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Controlling Depth of Field
The topic of depth of field (DOF) can be intimidating for many beginning photographers, but the concept is actually very simple. It is the range in a scene, from near to far, that is in sharp focus. The ability to control how much is in focus has an immense effect on your pictures. When shooting a landscape, for example, you'll want as much of the photograph in sharp focus as possible, so a significant depth of field is required. In other situations, as in taking a portrait, a shallower area of sharp focus will isolate your subject from distracting backgrounds.
Three things control how great the depth of field will be in a photograph: lens focal length, aperture size, and your distance from the subject. Other things being equal, shorter-focal-length lenses, smaller apertures, and greater lens-to-subject distance all increase the range of sharp focus, while longer lenses, wider apertures, and a shorter distance to your subject shrink depth of field.
When you look through your lens, however, you will not be seeing what's really sharp and what's not. With point-and-shoot cameras, this is because you're not looking through the lens, you're looking through a separate viewfinder. With DSLRs, you're looking through the lens at its widest aperture; it closes to a smaller f/stop only at the instant you press the shutter button. This is a problem if you're using a medium telephoto lens to take a portrait of Mom in gay Paree. As you look through the lens, it appears that the traffic and crowds in the background are out of focus. But because you have a small aperture set, when you get the picture back, zingo, Mom is lost in all that junk behind her.
The solution? Many DSLRs have a depth-of-field preview button; when you press it, the lens briefly closes to the shooting aperture, showing you the real depth of field. Be aware that when you press this button, the image in the viewfinder will temporarily darken. If too much is in focus, simply open the aperture, put on a longer lens, or move closer. Or do all three. If too little is sharp? Well, you can figure that out.Next: "Creative Camera Filters"