Neill Farmer , Dec 28, 2008; 03:28 a.m.
The simple answer would be to push the AF/MF switch to Manual (MF) and turn the ring on the lens until the image in the viewfinder is sharp, as your camera manual will cover.
But with the tiny viewfinders on crop DSLRs there is perhaps some technique to learn in order to get the best results. Choose a point on the view finder that you wish to focus on, then turning the ring wander it back and forth slightly until you get it in the middle of that sharp zone. Remember that the zone of focus will exist from in front of your selected point to behind it in roughly 1/3 to 2/3. The length of this "in focus" zone will depend on your lens settings, that is its focal length 24mm, 50mm, 200mm and its aperture settings, f2.8 a short distance, f8 a longer distance. Play around, experiment and read your manual and the odd photo guide or magazine.