Photography Depth of Field Explained

Over my nearly twenty years of teaching university photography classes, I've come to discover that one area that students often have the hardest time mastering is depth of field. Whereas beginning students usually manage pretty well to work with their cameras in manual mode which forces them to make their own selections of apertures and shutter speeds, they often seem to overlook creating images with a limited area of focus as the valuable tool it is for creating striking images.

Actually, I think one explanation for why this approach isn't explored more by many students is that they aren't active enough when they're out shooting. They see something in the distance that interests them and I fear too often they just take the shot from that point and don't investigate much further.

What, then, is "depth of field" (or DoF, for short)? Basically, it's the term we use to describe the area in the photograph which is in acceptable focus from the point nearest to the camera to the point furthest from the camera. What's interesting about this concept is precisely that this distance can change radically according to different settings and choices made by the photographer.

What Creates Different Ranges of Focus?

1. Aperture size. Large aperture openings like f 2, f 2.8 will have among the shortest or shallowest of range of focus. Smaller openings like f 11, f 16 and f 22 will have a progressively bigger depth of field as the opening is smaller (bigger numbers = smaller openings)

2. Lens type. Wide angle lenses will tend to have a greater range of focus than will longer telephoto lenses. The same will be true even with zoom lenses-when you're set to a more wide angle setting such as 35 mm or smaller, you'll have a bigger depth of field than if the lens is set at 100 mm or higher.

3. Focal distance. This may actually be the most important setting to impact whether you have a very narrow or very wide range of focus. If the absolute focus point is set on a point less than a meter (a few feet) from the camera you will have a much shallower depth of field than if your focal point is 10 meters or more from the camera.

Consequently, although an f 2.8 setting where you focus on a subject less than a meter from the camera will have a shallower range of focus than if you take the same picture with a smaller aperture (f 8, 11, 16, etc.), they will all be relatively shallow DoF precisely because the camera is focused very close.

Therefore, one is likely to find that the DoF will become narrower as one moves into tighter quarters which prevent focusing on objects at a greater distance. Now, why is all this important? Clearly, when something is in focus it draws the attention of the viewer more than something which is out of focus. In this way, a shallow range of focus can help direct the viewer toward parts of the picture which seem more important. Likewise, to take a portrait against a background that is a bit distracting, a shallow DoF which limits the area in focus primarily to the subject would allow the background to become softer and therefore less distracting.

Michael McCarthy
Atelier Vagabond Photo Tours & Workshops