What is AF fine tune and why should you use it?
AF fine tune is an option in the menu of your camera (if it’s supported by your camera, it was introduced with the D3 and D300, but it’s not supported by the D3000 and D5000 series). What it does is that it compensate that your camera and lens combination may be off by a little bit making the auto focus to miss to lesser or greater degree. Every camera body and lens is produced within a certain tolerances and if the body and lens are off to the same side, it will make the focus to miss. You might also be so lucky that they are off to opposite sides and by this cancel out the differences, but that’s not any guaranty. If you happen to own several lenses, I’m convinced that most of them need some minor fine tune and maybe a couple need considerable fine tune. Adding the correct fine tune will then compensate for any misalignment between the body and lens, making the auto focus focus correctly.