An in depth explanation of Depth of Field.

Here is an in depth explanation of Depth of Field from

Control over the sharp and unsharp parts of the image can be of considerable importance for the success of a photograph. Traditionally, the depth of field (DOF) is a great source of confusion among photographers. To blur or not to blur, that is the question. The latter is a matter of DOF, the former not per se.

Depth of field is defined as the range of object distances within which objects are imaged with acceptable sharpness.

Gromit faces a 28-mm lens @ f/4
 A: Gromit photographed with a 100-mm lens at f/4. B: Gromit photographed with a 28-mm lens at f/4. DOF is the same.
Gromit faces a 100-mm lens @ f/4