The Nikon D300 probably has the most advanced focusing system for DSLR. It is a 51-point autofocus system with 3-D tracking capability. It also uses Nikon's Scene Recoginiton System. Basically it uses artificial intelligence to select the subject in the scene to focus. When the subject moves, it will follow the subject and keep it in focus. In plain English, it will automatically keep the face in focus and can follow the running baby around never out of focus.
To use the system, first is to choose the AF-area mode. There are three: automatic-area AF, dynamic-area AF and single-point AF. The second is to choose the focus mode. There are three also: manual (M), single-servo AF (S) and continuous-servo AF (C). Thus 3 x 3, there are 9 combinations. In reality, we use only 2, with 2 more for special occasion.
1. Automatic-area AF; single-servo AF (S): The camera chooses by itself the subject to focus. Quite amazingly, the focusing points that covered the subject will light up in the view finder to let you know. We can also see the focusing points afterwards on the image using ViewNX on the computer.
2. Dynamic-area AF; continuous-servo AF (C): One single focusing point in the view finder shows up (it can be moved around in the view finder manually). Put the point on the subject, the camera will keep the subject in focus continuously.
3. Automatic-area AF; continuous-servo AF (C): Leave everything to the camera's artificial intelligent system to do the focusing, point-and -shoot. The focusing points will not show in the view finder.
4. Single-point AF; continuous-servo AF (C): One single focusing point for focusing continuously.
Method 1 and 2 are what we have been useing most often. Method 1 & 3 are the dummy modes, usually worked very well. Method 2 allows us to put the subject in the center where the focusing point is usually selected then recompose the scene. The focusing point will follow the subject and always keep it in focus.
I usually use method 1 or 2 only. However, with 50mm 1.4 lens, there were troubles. When the lens is wide open at 1.4, the depth-of-field is so narrow, the focus has to be exactly correct. If I use method 2, the image quite often not sharp. The reason is the dynamic-area AF actually shifting the focus around the selected focusing point and causing problems. The method 4 has to be used for precise autofocusing, in which continuous-servo AF is used instead of single-servo AF. Single-servo AF focuses instantly when we touch the shutter-release button. Any slight movement of the camera afterwards will put the subject out of focus. Thus continuous-servo AF has to be used, which continuously track any slight subject or camera movement. The above two images show how narrow the depth-of-field for the 50mm lens is at aperture 1.4. The focusing point was at the base of the chimney. From the close up image we can see anything just 1 mm away from the base is out of focus. 50mm 1.4 D is probably the sharpest lens that Nikon ever produced. I almost returned it to the dealer as unusable until I found out the correct way of focusing it from a knowledgeable Nikon sales representative.