Da erf is round,... so round mirrors show ebryting...
...
Square, round, trapezoidal,... not really the issue..
What matters is the amount of convex (wide angle) to the glass...
Too much, and everything is tiny specks... not enough, and you see nostrils...
Don't buy mirrors that show (actual size)... bike mirrors are too small to be useful that way. ...but, really wide angle mirrors are not useful either...
Best to hold them at arms length, and see what you can see...