One of the more confusing terms used in photography is Depth of Field. Simply defined, Depth of Field is the distance between the nearest object in sharp focus and the furthest object in sharp focus. The Depth of Field is controlled by the aperture size. The smaller the aperture, the greater the Depth of Field. With each change of the aperture by full f-stops, the Depth of Field will be halved or will double, depending on the direction of the change. Remember, the aperture is measured as a ratio of the size of the opening to the focal length of the lens focused at infinity. This is why the larger number equals a smaller aperture.
To clarify, a change from f 8 to f 11 will decrease the aperture size by half, allowing half the light to strike the film plane or the sensor, and will double the Depth of Field. Conversely, a change in the opposite direction, from f 8 to f 5.6 will allow twice as much light to reach the film plane or sensor, but will reduce the Depth of Field by half. Another factor in the Depth of Field is the focal length of the lens. Depth of Field decreases as the focal length increases. Thus wide angle lenses exhibit a greater Depth of Field than telephoto lenses.
Most single lens reflex cameras, whether film or digital, allow several choices for exposure settings. The two that I use more than others except Manual, are the Av (aperture variable) and the Tv (time or shutter speed variable). When viewing a scene, you must make a determination of how much Depth of Field is important to the scene you wish to record. If a greater Depth of Field is important to the image, I recommend using the Av setting and stopping down (decreasing aperture size) as far as necessary to attain the Depth of Field you desire. Many cameras have a Depth of Field preview setting, allowing you to see the Depth of Field more clearly through the view finder. Choosing the Av setting allows you to control the Depth of Field directly and lets the camera decide the shutter speed. This, of course, may require the use of a tripod if the shutter speed is reduced below the free hand comfort level for sharp images, which in most instance is 1/60 or slower. When shooting a landscape scene you will optimize the Depth of Field by focusing 1.3 of the way into the scene. This is the ideal focal point for the greatest Depth of Field in almost any
Portraits most often require a shallow Depth of Field, as this prevents the background from becoming distracting and drawing attention away from the subject. You can blur the background this way, allowing the viewer to realize, perhaps by a landmark, where the image was taken without the landmark dominating the the scene and overwhelming the subject. By choosing a larger aperture (smaller f number) you will reduce the Depth of Field, thus isolating the subject. I recommend experimenting. Try different uses of the Depth of Field. You'll see what a useful tool it is for improving your images.