Depth Of Field

Understanding aperture is a prerequisite for learning depth of field. Say you are taking a portrait of a baby. You focus on his face and take the photograph at the widest aperture (in example f/2.8) of your lens. The face will appear sharp but the surrounding objects like the furniture in the room will not. And when you stop down to f/11 to retake the photograph again, you will notice that everything from the foreground to the background appears significantly sharper. This changing zone of acceptable sharp focus on either side of the subject that you have focused on is known as depth of field.

Depth of field (DoF) can also be defined as the distance from the furthest to the closest points in a photograph that is in focus. Photographs where both foreground subjects and background elements remain in sharp focus are said to have a large (or deep) depth of field. Conversely, photographs that have a shallow depth of field are those where the subject remains sharp, but elements in front or behind it turn out blur. Three factors determine the depth of field in a photograph:

Aperture; the factor that has the most direct impact on depth of field, and is also the easiest to manipulate is the aperture. A wide aperture (small f-number) gives shallower depth of field while a small aperture (high f-number) creates a deeper depth of field, assuming that focus distance and focal length remain constant.

Subject distance: The second factor is the distance between the camera and the focus point. The closer you are to the subject, the lesser depth of field you have to play around with. For macro shooters whose lenses are often mere centimeters away from their subjects, the shallow depth of field available can make camera positioning and focusing a real challenge.

Focal length being the last factor is the focal length of the lens. The longer it is, the less depth of field it gives, even with the same aperture and subject distance. For instance, a 20mm lens at f/5.6 will produce images with a greater depth of field than a 105mm lens at the same aperture. This is due to the increase in image magnification at higher focal lengths.

The control of depth of field can be a very powerful creative technique. For example, you can choose a shallow depth of field to isolate a particular subject from others. This will draw the viewers’ attention to what you want them to see, rather than distracting them with unnecessary details. Examples of photographs that benefit from the use of shallow depth of field include portraits and floral photography. Whilst of course, maximizing depth of field is often needed if you are shooting landscapes or architecture.