You have the best focus performance at the middle of the lens' range. Wide open your focal depth is minimal.
You want like f8 for your sweet spot usually.
Edit:
Also if you really want to force the camera to stay on your chosen aperture, use aperture priority mode, set a minimum shutter speed. And leave on auto ISO.
I still would recommend stopping the lens down from wide open as that is definitely making your autofocus system work extra hard and giving you more out of focus shots.