Understanding Depth of Field

Depth of Field is an important factor to take into consideration when taking any kind of photograph.

When you look through the camera, you see objects near to you, in the middle distance and far away from you. Not all of these objects are in focus as far as the camera is concerned.

Aperture controls the depth of field. Stopping down (decreasing the f-stop number; e.g going from f/4 to f/8) has an important effect on your image apart from altering the exposure – it increases the depth of field. When you focus on a subject, say a person, for your photo and you use your lens at maximum aperture (say f/1.8), then only your subject will be in focus. When you get your photo back from the lab or see it on a monitor, you’ll notice that things that were closer to the camera are blurred and out of focus, as are objects behind your subject.

Using a lens wide open (maximum aperture) means that focusing is critical because only one thin plane will ever be in focus. As an example, suppose you’re subject was pointing at you. If you focus on their outstretched hand, the face will be out of focus in the final picture. If you focused on the face instead, then the hand would be out of focus.

Increasing the depth of field increases the depth of that focal plane so that some objects in front of where you’re focusing are also in focus and some objects behind your subject are also in focus. In practice, more things beyond your subject will be in focus than those in front.

Simple Rule

The more the aperture is stopped down the greater the depth of field.

A crude guide for depth of field is marked on most older lenses, under the distance scale. For any aperture setting, you can read off the max. and min. distances that will be sharp against the appropriate f-number on your depth of field scale.

Influence of Focal Length

The focal length of a lens also affects the depth of field. With a long-focal-length lens, such as a telephoto, to give the same exposure for any given f-stop, the actual diameter of the aperture must be larger than that needed by a lens with a shorter focal length.

Take a look at the above diagram. To get as bright an image, the aperture of the 180mm lens has to be much greater than that of the 28mm lens. Because the aperture is larger, the depth of field must be smaller.

Landscapes

Landscape photography frequently requires a large depth of focus as what you’re trying to do is get everything from just in front of the camera to the far distance in focus. If you’re trying to isolate a specific subject, say a single tree in a line of trees, then use a large aperture (and a small depth of field).

While setting a small aperture on your lens, f/11, f/16 or even f/32, will ensure great depth of field, if you have a foreground object near to the camera (e.g. a rock, tree stump, etc.), it may be too close to the camera to be in focus. In such cases, you should use hyperfocal focusing.