The first number in your visual acuity grade is always 20. That's because the 20 is a reference to the distance, in feet, you are standing or sitting from the eye chart. The distance is not a coincidence. Rays of light are just about parallel 20 feet (about 6m) from the eye chart, so that the muscle controlling the shape of the lens in a normal eye is in a state of relative rest when viewing the chart. Ideally, your eyes should be operating under optimal conditions during the eye test.
The second number represents the distance at which a normal eye should be able to see the letters on that line. The third from the bottom line on most eye charts is the 20-20 line. If you can see letters on that line, you have 20-20 ('normal') vision. A higher second number indicates your vision is subnormal. If you have 20-50 vision, you can discern letters that 'normal' observers could see from more than twice as far away, 50 feet (15m), if you achieve the highest score on the acuity test, a 20-10, you can spot letters that a normal person could detect only if he were 50 per cent closer.
We also got the answers to another imponderable we've always had about the vision test: are you allowed to miss one letter n a line and still get 'credit' for it? yes, all you need to do is identify a majority of the letters in a line to get full credit for reading it. it only our schoolteachers were such easy graders.
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