DEPTH OF FIELD

Depth of field is about how much of the picture is in sharp focus, and how much of the picture is not in focus. If a subject is in sharp focus then objects in front of and behind the subject, may also be in acceptable focus, this in-focus area is the depth of field.

If you are doing Landscape Photography you would want the total picture in sharp focus, from foreground to background (see this in the above photo.) however if you are doing Portrait Photography you would want the subject in sharp focus and the background out of focus.

To have maximum depth of field, everything in sharp focus, you must set the lens to it's smallest aperture (lens opening) f/16 or f/22, this is good for Landscapes etc. At the opposite end with the aperture set to f/2.8 depth of field may be a distance of a few feet. (selective focus) this is good for portraits.

The aperture you select is important to depth of field settings, so too is distance, the further away you focus from the camera position, the more the depth of field. For Scenic, Landscape Photography, focus on a point one third into the picture, this means if the most distant object in the picture (Infinity) is one mile away from the camera, focus on a point one third of a mile away.
Never focus on Infinity.