abacucat,
But if someone falls below that eyesight limit while holding a license THEY HAVE TO ACTIVELY REPORT IT TO DVLA. If they don't, and carry on driving, no-one will stop them unless / until they cause a serious or fatal accident, because there aren't e.g. 'renewal' tests: once you have a driving license you have it.
FIL has been blind in 1 eye since birth. He is due to have a cataract in the other removed in a couple of weeks. However, he continues to drive because 'last time I went to the opticians [to renew glasses, not to have the cataract examined, he doesn't count that] about a year ago, they didn't tell me I couldn't drive'.
He won't report himself to the DVLA.
SIL refuses to encourage him to do so, because 'he drives down here every weekend, he would be lonely if he didn't, and it takes too long to fetch him'.
DH can't influence him. At the moment, I am hanging fire on reporting him because he will have the op very shortly, though i have made dH - the only one of us who ever visits him, long story - never to allow FIL to drive him anywhere. If his vision doesn't improve very significantly, then I will report him, which will cause WW3 because everyone will know it is me.
It really isn't the case, sadly, that 'they are driving so their vision must be OK, because if their vision was poor they wouldn't be allowed to drive'