Canon has had AF Fine Tune for a long while, Nikon is relatively new to it. I am unsure if all camera bodies have this function. Just go to your menu system, in the Nikon world AF Fine Tune is under the Tools, or Set Up, menu.
Simply put, a lens out of the box put on a camera fresh out of the box is not likely to produce optimally sharp images.
It is real likely that the lens will focus either in front of your intended subject (front focussing) or behind the subject (back focussing).
It can miss the focus by just a couple inches or it could be a foot or so off. So if you get a lens/camera combo that isn't "sharp", be sure to check the AF Fine Tune.
It's a good practice to check your focus accuracy for every lens you have. Nikon cameras create a database of AF Fine Tune numbers. The camera talks to the lens and the camera knows what lens is mounted on it. If you use a third party lens, you can create a custom name for that lens and the camera will remember it.
Follow this method
1) put the camera with lens on a tripod
2) tape up some newspapers to a wall 10 or so feet away. Be sure the camera is perpendicular to the newspaper.
3) use the camera's auto focus to focus on the newspaper.
4) put your camera on self timer so your hand won't jiggle the camera during exposure
5) shoot with the zoom wide and again at the longest telephoto range.
6) evaluate your results on a computer and zoom in so your viewing your image at 100% and even 200% if you're really critical, the LCD screen on your camera won't give you enough information to make a good judgement.
7) use the camera's AF Fine Tune menu to adjust the focus correction and shoot and evaluate again.
8) repeat until you get the best results.
Here's a couple notes.
First, the menu has two adjustments, one is listed "default value" and the other is "saved value". Default value is applied to ALL lenses. The saved value is specific to the lens. This gives you a lot of flexibility. I tend to use the default value primarily and do a final adjustment using the saved value. It is altogether possible to have a negative Default Value and a lens that needs a positive Saved Value, in a case like that the two values basically cancel out each other. Most of the time Nikons seem to need negative values.
Second, the f-stop to use during testing. There are two schools of thought, one is to test wide open and the other is to test stopped down two stops. It's your call. I test wide open. I want to know the maximum possible tuning. Stopping down will generally make all images sharper on all lenses. So I think stopping down is cheating your tuning a little bit.
Third, once you get the close range test done, do another test at longer range, maybe 30 or so feet. I've never seen a lens tuned at 10 feet that needed any further adjustment at longer range, but it never hurts to test it.
Fourth, during this process it is possible that you'll notice that one side of your image is softer than the other side. This is not all that uncommon. Test again stopped down two or three stops and you should see that the sharpness has evened out. This is a fixable error but you have to send it a repair facility or back to Nikon. There's no guarantee that this issue will ever be solved though. At least this test will tell you what your minimum f-stop needs to be to get edge to edge sharpness, and that's good info to have.
4/14/2015