What is Depth of Field?

The difference between professional-looking photographs and amateur snapshots can often be recognized at a glance, based solely on the depth of field.

Gaining control over depth of field is what leads us to spend $1000 for a digital SLR, when for $90 we could get a point-and-shoot camera that takes nearly identical photos in terms of resolution and color. It's that important.

But despite its universal importance to photography, depth of field (hereafter DOF) remains widely misunderstood.

The misunderstandings arise partly because technical explanations of DOF are full of hair-raising mathematical equations and weird terminology like "circles of confusion," "hyperfocal distance" and other geek-speak.

But we need not worry about all that.

I'm going to talk about depth-of-field as a practical tool to be used when you have a camera in your hand, not a calculator. In order to do that, I'll have to sacrifice some mathematical rigor in favor of easy understanding, but you can find the math easily enough on the web, if you're into that sort of thing. What's hard to find is a simple explanation for beginning photographers, so that's my goal here.

Depth of Field is not the same thing as "Background Blur"

The most common misunderstanding among beginning photographers is a vague confusion between DOF and "background blur." Now, it's true that DOF is a tool we use to selectively blur parts of an image. But to think of DOF as just a way to blur a background is to slightly miss the point.

In fact, in one sense, it's looking at it exactly backwards. Because DOF does not refer to the blurry parts of an image, but to the sharp parts. Here's a working definition that I like:

Depth of Field (noun): The range of distance in an image where the focus is acceptably sharp.

The "depth" in depth of field is the distance between the nearest sharp object and the farthest sharp object in your image. It's the part that is "in focus." That distance may be two inches or two miles, depending on many factors involving your subject, your camera, and your lens.

If you keep the following three principles in mind, you'll avoid the typical confusions about DOF:

1. Depth of field defines how much of your image is sharp, not how much is blurry.
2. Depth of field is a continuous range, not an abrupt border. "Acceptably sharp" is a judgment call. There is no sudden transition from sharp to blurry but a gradual fade.
3. Depth of field applies equally to objects in front of your subject and behind it (well, not always equally, but we won't get into the complexities of that).

Consider the nearby diagram of a man photographing a tree. The red zone indicates the depth of field. There is actually only a single distance, a single plane, where the image is truly "in focus." (In this diagram, it's the exact center of the red area near the trunk of the tree). In front of that plane, and behind that plane, the image becomes gradually less focused, until eventually any objects outside that zone are seen as blurry. The farther outside the red zone, the blurrier the objects appear.

Isolating subjects from the surrounding area

First and most obvious, we can use depth of field to isolate a subject from other elements in a photo.

The classic example is blurring the background in a portrait to draw attention to the face of your subject.

Portrait Tip 1

If you want to blur the background, get your subject farther away from it. It's much easier to blur a background that is 10 feet behind your subject than one that is 2 feet. The corollary is, the closer you stand to your subject, the easier it is to blur the background. Think of it as a ratio: the distance between you and your subject, versus the distance between your subject and the background. You want to be close to your subject, and have the background be far.

Portrait Tip 2

If you can't move your subject farther from the background, move yourself back and zoom in with a telephoto lens. You can create a very shallow DOF (as seen in guitar photo #4, or the portrait of my friend Jennifer above) with a long focal length and wide aperture.

Portrait Tip 3

It's possible to overdo a shallow DOF for portraiture. You can make the depth of field so thin that you lose parts of your subject's face that you would prefer to keep in focus.

For example, in the photo at right, I chose a super-shallow depth of field at 55mm and f/2.8, because I wanted to blur the background as much as possible. However, when Jennifer turned her head slightly, her right eye (on your left) moved out the shallow DOF plane and became slightly unfocused. It's not very noticeable at this small size, but in a large print it's quite apparent. I could have avoided this by choosing a slightly smaller aperture, perhaps f/3.5, or f/4.

Foreground Blur

Foreground blur is sometimes overlooked by photographers when they think about DOF, probably because every photo has a background, but many photos are arranged to have nothing in the foreground but the subject.

The classic example is blurring the background in a portrait to draw attention to the face of your subject.

However, having blurred objects in the foreground can give a sense of framing, depth, or context to a photo.

For example, the blurred sunflowers in both the foreground and background of this photo give the feeling that the field goes on forever.

You can also use foreground blur to remove obstacles.

The race car photo at right was shot through a chain link fence. And no, I did not press the lens against the fence and shoot through a hole. I was standing about 10 feet away from the fence, as you can see from the smaller photo taken in the same position.

The mesh of the fence is present in photo of the car—it's just so blurred by my shallow DOF that you can't see it!

Actually you can barely see the grid like pattern of the fence as a pattern of light and dark in the photo, but it's subtle. I shot this at 200mm f/4, effectively erasing a fence that stood directly between me and my subject.

Bokeh

Bokeh (pronounced "bo" as in boat and "keh" as in Ken) is a term from the Japanese meaning "the visual quality of the out-of-focus parts of an image."

Photographers speak of lenses that give "good bokeh" or "bad bokeh" with a preference generally for smooth, circular bokeh.

Bad bokeh often has hard edges (even on blurry shapes) or donut-like rings rather than smooth circles.

Some lenses give better bokeh on background objects than foreground objects, or vice versa. In fact, there is necessarily a kind of trade-off between the two. So you may want to experiment with your lenses and see what kind of bokeh they give on foreground blur and background blur.

The easiest way to see the shape of the bokeh is by taking a photo with small lights in the background or foreground thrown out of focus.

Some photographers create custom-shaped bokeh by placing a cover over the lens (often homemade from black paper) with a hole cut in the shape of a star, or heart, or some other geometric pattern.

This creates bokeh in that shape, and the effect can be cute, but is easily overdone.