> But 50/50 would be effectively the same meaning as 20/20.

This isn't obvious; consider that having 3,000/3,000 vision ("I can't read text all that well when it's half a mile away") says pretty much nothing about how clearly you can see things that are 20 feet away.

Nearsightedness and farsightedness are both characterized by "normal vision" at certain distances which becomes markedly worse at certain other distances.

20 and 50 feet are both far beyond "nearsighted" range and into "normal distance," without being all that different from one another.

3,000 feet is quite different (for one thing, atmospheric effects start becoming relevant).

I should have been more clear. 20/20 refers to vision and is used in the traditional form of the expression and is supposed to mean "very good". 50/50 refers to percentages, meaning 50% chance of being correct and 50% chance of being wrong, which would be very bad.

I took "50/50" to mean like a coin flip odds.