach M (1997) Anti-aliasing and dithering in the “Freiburg Visual Acuity Test’. Spatial Vision 11:85–89
  32. Bach M (1996) The “Freiburg Visual Acuity Test” – Automatic measurement of visual acuity. Optometry and Vision Science 73:49–53