In the previous Aperture lecture we went over to use your camera’s Aperture to control your depth of Field.
An f/stop of f/16 gives you more depth of field over an f/stop of f/2.
Another component of Depth of Field is the Plane of Critical Focus.
The Plane of Critical Focus is the plane were the lens is focused.
Depth of Field is the area in a photograph where the photograph is acceptably sharp (focused).
In the image below by Eli Reed you can see the Plane of Critical Focus. The people are in focus and the background and the man in front are out of focus.
Below is a diagram of the Plane of Critical Focus.
Below is the same image, photoshoped to emphasize a smaller depth of field.
Here is the diagram of the new lower depth of field.
Focus (depth of field) in your photograph increases faster behind the plane than in front.
Usually the depth of field (objects acceptably in focus) extends about one-third in front of the plane of critical focus and two-thirds behind it.
Keep this in mind when you are really close to your subject. The closer you are to your subject the less depth of field you have.