[vandergus]

Alright, I'll just throw this out there as something you should check just to rule it out. The autofocus could be miscalibrated on your camera. You may not have noticed it with your other lenses because they are not as demanding of proper focusing as the 50mm is. How would this effect manual focusing? Well, the green dot focus indicator uses the autofocus sensors to tell whether or not you are in focus, so if your autofocus is off a bit, the green dot indicator will be telling you you the subject is in focus when it really isn't.

So break out a tripod and that book with all the text in it and set your lens to the widest aperture, f/1.8. Set up the camera and the book so the camera is looking at roughly a 45 angle. In the end you want some of the text to be in focus and the rest to be out of focus. Then pick one of the lines in the book and focus on it using the green dot as confirmation. Take a picture and zoom in to the focused area. Is the line you focused on sharp or is the line behind it sharp? If it's sharp, then great. You don't have to mess around with the inside of your camera or pay someone else to do it. Proceed with DOF learning and manual focus practicing. If not, well, we can talk about that later.