Basics of Depth of Field
Depth of field describes how much of a picture is in focus. For general photographs, you want the subject in focus. It also refers to how much of the scene in front of and behind your subject can be seen clearly. Objects closest to the point of focus will look very sharp; those farther away will be less sharp but will still be acceptable, as long as they're within the depth-of-field zone. The depth of field is shallower in front of a subject (in between the subject and the camera) than behind it.
Remember that three factors determine depth of field:
The lens's aperture
The lens's focal length
The distance from the camera to the subject
Wide aperture settings result in shallow depth of field. Pictures taken this way will have the subject in focus and the foreground and or background out of focus. As apertures narrow, depth of field lengthens, resulting in more of the picture being in focus. Limited depth of field can be a great way to ensure that a cluttered background does not distract the viewer's attention from the subject.
The depth of field rendered by each lens differs depending on its focal length. For now, just remember that depth of field is inversely proportional to the focal length of the lens. In other words, shorter lenses have greater depth of field; longer lenses have a shallower depth of field. A 50mm lens will capture more of the picture in sharp focus than a 100mm lens with exactly the same aperture setting.
Lenses are usually marked with scales that indicate their depth of field. If your camera or lens doesn't have depth-of-field markings, see if there's a depth-of-field scale in your camera or lens instruction manual. In manual mode, you can use these scales to determine how much of a picture will be in sharp focus from foreground to background before you snap the shutter.
Depth of field is also directly proportional to distance. In other words, a subject that's farther away from your camera will have greater depth of field than one that is closer. Changing the aperture setting won't have a huge effect on the depth of field for a distant subject or if you're using a lens with a short focal length. However, it can make a big difference in closeup work, where you're dealing with a much shallower depth of field, or when you're using lenses with longer focal lengths, which have shallower depths of field.
FIGURE 6-3 The moon was the primary subject of this shot. The leaves in the foreground are a much softer focus than the primary subject, allowing it to shine.
