I volunteered to do a modeling shoot, and I got to learn a bit about how what apertures to use on a 50mm f/1.4 lens. In this case, I was using a Canon 50mm USM f/1.4 lens.
I don’t think you ever want to use it wide open since it gets really soft. At f/2.0 it is still soft but it has a nice blur. If you take a head shot at f/2.0, you get some really sharp eyes (if you focus there) and all the wrinkles blur away.
Do not use f/2.0 for a bust or full body shot. Bad idea. Everything is blurry.
When you get to f/5.0, that’s a great f-stop for a couple. You still aren’t tack sharp yet.
At f/5.6, you get tack sharp photos.
Beyond f/5.6 sharpness begins to fade, but just slightly.