20:20 vision
Apologies for the thread drift, but, the accepted definition of 20:20 vision is as previously stated, however, both an optician and an AME have defined it to me as being, "20:20 means you can see at 20 feet what someone with perfect eyesight has, if your eyesight is worse the first number gets smaller, as in 20:40 means you need to be at 20 feet, and someone with perfect normal eyesight can see it at 40 feet".
As for the original question, it depends on so many things. How clear are the letters, (I've see some italic letters), it depends if the letters are actually standard size (some aircraft so appear to have smaller under wing lettering), whether the aircraft was straight and level, was it a normal SEP, could the person on the ground have had binoculars (depending what prompted the original question, whether a ground based person old have had cause to grab their binoculars!)