Now we're starting to see the compression and flattening of the image that you get when you zoom in with a long lens. The 106 mm focal length gives us a tight field of view the relatively wide aperture makes for a shallow DOF. However, both of these factors are offset by moving farther away from the subject. Crunching the numbers tells me that the DOF for this image is just over 2 feet, actually slightly deeper than in Image 2. Despite the big increase in focal length, the greater distance and slightly smaller aperture combined to give us a wee bit more DOF than in the previous image (but still not enough for anything but the red guitar to be in focus).

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.

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.

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.

Depth of fieldmicroscope

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.

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

Depth of fieldphotography

With a relatively small aperture like f/16 (remember, the higher the f-number the smaller the aperture), we have a very deep DOF. In fact, the DOF extends to infinity in this image. Even the buildings in the background, on the horizon, are in focus.

The video magnification is determined by dividing the TV monitor diagonal (mm) by the CCD chip diagonal (mm). A reference chart is given below which shows video magnification for various monitor and chip sizes. To determine “Video Magnification” on a computer monitor divide the actual image diagonal (as measured on the screen) by the CCD chip diagonal.

The combination of greater zoom (50 mm) and much wider aperture (f/2.8) create a much shallower depth of field. As you can see in the image, not only has the distant background gone out of focus, but even the second guitar is slightly blurred. If you do the math, the DOF for this image calculates to just over 1 foot. Everything outside of the 1-foot-deep zone around the red guitar will be out of focus, increasingly blurry with distance from that guitar.

Shallowdepth of field

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.

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.

There are other technical factors involving the sensor or film in your camera, but we're not going to worry about those, because they are basically a given and out of your control.

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.

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.

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.

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 fieldexamples

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.

You can also understand why a tiny point-and-shoot camera with a pinpoint aperture and extremely short focal length does not give you much control over depth of field. It's easy for such a camera to create extensive DOF (often infinite) but very hard for it to create shallow DOF.

I had to back up a few feet to keep the guitar framed at 50 mm. If nothing else had changed, moving farther from my subject would increase the DOF, but this factor was overwhelmed by radical changes in aperture and focal length.

You can move beyond that stage by consciously choosing how much DOF you want your image to have, before you put your camera to your eye—deciding exactly what you want to have in focus, and out of focus—then adjusting your position and camera settings until you get it.

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:

Maximum zoom and maximum aperture on my 70-200mm f/4 lens gives a very tight field of view and a very shallow depth of field. I can't keep the entire red guitar in the frame any more. And clearly, not only is the second guitar far out of focus, but the distant background has become a featureless smear of color and light. Crunching the numbers tells me that DOF is about 7 inches in this image. This kind of shallow DOF can really isolate your subject from the background.

Compare the following series of photos that I took of two guitars to show what happens to DOF when I change distance, aperture and focal length. In each case I'm focusing on the red guitar.

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.

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.

Depth of field vs field of viewcamera

What is the magnification on the TV monitor when using a 10X objective, 0.45x video coupler, a 1/2″ format CCD and a 19″ monitor?

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.

Field of viewhuman eye

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.

The approximate real size of a specimen can be determined by dividing the length of the specimen measured on the monitor screen by the total magnification on the monitor.

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.

Depth of fieldphotography examples

Depth of field vs field of viewphotography

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

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."

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

There's a tendency among many beginning photographers to exploit the "professional" look of shallow DOF by always choosing the largest possible aperture and then shooting away. The first time you acquire a big, fast lens this power can be intoxicating.

(If you want a fancy term to toss around at cocktail parties, when you focus on an object beyond what is known as the hyperfocal distance, you produce a depth of field that extends to infinity in the background).

In the series of guitar images, we've seen DOF go from infinity in Image 1 to a thin slice just 7 inches deep in Image 4. This is why we spend the money for SLR cameras and those big, heavy lenses with wide apertures. Among other benefits, that cumbersome professional gear gives us the ability to control the depth of field.

I'm not going to explain the math of why this happens. All you need to understand are the following general rules of thumb:

This article describes how to calculate the effective magnification and the size of your sample on your monitor when using a digital camera on a microscope.