eyesight

Eyesight is how well a person can see. Sharp, failing, or corrected by glasses.

It is a mass noun and it never takes a plural. The word belongs to opticians and to the driving test, where reading a number plate at a distance is the standard check. Good eyesight and failing eyesight are the common phrases. Vision means much the same thing but reaches further, since vision also means imagination and foresight, which eyesight never does. Sight on its own covers both.