I plucked this off an old dejanews archive(from sci.med.vision in the
mid 1990s, which feels remarkably like the s.m.v. of today!):

     log10 (VA) = -1.3534 (log10(-D)) - 0.5068,
where VA = decimal form of Snellen VA, D = myopia in dioptres

for all you refractionist wannabes who think only myopia exists