What is Depth of Field

Friday, 28 September 2012
What is Depth of Field (DoF) ?

Depth of Field is the amount of your image before and beyond your focus point that will be in focus. Depth of field tells you whether or not your subject and background can be sharply focused at the same time.

Depth of field is determined by several factors:
* Aperture/F-Stop
* Lens
* Subject Distance

While the camera can actually only focus on one tiny point in space, the depth of field determines how much of the image is in "acceptable focus" to the human eye.
In subjects such as landscapes, a large depth of field is often desired so that the entire scene appears to be in focus. With subjects such as portraits, a small depth of field is often used to blur the background and reduce distractions from the main subject of the image.

The Effect of Aperture/F-Stop on Depth of Field :
Aperture describes an adjustable opening inside your camera lens that controls the amount of light striking the film. As the size of the aperture changes, the angle of light striking the film also changes. It is this angle change, much like eyeglasses change the angle of the light, that creates changes in depth of field.

Aperture is measured by F-Stop on your camera controls. F-Stop settings represent a ratio derived from the size of the lens opening and focal length. Aperture has historically been confusing for new photographers (and some established photographers) because of the apparent conflict in description. A small F-Stop is a large aperture opening and a large F-Stop is a small aperture opening. Because a smaller aperture limits the amount of light entering the lens, a large F-Stop also requires more light to properly expose an image.

A simpler way to remember the relationship between F-Stop/Aperture and Depth of Field is:
 Large F-Stop = Large Depth of Field = More Light Needed Small F-Stop = Small Depth of Field = Less Light Needed

This means that larger F-Stops, such as F11, will require slower shutter speeds and produce images with larger depths of field. Smaller F-Stops, such as F4, will allow faster shutter speeds and produce images with shallower depths of field.

The Effect of Aperture/F-Stop on Depth of Field:
The focal length of your lens plays a big part in determining the depth of field (DOF) for your images. Think of your lens strength as a limiting factor for your aperture capabilities. The higher the magnification factor, the smaller the depth of field will be, even with large F-Stop settings.

Depth of Field Progression for a 70-300mm Lens
* 70mm = largest DOF
* 100mm = large DOF
* 200mm = small DOF
* 300mm = smallest DOF

This effect is especially pronounced in macro photography where close proximity to the subject and high focal lengths result in depths of field that are sometimes less than an inch.

The Effect of Subject Distance on Depth of Field :
Much like lens strength, subject distance, plays a big part in determining the possible size of depth of field. The closer you are to your focal point, or subject, the less depth of field is possible. To illustrate this effect, hold your hand at arm's length in front of your face. Even when focusing on your hand you can probably see a good bit of the surrounding environment in reasonably clear focus. Slowly move your hand towards your face until you reach the half-way point. Notice how much less of the area surrounding your hand is in focus. Continue moving your hand towards your face until it is as close as your eyes can focus on it. Very little of the area surrounding your hand can now be seen.

This same effect occurs with your camera lens. This effect, combined with high magnification factors, results in the tiny depth of fields seen in macro photography. It also makes the huge depths of field in many expansive landscapes possible when using a lower magnification factor lens.