Visual Assessment
Acuity measurement across ages and abilities, low-vision assessment, and potential acuity testing.
Notation, logMAR, and the Arithmetic of Acuity
At the technician level you are expected to move between notations without hesitating and to correct a measurement taken at a nonstandard distance. Everything follows from one idea: acuity is an angle, not a size.
The Snellen fraction is test distance over the distance at which a standard eye resolves that optotype. MAR (minimum angle of resolution, in minutes of arc) is simply the fraction inverted: 20/40 means a MAR of 2. logMAR is the base-10 logarithm of that: log10(2) = 0.30. Because it is a logarithm, every doubling of the visual angle adds exactly 0.30, and 20/20 sits at 0.00. Better-than-standard acuity goes negative — 20/16 is about −0.10. Decimal acuity, common on European reports, is the Snellen fraction carried out: 20/40 is 0.50.
In the full COT guide
- Notation, logMAR, and the Arithmetic of Acuity
- The Lane as an Instrument: Calibration, Contrast, and Artifact
- Beyond the High-Contrast Letter: Contrast, Glare, and Potential Acuity
- Nonstandard Patients: Pediatric, Nystagmus, Low Vision, and Inconsistent Results
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