Depth-of-Field (DOF) is “the portion of a scene that appears acceptably sharp in the image.”. wikipedia.com

The DOF is controlled by the lens Aperture diameter within the lens.  An aperture of F2.8 or less will create a very shallow DOF and is often used for Portrait Photography.  Opening Up (increasing your aperture) to F11 or greater will create a deep DOF and this effect is often used for Landscape Photography.  By understanding the Apertures on your lens, you will be able to Pre-visualize your Depth-of-Field and begin to control the effects within your photographs.

Depth-of-Field F2.8

The photograph above is an example of a Shallow Depth-of-Field where the subject is in focus but the foreground and background are out-of-focus.   The exposure was set at F2.8, 1/125 at 400ISO.

Montevideo, Uruguay F16

Above is an example of a Deep Depth-of-Field where the landscape photograph is completely in focus.  The exposure was set at F16, 1/60, 800ISO.

“Depth of Field.” wikipedia.org 2010. Wikimedia Foundation Inc. 9 February 2010.