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The problem is most likely that you are using the lens for a short distance shot and with a large aperture setting. This will give you a depth of field of only a few inches or even as little of a centimeter. Everything else will be out of focus. With this lens, shooting a photo of a persons face for example, you can easily have the front of the nose in focus, with the rest of the face out of focus.

It's tricky. Start with the center focus point selected and practice in a well light enviroment.