Depth of Field

The depth of field is a term used for whats in focus in a frame. There is a certain point away from a camera lens where a subject is totally in focus. If the subject is too far or too close to the lens, it will look blurry. Adjusting a camera's lens or zoom changes the depth of focus.

{http://en.wikipedia.org/wiki/Depth_of_field}

This pic demonstrates the depth of field and its 'limits'.



Aperture opening has a big effect on the DOF in a frame. A smaller opening reduces the amount of light entering the frame, and increases the diffraction through the lens (blurry at different levels).

In video, there's generally very little changing of the aperture, so to keep continuity from shot to shot. In still photography is when aperture adjusting is used more frequently for DOF changes.

As the aperture stops increase (size gets smaller), the DOF increases, but diffraction increases.