Depth of Field
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.
Three factors affect depth of field:
- Focal length
- Focus distance
The aperture is the factor that most impacts 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.
The figure below demonstrates the relation between aperture and depth of field. The area in red represents the depth of field.
Note: You always have more depth of field behind your plain of critical focus than in front of it.
Following the aperture, the second factor that most determines depth of field is focal length. The longer the lens, the shallower depth of field is. A wide angle lens, for instance, would render an image with more depth of field than one with a telephoto lens. Study the figure below. Notice that the f-number is kept the same and focal length is changed.
Focus distance is the last factor to determine depth of field. The closer the focus is to the camera, the bigger depth of field is. 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.