by Dave Johnson, PCWorld  

Folks new to photography tend to think that taking a good photo is all about sharpness, but that's only part of the truth. The subject should be in sharp focus, of course, but often great photos balance that with an out-of-focus background. Blurry backgrounds can bring attention to and emphasize the main subject. This week, I'd like to talk about the blur itself.

What is bokeh?

Not all blur is created equal. In fact, photographers spend a fair bit of time talking about the relative quality of the blur in their photographs, which is a property known as bokeh. (Bokeh is a made-up word that mimics the Japanese expression for haze.) Bokeh isn't the amount of blur. Instead it describes the appearance—even the "feel"—of the blur. If that sounds somewhat intangible and poetic, it is. Nonetheless, photographers tend to agree that there is both good (attractive) bokeh, and bad (unattractive) bokeh out there.

So what makes bokeh good or bad? What does good and bad bokeh look like? These are great questions, but before I answer them, let me take a step back and talk about how blur happens to begin with.

What is blur?

You probably already know that your camera lens can focus sharply only at one distance at a time. In a zoom lens, for example, each focal length can focus at a specific distance. Every other distance—closer or farther—appears blurry. That blur is caused when the light rays that bounce off objects in the scene and pass through the camera lens don't all line up properly on the image sensor. And as these light rays pass through the lens, they diffract around the lens aperture, causing each "blurry" point of light in the image to take on the distinctive shape of the aperture itself.

When you look at a photo with an out-of-focus background, you'll notice that it's composed of many blurry disc-shaped elements. The shape and attractiveness of these blurry background elements is what constitutes bokeh. Here I've zoomed in on a blurry detail in a photo so you can see what I mean.

The net result of this effect is that you can actually see the shape of the lens aperture in out-of-focus elements in your blurry backgrounds—but it's most apparent in bright reflections and light sources. Of course, lenses tend to be round, so these blurs are generally somewhat round as well.

You might think the effect would be subtle—and often it is—but bokeh can be quite pronounced. Bokeh generally isn't perfectly round, for example, because a camera's aperture is made from a bunch of metal blades that fan open and closed to make the lens opening bigger or smaller, as you can see in this photo of a camera lens.

Lens makers typically use lots of blades in their apertures to create a fairly circular bokeh. The fewer blades a lens uses, the more "geometric" bokeh can look.

Bokeh in action

Ready for some examples? I recently took some shots of my daughter that have a bunch of lights and reflections in the background, which makes this image a great playground for investigating bokeh. The following details show some of those reflections.

In this first shot, notice the roundness of the blurry elements, which can happen when the aperture is completely open, so the barrel of the lens (not the aperture) determines the shape of the bokeh.

Here's what the same scene would look like using a relatively inexpensive lens that has a five-blade aperture. Notice how the blue takes on an angular character.

Now contrast that with the same setup, only using a lens with a nine-blade aperture. The blur is much softer and almost as round as the first photo.

Here's a strange one. This photo shows donut-shaped bokeh—hollow circles. This is what you get when you use a catadioptric lens. The circles are hollow because catadioptric lenses use a telescope-like mirror and a small second mirror to reflect the image onto the sensor, and that mirror blocks the light from the larger primary mirror.

Good bokeh versus bad bokeh

So now that you've seen a few examples, perhaps we can answer the question: What is good bokeh?

Well, as in any artistic endeavor, it's subjective. But in general, photographers tend to like smoother, rounder bokeh—the kind you get from lenses with more aperture blades. It's no surprise, then, that high-quality short telephoto lenses (the kind favored by photographers for portraiture) typically feature nine-blade aperture blades, which yield very round bokeh.

While everyone is entitled to their opinion, I doubt you'll find many photographers who like the bokeh you get from a catadioptric lens. Not only are those donut-shaped blurs ugly, but they are too sharply defined. And that's another aesthetic consideration—is the blur distracting? The more sharply defined the edges of the blurry elements, the more distracting the blur will be in the photo. Blur should never steal your attention.

Come back next week, when I continue the discussion of bokeh—including ways to control the blur in your photos and how to simulate bokeh effects digitally in a photo editing program.