Jun 19, 2011

Photography 101 - Depth of Field

Depth of field or DoF relates to the parts of a photograph that are in full focus and show acceptable details. We, as photographers, have control over the amount of depth of field in a photograph and depending on our photographic or artistic goals we may vary one or all of the four factors that determine a shallow or extensive depth of field:

1. Focal Length
A wide angle lens such as 24 or 28 mm provides greater depth of field than a long telephoto lens with a focal length of 200 to 600 mm. The wide angle lens produces vast depth of field while the longer lenses produce a shallow depth of field.

2. Subject Distance or Magnification
The depth of field in a photograph deteriorates the closer we get to our photographic subject regardless of the chosen lens focal length. For example, in an extreme close up photograph the depth of field may only reach across a few millimeters while when photographing grand landscapes or city skylines from a distance the depth of field reaches across the entire frame.

3. Choice of Focus Point
Depth of field falls roughly one third in front and two third behind the focus point. For example if we lay the focus point at 3.5 meters away from the camera and we obtain a theoretical depth of field of 3 meters at an aperture of 8, then the selective sharpness in our image would reach from 2.5 meters to 5.5 meters.

4. Aperture or f-stop
A small aperture or f-stop setting (large f/numbers such as 11 and greater) will maximize depth of field and is mostly desired when photographing grand landscapes or city skylines. In such photos we strive for sharpness and detail from the nearest picture element in the foreground all the way to the horizon. A large aperture (small f/numbers such as 5.6 and less) will minimize depth of field and is mostly desired when capturing bird or flower with our photography. In these type of photos we strive for a quiet backdrop that beautifully isolates the main subject from any distractions in the background and solely lays the focus on the main subject.

Most cameras now a days have a depth of field preview mechanism where we can determine its acceptability with the chosen camera settings and set-ups. If depth of field is insufficient and does not span the entire frame one can either close the aperture further (larger f-stop number) or decrease magnification of the foreground by backing up or switching to a wider lens. On the other hand, if depth of field is to broad we may open the aperture further (smaller f-stop number), choose a longer lens or get in closer to enhance magnification.

In my next installment of this photography 101 series I will discuss exposure.