I joined the RAF at age 18 and passed up the opportunity to go to uni because I was worried about my eyesight. I had passed the medical at 15 (sixth form scholarship) but by the age of 17 needed specs (I am not telling you what sent me blind!).
At 17 I was still OK to get in but not by much. So I dumped uni and went straight to Cranwell because I did not want to take the chance of further deterioration over the next 3 years (drinking myself blind).
2 years after joining the RAF (whilst still under training) I was marginal on the eyesight but with some pretty weak corrective specs was back up to 20/20.
Over the next 10 years of service I had the expected age related deterioration but never got so bad that more than just a mild degree of correction was needed. I am now using -1.75 dioptre correction in both eyes - but have no idea how that relates to 6/6 or 20/20 or whatever scale is.
Bottom line for the service is that if your eyesight is less than perfect it will not get better with time. So, if they take someone who needs specs into the system they will need to provide specs for the rest of his acreer and there may come a point where his eyesight will reach a point that he will not be able to continue flying - even with correction. If you take soemone with perfect eyesight then there is no certainty that he will ever need specs - he may still have perfect eyesight at age 55.
It's a numbers game - and there has to be a limit for every physical characteristic: hearing, height, weight, blood pressure, cholesterol levels etc., and some people will be outside the limits.
By the way if the RAF has dropped it's eyesight limits to CAA limits is that old or new CAA limits? I ask because the CAA has had to relax it's limits to come in line with the JAA - so does the RAF now use JAA limits?
My optician said that wearing glasses I have better than perfect vision. So in wearing them I actually have better vision than most of the RAF pilots anyway![/
My eyesight was better with corrected specs than when I joined the RAFD - it was no longer marginal. Don't miss the point here - lots of RAF aircrew wear specs - it's just that very few did when they joined (this is one of the reasons for the annual medical, after all).