DoF = Depth of Field, e.g., the zone in front of the lens which will be in acceptable focus for any given focal length, f-stop and focus setting.

The longer the focal length of the lens (300mm) the shallower the zone. The shorter the lens (28mm), the deeper the zone. This is why you should always zoom in to focus and then set your frame by widening the shot, if desired.

The wider open the lens (f/2.8) the shallower the zone. The more stopped down (f/16) the deeper the zone. This is why you should always focus with the lens set to its widest aperture and then stop down to the proper exposure.

The closer the lens is focused (2 feet) the shallower the zone. The farther the focus (40 feet) the deeper the zone.

This is all based upon the Circle of Confusion at the film plane which is an arbitrary definition of sharpness of the image on the film. This is intimately connected to the depth of focus which is related to how all the above combine behind the lens onto the film to provide an image that is "in focus."

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