What Are the Advantages of Switching to Using the Logarithmic Scale as a Measurement Unit in Vision Tests?

  1. A LogMAR Eye Chart (Logarithmic Minimum Angle of Resolution) is designed to enable a more accurate measurement of visual acuity than other methods.
  2. If vision test results are to be consistent and comparable, health authorities recommend the use of a logarithmic structure for vision charts and reading vision test results.
  3. Our senses—both vision and hearing—are best measured according to a logarithmic principle. For example, when we think a sound has become slightly louder, it has actually increased tenfold, and if we turn it up a bit more, it becomes another ten times louder. The same applies to our visual ability.
  4. A logarithmic eye chart has rows of figures or letters (called Optotypes) in consistent logarithmic steps, so that the difficulty level decreases down the chart at the same rate. It becomes exactly ten times harder to see the row below. Note: This is not the case with the old Snellen charts, where the steps are often arbitrary and can vary significantly between different manufacturers of eye charts. This makes real comparison impossible. Simply applying logarithmic values to existing charts with arbitrary steps is not sufficient. Charts must be designed from the ground up according to logarithmic principles!
  5. Many doctors and health nurses ask why rows of letters or figures must be placed closer and closer together on a properly constructed logarithmic eye chart?
    The answer is that they must be placed closer to meet the criteria that it should be proportionally equally difficult for the human eye (and brain) to distinguish signs and figures at each step. For example, if there is relatively more and more space around letters in a row, it becomes proportionally easier for the eye to detect them, leading to inaccurate vision test results. This is called the Crowding Effect, and it must be respected for vision test results to be accurate.
  6. At the European level, with the participation of 32 countries, including Denmark, the USA, Australia, and England, an ISO standard (8596-2018-2020) for the construction of eye charts according to logarithmic principles has been adopted.

  7. Eye charts from Danish ISOeyes fully comply with this ISO Standard, which is expected to serve as a guideline for the Danish Health Authority’s upcoming new guidelines, anticipated to be published in their final form in 2023.

 Conversion Table from Old to New Reading Values

LogMAR Charts Explained – DOWNLOAD PDF

 Transition from the Old Charts to the New Logarithmic Charts

In the transition phase between the “old” measurement units and the logarithmic scale, the following conversion table can be used. On eye charts from ISOeyes, both the old and the new Logarithmic values are indicated to facilitate the transition. The doctor can thus continue to read values as either fraction or decimal until it becomes mandatory to switch to reading LogMAR values at a later time. 

Examples

Vision Test Measurements for Obtaining a Driver’s License

Where a decimal value of 0.5 is required to obtain a driver’s license, the value on the LogMAR scale is 0.3.

For a commercial driver’s license, where a value of 0.8 is required according to the decimal scale, the corresponding value on the logarithmic scale is 0.1.

Vision Test Measurements for Preschool and School Children

In health care, children are referred to an ophthalmologist if they have a value of 0.2 LogMAR or higher.

A More Accurate Measurement!

With the logarithmic system the value in the vision test result decreases with better vision and increases with worsening vision. In other words, it is the opposite of decimal measurement. The better you see, the lower your score in the logarithmic system.

Each sign or figure that can be identified by the test subject has a score value of -0.02 LogMAR units. Since there are 5 letters per line ( at least in the bottom 5 lines on the vision chart), the total score for a line on the LogMAR chart represents a change of 5 x -0.02 = -0.1 LogMAR units.

For example, if the test subject can see the entire line corresponding to the LogMAR value of 0.3 (corresponding to the “old” 6/12 or decimal 0.5) , but can also identify two additional signs or figures on the line below, the vision test result is reduced by an additional -0.02 points per identified sign or figure, so the final vision test result is 0.3 plus 2x -0.02 = a vision test result of 0.26 LogMAR.

DEFINITION: LogMAR VA = LogMAR value of the best line read – 0.02 X (number of additional optotypes identified)