Another major area of difference between a compact digicam and a digital SLR is in the ability to select focus modes. Whereas a compact camera focuses on a subject and then captures the image, a dSLR can easily shoot in Continuous Focus mode, tracking a moving subject and keeping it in focus until, and even after, you press the shutter button.
Continuous Focus, sometimes called Servo, or AI, or Predictive Auto Focus, is designed for sports and wildlife photography where the subject is moving and the photographer is panning the camera along with the subject prior to capturing the image. This mode works on most cameras by pressing the shutter button halfway to activate the metering and focus tracking. (Some cameras have alternative ways to activate focus.) Depending on your camera, one or more focus points (Figure 2.25) will lock onto the subject, and as it moves the lens will change focus to keep the subject sharp.
Figure 2.25. Most digital SLRs have multiple focus points to select from.
When you're ready to capture the image, press the shutter the rest of the way down. You need to be careful when using Continuous Focus mode. It's entirely possible and all too easy to end up with photos that are out of focus and blurry (trust me, I have hundreds of examples). Unless your subject is moving, I suggest you use Single Shot Focus mode (see the next section), which requires your camera to be in focus before taking the photo.
Single Shot Focus
As its name suggests, Single Shot Focus mode locks onto a focus point as the shutter button is pressed and then stays there. Unlike Continuous Focus Mode, which locks into, follows, and maintains focus on a moving subject, Single Shot Focus mode doesn't release the shutter until the camera has focused on the subject. Once you've locked focus on the subject, you can then press the shutter button halfway and recompose the scene, repositioning the subject in another part of the frame. As long as the shutter remains pressed halfway, and the subject hasn't moved, it will remain in focus.