Start by checking autofocus rather than assuming the camera body is faulty. Make sure the lens AF/MF switch is set to AF. Then half-press the shutter and watch the autofocus points in the viewfinder: one or more should light up red to show where the camera is focusing. If the camera is always using the wrong focus point, your subject may be out of focus even though exposure settings are unchanged. On the Rebel XTi, you can select the AF point with the AF-point selection button on the back and use the dial to change it; when all points are active, the camera can choose automatically. This may depend on being in creative modes such as M, Av, or Tv rather than a fully automatic mode. Since changing lenses didn’t help, the most likely issue from the answers is autofocus mode or AF-point selection, not the lens itself.