Depth of Field (DOF) is the area (front-to-back zone) in which the image is razor sharp.

Generally, at least one area in a photograph should be in focus. It can be a center of interest or the entire image.

As soon as the subject falls out of this range, it begins to loose focus at an accelerating degree the farther out of the zone it falls.

Depth of Field can be SHORT (Shallow) or LONG (Deep).

With any DOF zone, there is a Point of Optimum Focus in which the object is most sharp.

Different digital camera lenses have different focal lengths.

On a digital camera, focal length measures the optical distance between the lens and the image sensor (in millimeters).

Focal length determines the camera lens’s depth of field and the size at which your subject appears in the frame.

Higher the focal length number, the more magnification a lens has.

The closer the subject is to the camera - the shallower the DOF. The farther away the subject - the deeper is the DOF.

Changing the distance to subject is the least practical way to manipulate the DOF as you are changing your photograph composition.

Aperture - is the size of the lens opening.

f/1.4

f/2

f/2.8

f/4

f/5.6

f/8

f/11

f/22

It is controlled by metal leaves that mask a circular opening for light to enter the camera.

It is measured in f-stops.

The pupil of the eye operates in a similar way: the lower the light level - the larger the pupil becomes to let more light enter your eye.

The lower the light - the larger your pupil becomes to allow more light enter your eye.

In bright light - the pupil becomes smaller so less light enters your eye.

This is what happens when you change the aperture setting on your camera.

Background becomes more in focus as the Depth of Field gets larger.