Visual Acuity “Cheat Sheet” – high and low vision

Visual acuity (VA) is one of the most important measures of our visual performance. Further aspects like visual field, contrast sensitivity, color vision, motion perception etc. are 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 next section and the historical review by Colenbrander (2008).

All acuity measures 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:

The “20” resp. “6” in Snellen columns correspond to a testing distance of 20 ft resp. 6 m.
For the “opposite direction” of LogMAR versus the other acuity measures: think of LogMAR in terms of “visual loss”.

So which one is best?

• Decimal acuity and Snellen ratio are identical after you simply calculate the fraction.
• Decimal acuity and Snellen ratio are not useful for calculations, for instance you must never calculate the mean!
• LogMAR “runs the wrong way round”, think of it as measuring “visual loss”.
• LogMAR is approximately normally distributed. That means you can calculate means, SDs, t-tests, whatever, to your heart’s desire.
• As the first analysis step, convert everything to LogMAR. After analysis, one can convert back to Snellen or Decimal to reach the pertinent audience. I suggest to plot LogMAR on an inverted axis, so “good” acuity is up and/or right.
• “ETDRS”? →ETDRS is not a VA unit; ETDRS charts use LogMAR.
• “Letters”? →This term is not defined in any ISO Standard. It is used as equivalent to a step of 0.02 LogMAR.
• “Letter Score”? →This usually refers to the count of letters read (see previous item). Its absolute value is not well defined, as it depends on where you start counting, which depends on the exact type of chart, AND the distance. I avoid Letters or Letter Scores, they are needless additions since all can be expressed in LogMAR. You can still count letters on the last line read, should you wish for a (seemingly) higher resolution, on whichever chart (with a sensible progression and 5 optotypes per line) you use, and calculate “0.02 LogMAR” per letter.
• Line? →No ISO Standard term. Acuity charts are typically arranged in lines, with equal optotype grade along a line. The lines should have equidistant progressive acuity grades with 0.1 LogMAR increments. Then 1 Line is equivalent to 0.1 LogMAR.
  

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

• LogMAR values can be averaged in the normal arithmetic way [e.g. (val1+val2)/2] because LogMAR is an (approximate) interval scale
• Decimal acuity MUST NOT be averaged arithmetically (its “scale of measurement” is only ordinal). You could calculate the geometric mean, but best convert to LogMAR, then average. If desired you can then convert back to decimal notation.
• Snellen Fraction: convert to LogMAR, then average. If desired you can then convert back to Snellen Fraction.
  

For very low vision, categories are used. With the help of the FrACT Vision test, we were able to assign approximate values to counting fingers (CF) and hand movement (HM) (Schulze-Bonsel et al. 2006, Bach et al. 2007, Lange et al. 2009). The values for light perception (LP) and no light perception (NLP) are imputations (Bach et al. 2007).

• Schulze-Bonsel K, Feltgen N, Burau H, Hansen LL, Bach M (2006) Visual acuities “Hand Motion” and “Counting Fingers” can be quantified using the Freiburg Visual Acuity Test. Invest Ophthalmol Vis Sci 47:1236–1240   [PDF]
• 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
• 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 read→here, [PDF]

1. Bach M, Farmer JD (2020) Evaluation of the “Freiburg Acuity VEP” on commercial equipment. Doc Ophthalmol 140:139–145   [→PDF]
2. Bailey I, Bach M, Ferris R, Johnson C, Bittner A, Colenbrander A, Keeffe J (2020) Visual acuity. In: Ayton L et al. for the HOVER International Taskforce (2020) Harmonization of Outcomes and Vision Endpoints in Vision Restoration Trials: Recommendations from the International HOVER Taskforce. Transl Vis Sci Technol 9:25–25 [PDF]
3. Freundlieb P, Kramer F, Herbik A, Bach M, Hoffmann MB (2020) Scotopic and photopic conventional visual acuity and hyperacuity. Graefes Archives 258(1):129–135   [→read]
4. Wesemann W, Schiefer U, Heinrich SP, Jägle H, Bach M (2020) Neue DIN- und ISO-Normen zur Sehschärfebestimmung. Der Ophthalmologe 117(1):19–26
5. Rohrschneider K, Spittler AR, Bach M (2019) Vergleich der Sehschärfenbestimmung mit Landolt-Ringen versus Zahlen. Der Ophthalmologe 116(11):1058–1063
6. Bach M, Heinrich SP (2019) Acuity VEP: Improved with machine learning. Doc Ophthalmol 139(2):113–122. read→here
7. 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
8. 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
9. Bach M (2016) Dichoptisches Training bei Amblyopie. Der Ophthalmologe 113(4):304–308
10. Bach M, Schäfer K (2016) Visual acuity testing: feedback affects neither outcome nor reproducibility, but leaves participants happier. PLOS ONE 11(1):e0147803
11. Bach M, Reuter M, Lagrèze WA (2016) Vergleich zweier Visustests in der Einschulungsuntersuchung – E-Haken-Einblickgerät versus Freiburger Visustest. Der Ophthalmologe 113:684–689
12. 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
13. Heinrich SP, Bock CM, Bach M (2016) Imitating the effect of amblyopia on VEP-based acuity estimates. Doc Ophthalmol 133:183–187
14. Heinrich SP, Lüth I, Bach M (2015) Event-related potentials allow for optotype-based objective acuity estimation. IOVS 56:2184–2191
15. Marhöfer DJ, Bach M, Heinrich SP (2015) Objective acuity assessment with self-face P300 responses. Doc Ophthalmol 131(2):137–148
16. 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
17. 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
18. 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
19. Heinrich SP, Bach M (2013) Resolution Acuity versus Recognition Acuity with Landolt-style Optotypes. Graef Arch Clin Exp 251:2235–2241
20. Dehnert A, Bach M, Heinrich SP (2011) Subjective visual acuity with simulated defocus. Ophthalmic Physiol Optics 31:625–631
21. 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
22. 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
23. 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]
24. 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]
25. Wesemann W, Schiefer U, Bach M (2010) Neue DIN-Normen zur Sehschärfebestimmung. Der Ophthalmologe 107:821–826 [→Visustafeln zum Ausdrucken ]
26. Bach M, Dakin SC (2009) Regarding “Eagle-Eyed Visual Acuity: An Experimental Investigation of Enhanced Perception in Autism” Biol Psychiat 66:e19–e20
27. 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
28. 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
29. Bach M (2007) The Freiburg Visual Acuity Test – Variability unchanged by post-hoc re-analysis. Graefe’s Arch Clin Exp Ophthalmol 245:965–971
30. 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
31. 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
32. Bach M, Kommerell G (1998) Sehschärfebestimmung nach Europäischer Norm – wissenschaftliche Grundlagen und Möglichkeiten der automatischen Messung. Klin Mbl Augenheilk 212:190–195 (→HTML)
33. Bach M (1997) Anti-aliasing and dithering in the “Freiburg Visual Acuity Test’. Spatial Vision 11:85–89
34. Bach M (1996) The “Freiburg Visual Acuity Test” – Automatic measurement of visual acuity. Optometry and Vision Science 73:49–53