Depth of Field

Depth of field is the area of the image that appears in focus from foreground to background and is determined by a combination of the opening of the aperture and the focal length of the lens. A small aperture setting results in greater depth of field. Controlling depth of field is one of the easiest ways for a photographer to compose the image.
By limiting the depth of field of an image, the photographer can turn the attention of the viewer on the subject in focus. Often, limiting the depth of field of an image helps eliminate clutter in the background. On the other hand, when shooting a landscape, you want the image to have great depth of field. Limiting the depth of field to the foreground would not make sense.    .

Telephotolenses (with long focal lengths) tend to have shallow focus when the aperture is opened all the way, limiting the depth of field of an image. Wide-angle lenses (with short focal lengths) tend to create images with great depth of field regardless of the aperture setting.