Autofocus can sometimes be frustrating for the new digital SLR photographer, especially those coming from the point-and-shoot world. That’s because correct focus plays a greater role among your creative options with a DSLR, even when photographing the same subjects. Most non-DSLR digital cameras have sensors that are much tinier than the sensor in the D7000. Those smaller sensors require shorter focal lengths, which have, effectively, more depth-of-field. The bottom line is that with the average point-and-shoot camera, everything is in focus from about one foot to infinity and at virtually every f/stop. Unless you are shooting close-up photos a few inches from the camera, the depth-of-field, is prodigious, and autofocus is almost a non-factor.
The D7000, on the other hand, uses longer focal length lenses to achieve the same field of view with its larger sensor, so there is less depth-of-field. That’s a good thing, creatively, because you have the choice to use selective focus to isolate subjects. But it does make the correct use of autofocus more critical. To maintain the most creative control, you have to choose three attributes:
1. How much is in focus: Generally, by choosing the f/stop used, you’ll determine the range of sharpness/amount of depth-of-filed. The larger the DOF, the "easier" it is for the autofocus systems’ locked-in focus point to be appropriate (even though, strictly speaking, there is only one actual plane of sharp focus). With less depth-of-field, the accuracy of the focus point becomes more critical, because even a small error will result in an out-of-focus shot.
2. What subject is in focus: The portion of your subject that is zeroed in for autofocus is determined by the autofocus zone that is active, and which is chosen either by you or by the Nikon D7000. For example, when shooting portraits, it’s actually okay for part of the subject- or even part of the subject’s face to be slightly out of focus as long as the eyes (or even just the nearest eye) appear sharp.
3. When focus is applied: For static shots of objects that aren’t moving, when focus is applied doesn’t matter much. But when you are shooting sports or birds in flight, or children, the subject may move within the viewfinder as you are framing the image. Whether that movement is across the frame or headed right towards you, timing the instant when autofocus is applied can be important.
Reference: David Busch's Nikon D7000 Guide to Digital SLR Photography