When using the word "bokeh", one normally means the nature of out-of-focus rendering. How the lens renders objects that are out of focus, the bokeh, depends on a number of factors, for example, the lens itself, the objects photographed, and settings like the focus distance and the aperture.
It does not make sense to say that "a lens has bokeh", or that "there is bokeh in a photograph". Rather, the term can be used to describe the out of focus rendering, e.g., "this lens gives me good bokeh", or "in this photo, the bokeh is distracting".
First of all, note that the term bokeh is only used about the parts of an image which is out of focus. So the first thing we must discuss is simply focus.
It might sound counterintuitive that we talk so much about objects that are out of focus. Isn't photography about taking pictures of things that are in focus? Who would want to keep anything out of focus in the first place?
We shall see examples where having some parts of the image out of focus makes sense, and hence, that the bokeh, the rendering of out-of-focus areas, is important. The first example below is taken with the Panasonic Leica 45mm f/2.8 macro lens:
In this example, we see that in the first image, taken with a large aperture, the foreground is in focus, and the background is out of focus. The second image is taken with a small aperture, a large f-number, leaving the background more in focus. I think most would agree that the first image is the best, because the background is less distracting.
For the image taken with the large aperture f/2.8, I would say that the bokeh is smooth. By this, I mean that the background which is out of focus is rendered in a way which blurs it effectively, and does not cause any distraction.
This is commonly called selective focus. By using a large aperture, we can choose what should be in focus and what should be out of focus. In fact, this is one of the advantages of using a camera system with a reasonably large sensor.
A compact camera has other advantages, for example size, and that you don't need many accessories. Take the Panasonic Lumix TZ-series, for example. It has a generous zoom, going from very wide angle to long tele, and also a macro function. All in a very small package. But since the sensor is small, it cannot do selective focus as in the example above. A small sensor gives a large depth of focus, which is good for some applications, but makes it more difficult to blur the background.
In the second image, I used the smallest possible aperture, f/22. Generally, one should be careful with using so small apertures, due to diffraction.
Setting the focus is one of the important parts of taking a photo. It is usually the first thing you would do when composing an image. However, with modern cameras, this is almost always set automatically, through autofocus. Further, many modern lenses don't have any focus scales. So the concept of focus might seem more abstract to people who have not used manual focus camera systems.
In the image above, the focus distance is illustrated as the distance from the camera, more specifically from the sensor plane, to the object where the focus is placed. When taking a picture of a person, it is normal to focus on the eyes of the subject. Many Four Thirds Cameras have face detection, and automatically choose to focus on the eyes when photographing people.
Having set the focus distance, anything in the focus plane is in focus, while items closer or further away are out-of-focus. The focus plane is defined as the plane normal to the lens axis, at the focus distance.
However, focus is not a binary concept. An item is not either in focus or out of focus. So what I wrote in the paragraph above is not true: Any item outside of the focus plane is not simply out of focus. It can be in focus to various degrees, depending on a number of parameters.
Another mistake above is that not all lenses feature a focus plane. With some lenses, the plane of focus is curved, e.g., spherical. A curved focus plane makes sense for portrait lenses. With a portrait lens, you would normally focus on the eyes of the subject, while keeping the eyes in the center of the image, and then reframe the image. However, with face detection and multiple focus areas, there is not a big need for a curved focus plane anymore.
After setting the focus distance, the most common way to control what's in focus and what's not, is the aperture. A large aperture, e.g., f/1.4, will give a relatively shallow depth of focus (or depth of field, DOF), meaning that only points that are some short distance from the focus plane are in focus. On the other hand, a small aperture, denoted by a large f-number, makes more of the image in focus. This can be illustrated like this, using our previous example:
In this example, the left figure is holding the camera, and focusing on the eyes of the figure to the right. Using an aperture of f/5.6, the depth of field (DOF) covers the figure (green area), but not much of the foreground or background. Using a smaller aperture, f/11, the DOF now covers parts of the foreground and background as well (blue area).
This illustration is just an example, and not to be used in any real life application.
Older manual focus lenses typically include a focus scale with a depth of field indication. Here is one example, a Nikkor 50mm f/1.8 AIS lens:
This lens is a classic manual focus normal lens from Nikon. It is compact, reasonably good optically, and cheap. As seen above, it is connected to a Micro Four Thirds adapter, so that it can be used on M4/3 cameras.
In the image above, the focus ring is set to about 7 meters, 22 feet. The aperture ring is set to f/11. The silver ring in the middle features a crude DOF scale. Notice that the aperture f/11 is set in a yellow colour. There are two yellow markings in the DOF scale as well, indicating the near and far limits of the depth of focus. As the lens is set above, the image is in focus from about 4 meters up until infinity (the rotated 8 sign). Hence, in this case, 4 meters is referred to as the hyperfocal distance for this lens, using the aperture f/11. If I had set the aperture to f/22, the hyperfocal distance would be 2 meters.
Very few Four Thirds and Micro Four Thirds lenses feature any focus scale at all. And even fewer feature a DOF scale. One notable exception is the Olympus M.Zuiko Digital 12mm f/2 wide angle lens, which has both a focus scale and a DOF scale, a first for a Micro Four Thirds lens.
With other lenses, you must investigate the depth of field by looking it up on the Internet. There are a number of online DOF calculators out there, you can find them by searching for the term "dof calculator".
Using the online DOF calculators, you will find that the depth of field depends on the aperture (as we have seen above), the focus distance, the focal length, the size of the sensor, and the circle of confusion. The last term may sound a bit strange, but it simply says how strict the definition of "in focus" is. If you use a large circle of confusion, this means that you are not very strict about what you consider to be proper focus, and the depth of field will be fairly large. And vice versa.
Setting the aperture to affect the DOF
Based on what you have read above, or perhaps this was known to you already, setting a large aperture gives you a small depth of focus, and vice versa. This can be illustrated with an example.
For this example, I used the Pentax FA50, a 50mm f/1.4 film era lens. It is an autofocus lens, but when used with an adapter on the GH2 camera, the focus and aperture must be operated manually. It also has a focus scale and a simple DOF scale.
This lens is quite cheap, and is considered to be reasonably good. It has later been discontinued, and replaced by a newer 55mm f/1.4 lens, which is the portrait lens for Pentax DSLR cameras at the moment.
Using the lens on a GH2 camera, I took this series of images (click to enlarge them):
As you can see, the focus was set on the first plank, to the left. With the largest aperture, f/1.4, the depth of field is very thin, and only the tip of the plank is in focus. As the aperture gets smaller, more and more comes into focus.
A smaller aperture not only makes the depth of focus wider, it generally also improves the optical qualities of the lens. Generally, the sharpness and vignetting characteristics improve as you stop the lens down. However, due to diffraction, this rule does not apply for the smallest apertures. Usually, the optimal aperture in terms of sharpness is around f/5.6-8, due to diffraction effect at smaller apertures.
However, if you need very deep depth of focus (DOF), I think you should still consider using very small apertures, like f/16, despite the negative impacts of diffraction. As long as you don't print very large copies, the diffraction effects are generally not very disturbing.
In this example series, the out of focus appears to be quite ok, so it makes sense to say that the bokeh is pleasing. However, this example was perhaps not the best if the goal was to evaluate the bokeh. Let's look at another example.
Out of focus highlights
The most challenging out of focus type for a lens to handle, tends to be small highlights in the background (or in the foreground). I've made one such example. Here, the focus is set on the centre of the frame. There are out of focus highlights in the background, due to rays of the sun hitting the foliage:
In this case, we see clearly that at the largest aperture, the out of focus highlights are being rendered like small donuts. We say that the bokeh has ringing effects. Since the human eye/brain is very keen to find edges, this ringing effect is very distracting. The eye is constantly drawn to the background, which was not the intention. After all, the background was left out of focus to make it less dominating. So you can see that this is a highly undesired bokeh type.
The donut-shaped bokeh is typical of mirror tele lenses, also referred to as catadioptric telephoto lenses. In fact, this side effect of mirror lenses is one of the reasons why this type of lenses has not become very popular.
For the Pentax FA50 lens used here, the problem appears to be mostly for the very large apertures. Here is an enlargement of the top left corner for the apertures f/1.4-4:
With the latest example of poor bokeh, a short summary of this first article in the series about bokeh is:
One advantage of large sensor camera systems like Micro Four Thirds is the possibility for selective focus, letting the background or foreground go out of focus to make it less distracting.
However, look out for the bokeh, the rendering of the out of focus areas. The bokeh may have artifacts which makes the out of focus areas stand out, rather than blur, which is normally the intention.
Getting an adapter and a large aperture legacy normal lens is a common way to explore the area of thin depth of focus (DOF). But if the lens produces bokeh which is distracting at the large aperture, you may not be able to use it anyway. So it pays to research this aspect before buying.