why are only certain letters used for the eye chart?

Answer 1

Some letters are easier to see than others. Like 'I' or 'A'. They don't look like anything else. 'B' and 'R' are easy to confuse if you can't see them well. Or 'O' and 'Q', or 'N' and 'M' or even 'H'.

When I was a kid the eye chart we had at school didn't have letters, it only had 'E's, but they were square, I mean as wide as they were high, and they pointed in all four directions, and you pointed which way you thought they pointed. I'm guessing now that this was so kindergartners could be tested who didn't know the alphabet. But it had the advantage that every letter was equally easy to see!

At the DMV here in California they have a chart with diamond shapes, each divided into quarters, with one quarter a different pattern. That's pretty cool too!

Answer 2

These letters used are often the most easily confused one by the human eye. Some letter are so distinct by their shape that even someone with lower vision will be able to guess it. Thus the evaluation will not be correct.

Answer 3

The snellen eye chart is the most common used. The web site below explains it.

Answer 4

Because they can look like other letters to someone with a vision impairment. Like a lower case h and a lower case b are awful close, c's and o's, capital E's and F's, etc.

Answer 5

They are looking at letters that look alike so it isn't easy to tell them apart. Most people will mistake a letter for them same thing if they are near sighted or whatever.