I mean with just one eye, being able to consciously refocus that eye.
For example, looking at this writing on your screen able to make that go in and out of focus.
I can bring my finger into focus, drawing my focal point in front of the screen, and if then remove my finger I can hold that focus (with about a 50% success rate). Perhaps that is cheating a little ;).
I can do this. It took me a very, very long time to figure out how to explain it to people who have "normal" vision. For what it's worth, I also have diplopia and I'm convinced that they are related.
If so, this isn't really what I mean.
I mean with just one eye, being able to consciously refocus that eye. For example, looking at this writing on your screen able to make that go in and out of focus.