Photo Glossary - 07 - Depth of Field

Objects that are not on the focal plane (i.e. closer to or further away from the camera than the focal point is) will always be out of focus.  The further away from the focal plane an object is, the more out of focus it will be.  Objects close to the focal plane will appear to be in focus for the purposes of the photograph.  The distance from the closest object that appears to be in focus to the furthest object that appears to be in focus is called the depth of field.

There are three factors that affect the depth of field of a photograph:

 1.       Aperture of the lens – a large aperture will produce a shallow depth of field and a small aperture will produce a deep depth of field.

 2.       Focal Length of the lens – a long lens (zoomed in) will produce a shallow depth of field while a wide lens (zoomed out) will produce a deep depth of field.
 3.       Distance to the Focal Plane – Focussing close to the camera will produce a shallow depth of field, while focussing far from the camera will produce a deep depth of field.

A deep depth of field is useful when you want to capture an entire scene, for example in landscape photography.  A shallow depth of field is useful when you want to draw attention to your subject only, for example in portrait or wildlife photography.

Additional Info

Although the following doesn’t strictly change the depth of field, it allows you to get the most out of the depth of field that you do have.  Strictly speaking, depth of field is a measure of what distance an object can be from the focal point while remaining in focus.  What we really see when we look at a photo however is the apparent depth of field, which is influenced more by the difference between the sharpest and softest areas of the photo, than the actual, technical depth of field.
  

The secret to manipulating the apparent depth of field is in controlling the distances from your camera to your subject and from your camera to the background.  In fact it’s the ratio between the two distances that matters.
  

In order to get an apparently shallow depth of field put your subject close to the camera with the background far behind it.  The bigger the ratio between the distances, the more out of focus the background will be and the shallower the apparent depth of field.  The narrowest depth of field can be obtained by focussing on either the closest or furthest object in the photo.

In order to get an apparently deep depth of field put your subject far from your camera and close to the background.  The smaller the ratio between the distances the less out of focus the background will be and the deeper the apparent depth of field.

The widest depth of field can generally be obtained by focussing at about 2/3 of the distance between your camera and the furthest object in the photo.