Understanding Depth of Field

In the olden days of film photography, depth of field was discussed quite extensively amongst photographers. Somehow, up and coming digital photographers seem to have lost their focus (sorry, I couldn’t resist) on the importance of understanding how certain camera settings greatly effect depth of field. I believe that in days past, a significant division existed between those who were SLR photographers and those who were not. Back then, auto focus was unheard of on a SLR camera. After all, where is the skill in letting the camera do all the work? Instead, the photographer had to set focus by aligning a split horizon and depth of field was determined by simulating the effects by pressing a DOF button, which was quite a nice feature that was available on some systems. So, to pay due tribute to those who long for their old SLR systems, let’s take a look at some of the principles relating to depth of field.
To begin, let’s define the meaning of depth of field. Depth of field is a principle where the distance between a focal point and the point at which the background or foreground loses focal quality. As the distance between the two points increase, the shot setup is said to have an increased depth of field. Conversely, as the distance between the focal point and the point at which the shot becomes unclear is reduced, the shot is said to have a narrow depth of field. Unlike the human eye where most people are able to simultaneously focus on objects that are close by and those object that are far away, camera lenses require focal points to be effective. So why do shots, such as landscapes, appear to be entirely in focus? That’s an easy question to answer; the focal point in most landscape shots are set to infinity, therefore, the shot has a very wide depth of field.
Generally, there are three ingredients that affect depth of field: focal length, aperture size, and the distance between the lens and the subject.
Focal Length
Telephoto lenses tend to have a much narrower depth of field than wide angle lenses. This becomes more evident as the focal length increases. Telephoto lenses are generally used to focus on a single subject at greater distances. As the focus becomes more concentrated, the depth of field is greatly reduced. However, when using a short lens, say about a 28mm, less attention is placed on a single point, rather the general area becomes the targeted subject, which increases the depth of field.
Aperture Size
Depth of field is dramatically reduced as more light enters the lens. Light, as you may recall, is generally controlled by the aperture. A wide aperture (low f-stop numbers) increases the amount of information the camera must process, thus those items that are outside the focal area lose focus and the depth of field is reduced. However, reduce the size of the aperture (high f-stop numbers) and depth of field is restored.
Distance
The depth of field is increased by distance between the subject and the lens. This can be easily demonstrated by comparing a shot taken at the minimal focal distance and a subject that is far away without changing the aperture settings (let’s be fair about this). To demonstrate this more dramatically, examine a macro shot that was taken at a 1:1 ratio. You will see how the macro shot has barely any room for error, which is why a stabilizing system is so important.
Now that you have a better understanding of depth of field, don’t be shy about bringing up the subject around a few of the old timers as they follow butterflies in the park with their legacy systems. After all, you may earn a little recognition in the process, but be prepared for some pre-digital war stories.