Saturday 3 December 2011

Measure for Measure – what do the eye measurements really mean?

We all need to understand what a child’s eyesight measurement means. I am frequently asked what the fractions or figures describing a child’s eyesight actually mean. For example I can say little Enid is 6/24 or Frank is 6/60 or Mabel is maybe 6/250. But what does all this really mean?

It is further complicated there is a new system of measuring eyesight called LogMAR. The standard eye test was invented by Mr Snellen and consists of increasingly smaller letters on a chart. The biggest at the top has one letter (this is 6/60 or 20/200), while at the bottom are eight small letters (6/6 – 20/20). However with the LogMar test your child can have a visual acuity measurement of zero and that might sound scary but it means they have perfect vision or an equivalent of 6/6 in Snellen metric measurement (metres) or 20/20 in feet (USA).

The conversion chart shows that 6/18 is LogMAR 0.48 and Snellen 6/60 is LogMAR 1. But zero vision does not sound quite the same as 20/20 or 6/6 vision and LogMAR 1 meaning severely visually impaired really needs to be unpacked. I am sure that LogMAR is more precise or accurate but I think it is confusing to the layman. And that is perhaps why many teaching professionals who are vision specialists are slow to convert over to LogMAR. At least that is my experience. It is still easier to explain that 6/60 (20/200ft) means that the person sees at six metres (20ft) what a person with ‘standard vision’ can see at 60 metres (200ft) even if technically it all refers to an angle. Or I might explain it by saying the person can just about see the top line of the eye chart, which is even easier to follow.  Registered blind vision is 3/60 which is in LogMAR a value of 1.3.

LogMAR /  

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