Oh dear, doesn't take much of a leap to realise that 6/6 refers to meters, the European metric measure of normal eyesight, and 20/20 to feet and the American imperial measure. So 5/6 is the same as 16/20 or 17/20.
In either case the second number refers to the distance that a normal person with perfect visual acuity can read the specific line on the snellen chart (eye test chart). The first number refers to the distance at which the test subject needs to be to read the same line.
So 5/6 is less than perfect eyesight. However, that description is one of the old measurment of visual acuity (old as in about 10 years) and the newer one works slightly differently, i.e. the indictions are reversed.
So 6/6 still refers to perfect vision, 6/9 to vision whereby a subject needs to be at 6 meters to read script that a normal person could read at 9. And so onward. 6/12, 6/18 etc.
Confused yet? Ha ha!
The RAF use a further coded number system to refer to sight requirements for various jobs within the RAF, whereby the first number refers to unaided vision and the second to corrected vision (i.e. glasses). eg. 5/2 is unaided vision of 6/24 and corrected vision of 6/9.
Pilot is 1/1, so no messing about there you must have uncorrected perfect vision.
Navigator (now Weapon Systems Officer) is 5/1 so unaided 6/24 and with glasses 6/6. etc. etc.
Got a list with all the requirements for various branches on but not going to repeat it all ad nauseam. There are further requirements concerning colour perception, refraction range, stereopsis and convergeance, but the the whole subject is very very dull and quite frankly i don't understand any more than i've just written.
Hope that clears up any further or past questions about eysight.
Also, a history of refractive surgery is a bar to all aircrew and specialist branches, no matter if you now have 6/6 vision. (Is permitted in non-specialist ground branches).
Cheers