Lens - Depth of Field
When the lens is focused at any point, there is a certain distance in front and behind of that point that appears to be in focus.
Thus:

Depth of Field is an area of apparent sharpness with the Point the lens is focused on within it.

In motion picture work it is essential to know the effective Depth of Field when shooting, so that all the important details of the
scene are sharply reproduced and unimportant features as the background can be made less conspicuous by appearing out of
focus. The size of D of F can be calculated but it is more convenient to use tables showing these limits.
Size of Depth of Field depends on three factors:
1. Focal Length   2. Aperture Opening (F-stop)   3. Focus Distance
At any given situation cinematographer by manipulating between these factors can obtain desired Depth of Field.
Long (Narrow) Angle  Lens            Short (Wide) Angle Lens  
     Depth of Field / Focal Length
     Depth of Field / Aperture
   f 16                                              f 2.0
     Depth of Field / Focus Distance
The Shorter the Focus Distance - the Shallower the Depth of Field
The Smaller the F-Stop number (bigger aperture opening) -
the Shallower the Depth of Field
The Longer the Focal Length - The Shallower Depth of Field
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