Understanding the dept of field (DOF)

Definition: Depth Of Field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.
The above illutration using the same focal length (of focus distance) and different aperture (ie. 2.8, 5.6 and 22)
To practice this effect,
you should set your DSLR at AV mode (aperture priority) and try to shoot a static object with different aperture.

To get better result, you should:

• Understand how to manually change the aperture on your camera.
• Use lots of reflected, diffused light – either natural or studio.
• Your camera should be mounted on a sturdy tripod at the same level as the item to be photographed.
• Understand the rule of thirds and how it applies to composition.
• Know that automatic focus, shutter speed and ISO settings can simplify the exercise.