What would be visual acuity of a person if he can can read all letters of 20/25 and most of 20/20 on Snellen chart?

Question

Suppose this is the Snellen chart:

And you measure your normal visual acuity using correct rules (i.e., placing it at 6 meters distance, in observation of an examiner in hospital.

And suppose you can read 6 out of 8 of last row and 8 out 8 letters of 2nd last row from 6 meters.

Further, again you use a different type of Snellen chart (for example sans-serif letters) for further clarifying. And again you get similar results.

Does this mean (theoretically) you have a visual acuity of 20/20

OR would it be called 20/25 instead, since you couldn't read 2 letters in last row?

Further, do doctors recommend correctness of such eyesight with glasses/lens?

Answer 1

The 20:S notation means that a person with normal vision standing S feet away can see that line on the Snellen chart clearly, but fails to see smaller lines.

Size of Letters Required for Visibility as a Function of Viewing Distance and Observer Visual Acuity

3.1 MEANING OF THE SNELLEN NOTATION (page 12)

At 20 feet (6 metres), this person with normal vision can see all the chart.

Therefore the person being tested moves forwards until they get a perfect score, say to x feet.

Now their vision is x:20, which, when standardized, becomes 20:(400/x) vision.

Answer 2

If the patient cannot read only minority of the letters in a line. Then the visual acuity(VA) is recorded as: 20/x-no. of letters not read where x is the line which the patient is reading.

And when the patient cannot read majority of the letters in a line. Then VA is recorded as: 20/x+no. of letters read where x is the previous line which the patient has read completely.

In your example, since the patient cannot read only minority of the letters, the VA should be: 20/20-2

In regard to your second question, I don't think anybody would prescribe glasses for such a small defect.

Source: https://www.urmc.rochester.edu/MediaLibraries/URMCMedia/eye-institute/documents/Checking-Vision.pdf

Note: I don't know if this is the universal way of recording. It may change according to your region or the clinicians may not give that much importance to it.