I've always focused on the front sight, but when I was a helluva lot younger, the rear sight and the target were pretty much in focus as well.
Fast forward several decades and I still focus on the front sight.
Being near sighted, I typically look over the top of my glasses to get good focus on the front. The rear looks pretty decent for aligning, and of course, the target is just fuzz.
However, I find that if I aim at the center of the fuzz, I actually do quite well. It's kinda like aiming at "center of mass" in a real scenario with thing happening fast.