It sounds as though your camera is set to a deep depth of focus. This can occur from using a fully automatic setting, a shutter priority setting, or a manual setting with a high aperture number (f:22 for example). In order to narrow your depth of focus, and be able to tell just where the center of focus is located, be sure to use a fully manual setting or an aperture priority setting, and the smallest aperture value (often f:4 or f:2.8) available. This will open the aperture on your lens, and reduce the depth of field. Once you have calibrated the camera, feel free to use the lens at any aperture setting, or at any automatic setting, the correction you have made will also apply at deeper depths of field.