Help, any vision experts here.

[Abigfeckingspider]

Hi I am looking to see how to covert a snellen score to logmar.

the Snellen score is 1/500 , all the charts I see start with a 6 so 6/ etc. I take it this is a poor score. Any guidance would be great. I need the logmar score for a form. Thanks

The Snellen ratio is the distance you are viewing the chart from over the distance from which someone with normal vision can read the smallest line of optotypes (the letter-like symbols on the chart) you can read.

You sit 6 metres from the chart (or 3 m from a mirror to view a chart above your head, so a total light path distance of 6 m). You then read the optotypes/letters from the largest (usually labelled 20 or 24: the top row) down to the smallest you can read. The smallest of all will be labelled 5 or 4. If you can read the line labelled 6, you are able to read from 6 m the line that someone with normal vision can read from 6 m: you have 6/6 vision (what Americans call 20/20 because their distances are in feet). If you can read the line labelled 5 from 6 m, you are able to read from 6 m the line that someone with normal vision can read from 5 m: you have 6/5 vision (better than normal). If you can only read the line labelled 9, you have 6/9 vision (worse than normal).

The LogMAR score is the logarithm of the minimum angle of resolution: the smallest angle between which you can resolve details. The visual acuity defined as normal (Snellen ratio of 6/6) indicates an ability to resolve details separated by 1 minute of visual angle (i.e. 1/60 of a degree). A snellen ratio of 6/12 indicates an ability to resolve details separated by 2 minutes (i.e. 1/30 of a degree).

The logarithm of 1 is 0, so the LogMAR score for 6/6 vision is 0. The logarithm of 2 is 0.3, so someone with 6/12 vision has a LogMAR score of 0.3. You should be able to work out the rest, but if you tell us what your Snellen score is, someone will calculate your LogMAR.

Using an actual LogMAR chart gives more accurate measures, but if you only have a Snellen score you'll have to make do with this calculation.

Sorry, I missed the detail that you are working from a Snellen score of 1/500. I have no idea what that could possibly mean. If it's 10/500, that would be an Americal 10 ft/500 ft, which is 6/300 or a LogMAR of 1.70. If it genuinely someone who can only read from 1 ft what someone with normal vision can read from 500 ft, that would be a LogMAR of 2.70.

I presume the form is for someone with severe visual impairment.