Getting into Focus

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Learning to use the Canon EOS 40D's autofocus system is easy, but you do need to fully understand how the system works to get the most benefit from it. Once you're comfortable with autofocus, you'll know when it's appropriate to use the manual focus option, too. The important thing to remember is that focus isn't absolute. For example, some things that look in sharp focus at a given viewing size and distance might not be in focus at a larger size and/or closer distance. In addition, the goal of optimum focus isn't always to make things look sharp. Not all of an image will be or should be sharp. Controlling exactly what is sharp and what is not is part of your creative palette. Use of depth-of-field characteristics to throw part of an image out of focus while other parts are sharply focused is one of the most valuable tools available to a photographer. But selective focus works only when the desired areas of an image are in focus properly. For the digital SLR photographer, correct focus can be one of the trickiest parts of the technical and creative process.

To make your job easier, the EOS 40D has a precision nine-point autofocus system that uses a separate CMOS sensor in the viewing system to measure the contrast of the image. When the contrast is highest at the active autofocus point(s), that part of the image is in sharp focus. These points are of the advanced "cross" type (that is, they measure in both horizontal and vertical directions) and can be selected automatically by the camera, or manually by you, the photographer. They're represented by the nine sets of brackets visible in the viewfinder (see Figure 5.9).

Your camera's autofocus sensors require a minimum amount of light to operate, which is why autofocus capabilities are possible only with lenses having an f/5.6 or larger maximum aperture. If necessary, the AF assist beam built into the 40D and Canon's dedicated flash units provide additional light that helps assure enough illumination for autofocus.

Figure 5.9

Any of the nine autofocus points can be selected by the photographer manually or by the camera automatically.

Figure 5.9

Any of the nine autofocus points can be selected by the photographer manually or by the camera automatically.

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