Compression of Space. Camera-to-subject distance can dramatically compress the perceived distance between the subject and the background in a shot. It is, in fact, the camera-to-object distance that changes the depth-of-field and not the focal length of the lens as is commonly thought (Whittaker, 2003, http://www.cybercollege.com/myths.htm)

These four shots all have approximately the same framing but the camera-to-subject distance changes from small to large, compressing the distance in the background.

Lens focal length – a shorter focal length allows the camera to get closer to the subject allowing a greater depth of field.