The word bokeh has been borrowed from the Japanese dictionary. It signifies the quality of the out of focus effect rendered to the background (and the foreground) in an image. It does not merely suggest the out of focus effect itself. So, there is a subtle difference between the two. You may have seen this effect in your own images and are quite familiar with it. Sometimes, inadvertently blurring the background and or the foreground when using a large aperture like f/1.8 or faster.
Bokeh, and therefore the quality of the blur, depends on more than one aspect. In this discussion, we look at the parameters that govern bokeh.
Aperture
The Size of the Aperture has a bearing on Bokeh
The first and the most important aspect that controls bokeh is the aperture of the lens in use. Aperture denotes the size of the opening of the lens. With a lens that has a wide aperture, such as f/1.8 or faster, the chances of getting a better quality bokeh is higher. The reason is with a wide-open aperture the lens offers a very small depth of field, which accentuates the whole background and foreground blur.
With smaller apertures, anything starting at f/3.5 and smaller, you get a larger depth of field. And that defeats the whole purpose of trying to capture bokeh.
Depth of Field
A term that we just used above is the Depth of Field. When you focus on a subject that subject appears tack sharp in the image. This is easy to understand. However, if you notice carefully, there is an area just in front and behind the point of focus which also appears to be acceptably sharp. This ‘acceptably sharp’ area will increase or decrease depending on the aperture that you use. This area is known as the Depth of Field.
Subject to Background Distance
Further the background from the point of focus, blurrier it will be
The more the distance between the background and the subject, the better the quality of bokeh that you are going to get. This is because the plane of focus (which is an imaginary line running through the point of focus) becomes quite distant from the background. In other words, the background falls outside the depth of field and is therefore completely blurred.
If you have no way to change the subject to background distance, step closer to the subject to change the ratio between the distance from the camera to the subject and subject to background. That also has the same effect as increasing the distance between subject and background.
Aperture Blades
Believe it or not, the number of diaphragm blades that make up the aperture also has a significant bearing on bokeh. More the number of blades, the rounder and smoother is the quality of the bokeh. Why? Because with a higher number of aperture blades it is much easier to produce a ‘rounder’ circle.
We can cite an example to explain this. Let’s take an image of some Christmas lights. But don’t focus on the lights themselves. Focus on something in the foreground. And use the widest aperture possible on the lens.
Scrutinize the image. Enlarge the image on your computer screen to its full size. Take a look at the lights. Can you see the blurry Christmas lights? If you look closely you can count the number of sides. These are commensurate with the number of aperture blades in your lens. The more the number of blades the smoother the blur would be.
Cheaper entry-level kit lenses don’t have as many lens blades as the more expensive ones do. That is why they create a weirdly shaped bokeh. The best bokeh is usually produced by prime lenses that have 8 or higher aperture diaphragm blades.
Solid Background vs. Textured Background
Textured backgrounds are the best when it comes to bokeh. Why? Because textured background produces a nice uneven background and therefore produces a much more interesting background than one with a solid one.
Shooting Mode
Apart from choosing a fast wide lens and a prime to be precise, it is imperative that you choose aperture priority mode or manual mode to shoot your images. Why? Because these are the two modes that allow you to select your own aperture for shooting. As has been detailed above, unless you are shooting wide open (fast aperture) you cannot hope to get a decent bokeh going. For that, it is imperative that you have a fast wide lens and you are able to control the exact aperture being used.