What is DOF or Depth of Field

DOF or Depth of field is the amount of distance between the nearest and farthest objects that appear in acceptably sharp focus in a photograph. Or - how deep is your focus. The deeper the focus - more objects are in focus and the shallower the focus - fewer of the objects are in focus.

For example, if your subject is standing in front of a far off mountain and your subject and mountain both are in focus then you have a "deep" DOF. If only the subject or only the mountain is in focus then you have a "shallow" DOF.

Depth-of-field is an extremely important topic. Having the ability to wisely control and use the depth of field makes for extremely rich and stunning compositions.

Area of Sharpness

In any photo, only one focal plane is sharp. The area of sharpness is also 1/3rd in front of and 2/3rd behind the focal plane. So, if you have a person standing 10 feet away and that person is in focus then you may have 2 feet before him and 4 feet behind him in reasonably sharp focus.

Most dramatic effects are achieved by a shallow DOF. However, for specific cases (such as a subject in front of the mountain and you want the mountain and the subject both in focus) you can choose deep DOF.

Controlling Dept-of-field

The following 3 aspects work simultaneously to contribute to the depth of field. Increasing or decreasing one can increase or decrease the depth of field compensating for the other.

Aperture:
- Larger the aperture (smaller f-stops like f/1.4, f/2, f/2.8, etc.) the shallower the depth of field
- Smaller the aperture (larger f-stops like f/8, f/11, f/16, etc.) the deeper the depth of field
Relative distances between the camera and the subject and the subject and other objects
- The closer the camera is to a subject, the shallower is the depth of field
- The greater the relative distance between your subject and the objects, the shallower the depth of field
Focal Length
- The Wider the angle the shallower the depth of field
- The narrower the angle the deeper the depth of field

The role played by Aperture in depth of field is very important and hence lenses with lower apertures are desired.

Since both distance and focal length can be constrained by physical measures aperture can solely be controlled by you via your lens
Aperture can change depth-of-field without changing the overall composition - something that changing the focal length or the relative distances generally cannot.