What are the benefits of switching to logarithmic scaling?
as a unit of measurement in vision tests?

  1. A logMAR Vision Chart (Logarithmic Minimum angle of resolution) is designed to enable a more accurate measurement of visual acuity than using other methods.
  2. If vision test results are to be consistent and comparable, health authorities recommend the use of Logarithmic structure of vision charts and reading of vision test results.
  3. Our senses – both the sense of sight and the sense of hearing  -  is best measured according to a logarithmic principle. When we think a sound has become one notch higher - it has in reality become 10 times louder - and if we turn it down just one more notch - yes, then it has become another 10 times louder.  In the same way with our eyesight.
  4. A Logarithmic sight chart thus has  rows of figures or letters (called Optotypes) in unique logarithmically measured steps,  so that the degree of difficulty down the chart occurs at the same rate of increase throughout.  Yes, it becomes exactly 10 times more difficult to see the row below. NOTE: This is precisely not the case with the old Snellen charts, where the jumps are often arbitrary, and furthermore  can be very different from make to make of sight charts. This makes real comparison impossible. It is therefore not sufficient to apply logarithmic values ​​to the existing tables, with arbitrary jumps. Charts must be designed from the ground up according to logarithmic principles!
  5. Many doctors and health care workers  asks why rows of letters or figures must be closer and closer in a correctly executed logarithmic sight chart?
    The answer is that they have to in order to meet the criteria of whether it must be relatively equally difficult for the human eye (and brain)  at each step to distinguish signs and figures from each other. Will there be e.g. relatively more and more space around letters in a row, it becomes relatively easier for the eye to detect, and this does not lead to an accurate vision test result. You call it The crowding effect, and it must be respected if the vision test result is to be accurate.
  6. On a European level and with the accession of 32 countries for the time being, including Denmark,  The USA, Australia and England have adopted an ISO standard: 8596-2018-2020 for the production of sight charts according to logarithmic principles.

  7. Vision charts from Denmark ISOeyes complies in all respects with this ISO Standard, which is expected to be a guideline for the Danish Health Authority's announced new guidelines, which are expected to be published in their final form in 2023.

 Conversion table from  old to new reading values

 Transition from the old tables to the new logarithmic tables

In the transition phase  between the "old" measuring units and the logarithmic scale, the following conversion table can be used. On the Vision Charts from ISOeyes , the old as well as the new logarithmic values ​​are listed,  to ease the transition. The doctor can thus continue to read values ​​as either fractions or decimals until it becomes a legal requirement,  and move on to reading LogMAR values ​​on a later occasion. 


Vision test measurements  regarding the acquisition of a driver's license

Where with decimal numbers 0.5 is required in value to be able to acquire a driver's license, the value on the LogMAR scale is 0,3

For a commercial driver's license, whether the value 0.8 is required cf. The decimal scale, the corresponding value on the logarithmic scale is 0,1.

Vision test measurements  of pre-school and school children

In health care, a value of 0.2 LogMAR or higher is referred to an ophthalmologist.

A more accurate registration!

With the logarithmic system  the value indication decreases  i  the vision test result with better vision,  and increases with impaired vision. So the reverse of the decimal measurement.  The better you see, the lower your score in the logarithmic system.

Each sign or figure which can be identified by the person taking the visual test  has a score value of -0,02 logmar units. Since there are 5 letters per line (  minimum in the bottom 5 lines of 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.

Can the test person, for example, see the entire line corresponding to  The LogMAR value 0.3 (corresponding to the “old” 6/12 or decimal 0.5)  , but can also identify two more of the characters or figures on the line below – then the sight test result is deducted   extra -0,02  point  per identified  sign or figure,  so the final vision test result will be 0,3  more  2x -0,02 = an eye test result of  0,26 LogMAR.

DEFINITION: LogMAR VA = LogMAR value of the best line read – 0,02 X (number of extra optotypes identified)