Even the most sophisticated autofocus systems need you to take control in order to focus on off-centre subjects
Positioning your subject off-centre is one of the best ways to improve the composition of your shots, but you need to make sure that your Nikon focuses on this area rather than what’s in the middle of the frame, otherwise you could end up with your subject out of focus and your background pinsharp (a problem common to compact camera users). With static subjects, the easiest way to do this is to use a technique known as focus lock. composition by positioning the subject in different areas of the frame. Once you have locked the focus, you can position the subject anywhere in the frame, and as long as you remain the same distance away, it will be sharp.
This is useful on cameras which have a limited number of AF points, as they only allow you to focus on a limited number of areas within your scene. Using focus lock, you can place the subject in areas of the frame where there aren’t any suitable AF points. (AF-S). Then, frame the scene so your subject is in the centre and half-press the shutter release until the camera focuses. You can then recompose your image so that the subject is anywhere in the frame, but you must keep your finger half-pressed on the shutter release, and make sure that you don’t change the distance between you and the subject. Then, once you’re happy with the composition, you can press the shutter fully to take your shot. Like many automatic functions, the automatic AF point selection can work perfectly well, but it doesn’t always pick the AF point that corresponds with the thing that you want to focus on. You’ll get more reliable and consistent results by choosing the AF point manually (see Taking it further), or simply leaving it set to the central AF point and using the focus lock technique described above.