Nikon D7000 manual Adding Circles of Confusion

Chapter 5 ■ Mastering Autofocus Options 139

Figure 5.7

Focus in Con- trast Detection mode evaluates the increase in contrast in the

edges of sub- jects, starting with a blurry image (top) and producing a sharp, con- trasty image (bottom).

be focusing on, by choosing a focus zone or by allowing the camera to choose a focus zone for you. I’ll address that topic shortly.

As the camera collects focus information from the sensors, it then evaluates it to deter- mine whether the desired sharp focus has been achieved. The calculations may include whether the subject is moving, and whether the camera needs to “predict” where the subject will be when the shutter release button is fully depressed and the picture is taken. The speed with which the camera is able to evaluate focus and then move the lens ele- ments into the proper position to achieve the sharpest focus determines how fast the autofocus mechanism is. Although your D7000 will almost always focus more quickly than a human, there are types of shooting situations where that’s not fast enough. For example, if you’re having problems shooting sports because the D7000’s autofocus sys- tem manically follows each moving subject, a better choice might be to switch Autofocus modes or shift into Manual and prefocus on a spot where you anticipate the action will be, such as a goal line or soccer net.

Adding Circles of Confusion

But there are other factors in play, as well. You know that increased depth-of-field brings more of your subject into focus. But more depth-of-field also makes autofocusing (or manual focusing) more difficult because the contrast is lower between objects at dif- ferent distances. So, autofocus with a 200mm lens (or zoom setting) may be easier than at a 28mm focal length (or zoom setting) because the longer lens has less apparent