Depth of Field

Ah, depth of field. The one concept from my photography class way back when that I can't seem to explain very well. Until now (I hope).

Depth of field is the distance between the nearest object and the farthest object in a photograph that are sharp or in focus. Essentially your lens can only focus precisely at one distance at a time, which means anything before or after that particular point in your image is not as sharp, or even fuzzy.

Look at the photograph above - the bell on the bicycle appears sharp and in focus, but when you look beyond the bell, most of the image is fuzzy or out of focus. So the depth of field is essentially the image immediately around the bell. Notice the change from focused to unfocused is gradual.

Here is a photograph that illustrates depth of field very distinctly:

Depth of field is an important concept. It helps you, the photographer, determine how much of an image will be sharp, and you determine what is "significant" in any given photo. Sometimes you want to focus on a single object and blur the background, such as when you are photographing a baby. Sometimes you want to ensure the entire image is sharper, such as with a beautiful landscape photo.

In a nutshell, there are 2 things that will affect depth of field: (1) focal length (or focusing distance), and (2) aperture setting. Longer focal length and/or larger aperture setting (remember, this means the f-stop number is smaller) will result in a more shallow depth of field (and vice versa). Here's an example:


All 3 of these photographs were taken with a 200mm lens, but at various f-stops (from left to right: f8, f5.6, and f2.8). Notice the differences with respect to how much of the image is sharp versus fuzzy. The depth of field in these images would also change if the photographer changed his focal length.

I found this tutorial on youtube which I think explains it very well. Look - I even provide audio/visual guides in my lessons!