on the cheap ones i think it is f8. never tasted as well though.
but on the corrected lenses like meogon and i think the nikkors as well it is 5.6 (we are talking about the 50mm here)
on the german top lines u limit to the optimal only when u exide seriously the with the enlargement - something like x15+. but generally those germans will performe close to their best even at f4 or f8.
it also depends on the light sourse. if u use the cold light or deffused all the deferances are more pronounced than with the condensor.