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).
Definitions and conversion
All acuity measures relate to the visual angle of the smallest perceived (or discriminable) structure. Whichever way we define “smallest perceived structure”, let’s call it “MAR” for “minimum angle of resolution”, and let its dimension be minutes of arc. Then:
Snellen Ratio (ft)
Snellen Ratio (m)
Letters, Letter score
don’t use (see below)
1 / MAR
20 / (20 · MAR)
6 / (6 · MAR)
VASnellen ≡ VAdec
VASnellen ≡ VAdec
1 line = 0.1 LogMAR
1 letter = 0.02 LogMAR
The “20” resp. “6” in the 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?
Snellen ratio and decimal acuity 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”, unless you 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.
Lines? →This is not an ISO Standard term. Acuity charts are typically arranged in lines, with equal optotype grade along a line. Following ISO the lines should have equidistant progressive acuity grades with 0.1 LogMAR increments. Then 1 Line is equivalent to 0.1 LogMAR.
“Letters”? →This is not an ISO Standard term. 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 you use (with a sensible progression and 5 optotypes per line), and calculate “0.02 LogMAR” per letter.
“ETDRS”? →ETDRS is not a VA unit; ETDRS charts use LogMAR.
For studies, visual acuity data frequently needs 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.
Normal acuity in age range 20–65 is ≈–0.2 LogMAE = 1.6 decimal: Frisén L, Frisén M (1981) How good is normal visual acuity? Albrecht von Graefes Arch Klin Ophthalmol 215:149–157
My papers on behavioural acuity
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]