Originally posted by tabl10s If this is the case, what happens when you use a 1.2 at it's max aperture and the hexagon indicates you're in focus when you actually aren't?
First of all, you should not completely trust the focus indicator. Why? Because...
1) ...at f1.2 your DoF (depth of field) can be so shallow that as you focus not everything you want to be in focus will be.
2) ...when you shoot wide open like that you will probably not get everything tack sharp anyways. All lenses get a tad soft wide open, so it is a good idea just to never (unless you have a pressing need to) shoot wide open.
With that said, it is a good idea to open the lens wide open and then purposely stop it down one stop of so to do your actual focusing. That aids in the accuracy of your focusing, because the image will naturally come in sharper as you view it.
3) As you manually focus, especially with a lens wide open, you can actually overshoot the mark as it were before the camera has a chance to give you the in focus indication. In other words, you can miss the focus before the camera can tell you to stop because you are moving faster than the camera can react to.