So, what is bokeh? And why is it so cool all of the sudden? (Well, to some of us, it's all of the sudden.)
Bokeh is nothing new. Calling it bokeh is relatively new, though. It's been around since the beginnings of photography. Until recently, it was called "circles of confusion." Now, it's bokeh. It is pronounced bo'-kay, bo'-keh, or bo'-kuh. It's a Japanese word used to describe circles of confusion in an image. You can see it in this image, cropped out of a larger image for illustration:
What is it? What causes it? It's simply the by-product of focusing a cone of light through a round lens. In other words, the whole image is made up of circles of confusion. Those circles on the parts of the image that are considered "in focus" are very tiny and compacted (relative to the final size of the image and the distance from which it will be viewed). As you move out of focus, toward the front and rear of the subject, they grow larger. They appear to the eye simply as blur when they are small. However, as you move further from the camera and into the background of the image, they typically become larger. There are several factors that can influence this.
The first step is to understand depth of field. That describes the area of an image that is in focus. If just a small area of the subject is in focus, and ... No, on 2nd thought, the first step is to understand the purpose of a photograph, particularly a portrait.
Are you just wanting to record your subject's face? A photograph should tell a story. What story do you want to tell with the image you're creating? Are you wanting to say, "I'm a cool photographer. Look what I can do. I can make bokeh!"? Or are you wanting to tell something about them, who they are, what they do, what they value, what's happening at this point in their lives?
Okay. So what is it going to take to communicate that story? Do they need to be surrounded by the things that are important to them? The people, their home, their pet, some of their possessions, a favorite place, a beautiful landscape? How much stuff? How much of it needs to be in focus? How much needs to be toned down to make sure they are still the subject? How do you want to do that? Do you want to use more or less light in different areas of the image? Do you want to use greater or smaller depth of field? However you do it, you want to simplify. You want to call attention to your subject. Sometimes the story is all the stuff surrounding them. Sometimes the story is the bright print clothing they are wearing. Generally, though, that's not the case. That's why you want them to wear less distracting clothing, and you want to control the environment around them by the way you crop the image, the lighting you use, and the depth of field.
A camera can really only focus on one distance at a time - one point, or plane. However, you can control how much in front of and beyond that point is acceptably in focus, and/or appears in focus by controlling the depth of field. You do that primarily with the aperture settings or f-stops on your camera. Since f-stops are actually based on mathematical ratios, a small f-number represents a larger aperture opening in your lens. A larger number means a smaller opening. In other words, f-4 is a larger aperture than f-16. A larger opening (smaller f-stop) means smaller depth of field - less of the image will be in focus.
Another factor that influences this is the size of the lens. A wide angle lens and a telephoto lens have different effects. Outdoors or in the studio, a longer lens gets you further from your subject, and still controls depth of field. You can have shallow depth of field (and even bokeh) at higher f-stops with a longer lens. There are a couple of big advantages. You don't have to be right in your subject's face, which tends to make them more comfortable. You also get a more pleasing, realistic image - facial features aren't distorted as they are when you get close to your subject with a smaller lens (the old "dog-nose effect" where the nose is disproportionate to the rest of the face is fine for cartoons and humorous greeting cards, but not so good in a portrait). And, you reduce the amount of background clutter significantly. (By nature, a long lens compresses distance - makes things in the foreground and background appear closer to each other.*)
Any of these factors can result in texture in the background appearing as circles of confusion, or bokeh. The goal of a portrait, or any photograph, is to call attention to the subject and tell a story. If the side effect of doing that is circles of confusion, fine. But unless your goal is to show some cool effect created by blurring an image for the purpose of creating bokeh, bokeh in itself doesn't really mean a thing - it's the by-product, not the goal.
*When you're watching a chase scene in an old western, have you noticed how close the riders appear to one another when they're coming at the camera, then how far apart they are from a side view? That's because a telephoto lens was used for the head-on images so they don't have to keep moving the camera to stay ahead of the action. The side view is done with a normal or wider lens.
