What can reading letters in an eye test tell?

[Belville]

I just wonder how the doctor can tell whether you are nearsighted or farsighted and whether you need glasses or not just by reading the letters.

[Belvue]

If you are short-sighted, you will not be able to read small letters in the distance. If you are farsighted, you can read all the letters in the distance and can't read the letters at a very close distance.

[Belwood]

The doctor can determine if you are short sighted by asking you to read the certain row on the letter chart. Most nearsighted people can't read the small letters at the bottom of the chart but farsighted people can. Also the optometrist will also ask the patient to read something on a piece of paper. If you need the paper to be further away to be able to read it, then you are farsighted.