Choosing the right autofocus mode and the way in which focus points are selected is your key to success. Using the wrong mode for a particular type of photography can lead to a series of pictures that are all sharply focused—on the wrong subject. When I first started shooting sports with an autofocus SLR (back in the film camera days), I covered one game alternating between shots of base runners and outfielders with pictures of a promising young pitcher, all from a position next to the third base dugout. The base runner and outfielder photos were great, because their backgrounds didn't distract the autofocus mechanism. But all my photos of the pitcher had the focus tightly zeroed in on the fans in the stands behind him. Because I was shooting film instead of a digital camera, I didn't know about my gaffe until the film was developed. A simple change, such as locking in focus or focus zone manually, or even manually focusing, would have done the trick.
To save battery power, your 40D doesn't start to focus the lens until you partially depress the shutter release. But, autofocus isn't some mindless beast out there snapping your pictures in and out of focus with no feedback from you after you press that button. There are several settings you can modify that return at least a modicum of control to you. Your first decision should be whether you set the 40D to One Shot, AI Servo AF, or AI Focus AF. With the camera set for one of the Creative Zone modes, press the AF-DRIVE button and spin the Main Dial until the choice you want is displayed on the LCD status panel (see Figure 5.14). (The AF/M switch on the lens must be set to AF before you can change autofocus mode.)
Rotate the Main Dial until the AF choice you want becomes visible (all three choices are never visible simultaneously, as in this illustration).
One Shot AF
In this mode, also called Single Autofocus, focus is set once and remains at that setting until the button is fully depressed, taking the picture, or until you release the shutter button without taking a shot. For non-action photography, this setting is usually your best choice, as it minimizes out-of-focus pictures (at the expense of spontaneity). The drawback here is that you might not be able to take a picture at all while the camera is seeking focus; you're locked out until the autofocus mechanism is happy with the current setting. One Shot AF/Single Autofocus is sometimes referred to as focus priority for that reason. Because of the small delay while the camera zeroes in on correct focus, you might experience slightly more shutter lag. This mode uses less battery power.
When sharp focus is achieved, the selected focus point will flash red in the viewfinder, and the focus confirmation light at the lower right will flash green. If you're using Evaluative metering, the exposure will be locked at the same time. By keeping the shutter button depressed halfway, you'll find you can reframe the image while retaining the focus (and exposure) that's been set.
This mode, also known as Continuous Autofocus is the mode to use for sports and other fast-moving subjects. In this mode, once the shutter release is partially depressed, the camera sets the focus but continues to monitor the subject, so that if it moves or you move, the lens will be refocused to suit. Focus and exposure aren't really locked until you press the shutter release down all the way to take the picture. You'll often see Continuous Autofocus referred to as release priority. If you press the shutter release down all the way while the system is refining focus, the camera will go ahead and take a picture, even if the image is slightly out of focus. You'll find that AI Servo AF produces the least amount of shutter lag of any autofocus mode: press the button and the camera fires. It also uses the most battery power, because the autofocus system operates as long as the shutter release button is partially depressed.
AI Servo AF uses a technology called predictive AF, which allows the 40D to calculate the correct focus if the subject is moving toward or away from the camera at a constant rate. It uses either the automatically selected AF point or the point you select manually to set focus.
This setting is actually a combination of the first two. When selected, the camera focuses using One Shot AF and locks in the focus setting. But, if the subject begins moving, it will switch automatically to AI Servo AF and change the focus to keep the subject sharp. AI Focus AF is a good choice when you're shooting a mixture of action pictures and less dynamic shots and want to use One Shot AF when possible. The camera will default to that mode, yet switch automatically to AI Servo AF when it would be useful for subjects that might begin moving unexpectedly.
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