DEPTH OF FIELD
A basic definition of depth of field is: the zone of acceptable sharpness within a photo that will appear in focus. In every picture there is a certain area of your image in front of, and behind the subject that will appear in focus. This zone will vary from photo to photo. Some images may have very small zones of focus which is called shallow depth of field. Others may have a very large zone of focus which is called deep depth of field. Three main factors that will affect how you control the depth of field of your images are: aperture (f-stop) distance from the subject to the camera, and focal length of the lens on The closer your subject is to the camera, the shallower your depth of field becomes. Therefore, moving further away from your subject will deepen your depth of field.
These pictures i have taken show a shallow depth of field.
These pictures show a medium depth of field.
These pictures show a large depth of field.