Disclaimer: I am not a doctor, this is not medical advice, this is not a replacement for an eye exam and it will not tell you if you have any eye diseases. This is also not as accurate as the measurements you would get in an optometrist’s office even though it does follow the same principles. Consider this guide as educational or entertainment only.
You’re going to need a few tools to do the job. Fortunately they are either free or cheaply purchased.
A Snellen Chart
A Rosenbaum Pocket Vision Screener
An Astigmatism Dial
A ruler or tape measure with centimeters
A journal or spreadsheet to keep track of your measurements over time
Just google the first three and either print them out or buy them from wherever you like. I’d suggest getting a Snellen chart that includes both meters and feet for the sake of simplicity. I would also print/buy two of them. One for taking measurements outdoors in full sun and one for taking measurements indoors in both natural and artificial light. The other two charts can be used in your hand so one is enough.
Measurements for Myopia
For the Snellen chart just follow the instructions that came with it. Usually you’re told to attach it to a wall at eye height and stand or sit 3 or 6m (depending on the size of the letters) away from it. Then just read down the lines from the top until you can’t read more than half of the letters on a line. That’s your distance visual acuity. For a little more accuracy you should do this outdoors in full sun, indoors in natural light during the day, and indoors in artificial light at night and average the three. You should do the same with the other charts as well.
For the Rosenbaum simply hold the chart in your hand with one end of the ruler or tape against it and the other end against your temple. Then move the chart away from your face until it just reaches the edge of perfect clarity*.* Remember you’re measuring only how far you can still see the chart clearly and no further. Now take note of the distance in centimeters.
If you would like to convert this so you know how many diopters of correction your glasses need to be to see to infinity clearly. Then all you have to do is divide the measurement you just took into 100.
Example: If you measured 30cm from the chart then 100/30 = 3.33 diopters. In the case of myopia the correction is always negative (-) because the image has to be projected closer to you to see it clearly. For hyperopia it’s the opposite. Repeat the measurements for each eye separately and together and make note of them in your journal.
Measurements for Astigmatism
With the Astigmatism Dial and ruler/tape in hand you’re going to do the same thing you just did with the Rosenbaum, there’s just a couple extra steps.
First you need to do this one eye at a time. So close or cover one eye and move the dial away until the first line on the chart starts to become blurry. That’s the Axis of your astigmatism, make note of it. Also make a note of the distance you just measured. Now continue to push the dial away until the last line becomes blurry and make note of that measurement.
Now you need to convert both of those measurements into diopters the same way you did above and then deduct one from the other.
Example: First line becomes blurry at 21.5cm. 100/21.5 = 4.65. Last line becomes blurry at 26cm. 100/26 = 3.84. Now take the last off the first. 4.65 – 3.84 = 0.81. So 0.81 diopters at the Axis you took note of earlier is how much cylinder you need to correct for your astigmatism.
Hope you found this as interesting as I did when I learned how to do it. For me it really helped to demystify how my optometrist arrived at my prescription and I found that it is perfectly accurate to less than 0.25 diopters (the lowest correction put into glasses) for both the myopia and astigmatism correction given to me by my optometrist.