Depth of field (deep and shallow focus)

Definition of depth of field: In film depth of field is the distance between the nearest and farthest objects in a scene that appear sharp in the image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so within the depth of field, the unsharpness is hard to see under normal viewing conditions.In some cases the entire image sharp must be sharp, and a large depth of field is used which is called deep focus. In other cases, a small depth of field is used which emphasises the subject while de-emphasizing the foreground and background, this is called shallow focus.

Example of the use of depth of field:

Depth of field is used here to make one object stand out while you are still easily able to see the other objects. This can let the audience see the distance between them or could  let them think the more focused glass is more important.