Is the visual acuity expression 20/20......?

Question

applicable only for both eyes? Or can you say something like 'my left eye is 20/20 and my right eye is 10/20'?

Answer 1

Each eye can be rated separately, but a binocular value can be given as well.
The expression actually includes a reference to the distance at which the test was done, so the first number will normally be 20 (ft) in the USA and 6 (metres) elsewhere, giving 20/20 or 6/6 for nominally normal vision. Consider the expression as a fraction: 20 over 20, =1.

If the vision is worse, the result will not usually be recorded as your example10/20, but 20/40
"The ability see at 20 ft a target that a normal eye could see at 40 ft"

There are historical reasons why the 20/20 became "normal good vision" even though it is a little pessimistic. Many people have, or can have with optical correction vison signnficanlty better then 20/20, 6/6. The fraction then appears bigger than 1. 20/15, 6/5...

Occasionally tests are not done at the standard distances.
Home visits might be done at 10 ft or 3 metres, and the acuity could be recorded as 3/6, 10/30.....

Many people with binocular vision see better when using two eyes than with each eye individually.
someone with 20/20 in each eye individually might achieve 20/18 using the two eyes together.
(This can be especially marked with the users of soft contact lenses)
Formal reports often record up to six visual acuities:
each eye, with and without correction, and binocularly, with and without correction.

Answer 2

Both is right, when you are tested most time you will be tested with both eyes together and then individually. Typically when someone says I see 20/20 that means they see that with both eyes. Even though one eye may be 20/25 and the other is 20/30.

Answer 3

It's really quite simple once you learn what 20/20 stands for. It means that everything you see at 20 feet, looks like it's 20 feet away. Where if you had say 20/60 vision, that would mean that everything you see at 20 feet looks like it's actually 60 feet. Further away and less visible. Hope that helps.