Depth of Field

To understand it, you need to realize one particular fact in which your eyes have been deceiving you.

When you look at most photographs, objects at varying distances from the camera, still all appear to be in focus.

Take this photograph for example. The pelican and the trees all appear to be in focus.

In reality, only the pelican is in perfect focus. Here is the basic photography truth: There is only one point in any photograph that is in perfect focus.

The exception to this rule is if there are more than one object in the same plane and distance from the camera. In that case, all objects in the same plane and distance from the camera would be in focus.

Other objects in the photograph which seem to be in focus, technically are not. They are within what could be called "the zone of sharpness," and hence they appear sharp or, "in focus."

For example, if the closest object in a particular photographic print that seems sharp is at 5 feet, and the most distant object is at 15 feet, then the depth of field extends from 5 to 15 feet from the camera.

Realize that DOF is not an absolute measure, as it depends on the size of the print, the viewing distance and the visual acuity of the observer.

Why are we spending all this time on depth of field? Without an understanding of how to blur what you want and not blur what you want sharp, you will only produce the desired results about 1/2 the time.

When you see an object/subject that you want to capture (and, have in focus), you can do so by making the subject remain in sharp focus while the background is blurred (or "thrown out of focus").

Or, you can photograph the same subject and blur it, while keeping the background in focus.

Two obvious examples involve taking a picture of a beautiful rose in front of an unsightly garbage dump. You would want to blur the background as much as possible, while the rose is clear and sharp.

On the other hand, a photograph of your family in front of the Grand Canyon loses most of its impact if you can't tell it's the Grand Canyon, right? In that case, you want to have BOTH foreground and background as sharp as possible.

So, how do we create the effect we want? There are several factors that go into DOF... lens focal length, camera-to-subject distance, and aperture.

We will focus on the basic points for achieving the desired result...

• To have everything in the photo SHARP: For landscapes and large groups, use a normal or wide angle lens (not telephoto) and as small an aperture as possible (F22 for example).

• To have the main subject sharp while the background is out of focus: Typically for portraits, use a mild telephoto lens (minimum 100mm) in combination with a very large aperture (example: F/2.8). If a telephoto is not handy, use as large an aperture of whichever lens you are using.