Visual test?!

Visual test?

I am wondering what does that 3 rows of visual test means, i mean from the 1st row which is big characters to the 3rd rows which is relatively small. how did the nurse deduce 6/6, 6./7.5 to 6/120 etc?

Any additional helpful information on this particular test will be most helpful. Many thanks!

The chart has various letters made in standard sizes for standard test distances. Standard test distance is 6 metres. The smallest letter the average human eye without eye disease can see subtends about 1 degree in arc. The "6" size letter subtends 1 degree if viewed from 6 metres, the "7.5" subtends 1 degree if viewed from 7.5 metres, the "120" subtends 1 degree at 120 metres and so on.

Your visual acuity is recorded as the smallest size you are able to correctly read at the standard 6 metre test distance. So if you could read the "6" line you have 6/6 vision, if you could only read the "120" line you have 6/ 120. The nurse doesn't have to deduce anything, she records the letter size.