The LogMar chart is also commonly used for measuring visual acuity. It has a number of advantages over the Snellen chart that make it a more reliable and accurate test. The results are written as a decimal number and not a fraction, eg: 0.78 which is equal to a Snellen of 6/36 or 20/120. The patient stands 6 metres from the chart and reads from top to bottom. The more letters and whole lines that can be read the lower the LogMar score is. For instance a LogMar of 0.0 (zero) is equal to a Snellen of 6/6 (or 20/20) whereas a LogMar of 1.0 is equal to 6/60 (20/200). Click here for a conversion table.
The name, LogMar can be split into two, the ‘MAR’ part refers to ‘Minimum Angle of Resolution’. Put simply, this refers to the smallest sized image that can be seen and recognised (or the smallest letter in this case). The ‘Log’ part of the name refers to ’Logarithm’ which is the mathematics used to calculate the score and convert it into a decimal number. The maths can seem over complicated but once a score has been calculated it can be used to make more accurate comparisons.
The terms ‘best corrected vision’ and ‘best corrected visual acuity’ refer to the results from tests of visual acuity taken when a patient is wearing lenses that correct for refractive errors and astigmatism.
How is visual acuity measured.
Patients and parents may find that ophthalmologists and opticians quote different results from eye tests. This is because visual acuity can be measured with a number of different test charts. Each chart has its own scoring system. It is possible to convert and compare results between the different approaches. Click here for a conversion table.
The most common approach is known as the ‘Snellen chart’. The results are expressed as a fraction.
For example, good visual acuity is 6/6 (or 20/20) when a person can read the eighth line of a Snellen chart. In the case of HPS:
The term ‘Visual Acuity’ refers to how much detail can be seen at a set distance (usually 6 metres). The better a persons’ visual acuity is the more fine detail they can see at the set distance. Acuity is typically low for people affected by HPS and albinism.
The amount of detail that can be seen is directly related to the number of cells found in the retinas (photo-
Visual acuity is low in normal newborn babies and typically improves dramatically during the first 18 months in line with expected development. It typically continues to improve up to the age of around 4 years and remains stable throughout life -