The area in front of the camera that appears sharp in the frame is called depth of field. Depth of Field is defined as the range of acceptable focus on a shot or photograph. Depth of field is an important concept for cinematographers and camera operators to master because they often need to manipulate focus to achieve a desired effect. Selective focus, for instance, draws the audience’s attention to a specific portion of the frame.
Depth of field can be either shallow or deep. Shallow depth of field is the kind in which part of the frame is soft or out of focus. Thus, the areas of focus or sharpness are limited. Deep depth of field, on the other hand, is the kind in which the entire frame, from the foreground to the background, is sharp or in focus.
These three parts of the camera affect depth of field:
- Focal length
- Focus distance
The aperture is the factor that most influences depth of field. The wider the aperture is, the shallower depth of field will be. A low f-number such as f/2.8 will likely render an image with some soft focus. Conversely, the narrower the aperture is, the deeper depth of field will be.
Note: In the image above, other settings were altered to maintain normal exposure.
The figure below illustrates the relation between aperture and the amount of depth of field. For this illustration, image that the camera is on the left hand side of the image, represented below by the aperture. The 100mm value represents the focal length of the lens, which does not change in this scenario:
The area in red represents the depth of field, or the distance in front of the camera in which the picture is on focus. The black represents soft focus. Notice how the depth of field changes together with the f-number on the left.
The black vertical line represents the area that is sharpest in focus, which we call critical focus. Note that you always have more depth of field behind your plain of critical focus than in front of it.
Next to the aperture, the second factor that most affects depth of field is focal length. The longer the lens, the shallower depth of field is. A wide angle lens (a short lens), for instance, would render an image with more depth of field than one with a telephoto lens (a long lens). Study the figure below and notice that the f-number is the same while focal length changes:
Focus distance is the last factor to determine depth of field. The closer the focus distance is to the camera, the less depth of field you have. You probably have noticed this. On close-ups of actors or objects, part of the frame is usually blurrier than on wide shots of landscapes.
Though focus distance does play a part in the equation, you are better off relying on f-number and focal length to just depth of field, which, as you have learned is the amount of focus in the shot.