Boke, sometimes incorrectly written as bokeh, is a Japanese term for the visual appeal of the out of focus background in an image.
When you photograph a subject up close and include some background, the background will be in focus depending on distance, the length of the lens, and the aperture of the lens. Boke, the Japanese word for blur, is used to describe the background blur and how enjoyable the blur might be.
Boke is Japanese and is pronounced with the bo as in bone and the ke as in Ken.
Boke was also occasionally used in old English for book, is the name of a town, a geographic region, a sport, used to mean
thrust, and has some slang meanings.
You control background blur, boke, by controlling aperture. Place your camera on a tripod then aim the camera at a row of plants and trees leading from your feet to the horizon or some long range of distances. Switch your camera to aperture control and set the camera to the maximum aperture. Snap. Now switch the aperture back one click. Snap. Step through the whole range of apertures on you camera. Look at the result to see the parts in focus and the parts on boke.
It does not matter about the total range or how many click stops you have. A click stop might be half an aperture increment or one third and the difference is rarely important. The minimum aperture is usually enough to keep everything in focus and the largest aperture on a good lens will through the background out of focus.
Wide angle lenses rarely throw anything out of focus so use a medium or long lens. really cheap cameras with tiny apertures rarely throw anything out of focus and boke is rarely an issue outside of professional lens. You want a medium to long lens with a medium to high price tag to have the option to induce significant background where the quality of the boke might become relevant.
Background blur has subjective value in context. Strong blur helps to isolate the central subject of an image but destroys the context for the subject. Think of a picture of an insect. If the background shown bright flowers then you know the insect lives on flowers. The same leaves in the background of a wedding portrait might distract people from the wedding couple.
Now think about an expensive wedding on an exotic tropical island versus a wedding in the local botanic gardens. The people using the botanic gardens might not want all of the background in focus especially if there are rubbish bins and public toilets. By contrast, the person paying for the tropical island wedding might insist that all photographs include the tropical flowers in the background.
Background blur is subjective
Pretentious camera reviews can use boke as an excuse to dismiss useful medium price lenses and promote expensive brand names. In 99.89 percent of photographs there are things far more important than boke. 99.89 percent of camera lens reviews have failed if they resort to listing boke as a deciding factor.
When you are out there taking images, critical issues are how fast you can focus and a bunch of other really important issues. Using or not using background blur is not important if you miss the photograph.
The one big difference between background blurs is a curious set of circles produced by mirror lenses. Mirrors lenses are usually used in telephoto lenses to save length and weight. When you are photographing the one in ten years appearance of exotic birds on the island in the northern part of Lake Eyre, do you want to use a large fast mirror telephoto lens that occasionally produces little circles in the background or do you want to use a conventional telephoto lens with good boke but is physically too heavy for you to carry on the long trek across to the island and costs so much money that you have to skip this year's opportunity because you cannot afford the lens? The boke of mirror lenses is far less important than the physical ability to take the photograph.
The length of the lens decides how close you can get to the image without actually being close. You decide on the lens length based on a lot of reasons but not boke.
Long lenses, when used at medium distances, produce stronger background blur typically because there is a bigger distance between the subject and the background. Think about elephants out on the open plain with trees in the distant background.
Long lenses used at maximum distance rarely produce boke because the subject is close to infinity as is the background.
Lens maximum aperture
The maximum aperture of a lens places several limitations of your use of a lens. A large aperture is the most flexible but is also the heaviest and most expensive. Weight will limit the maximum aperture for wildlife photographers and price will limit most non professional photographers. The boke of the lens is the last thing you need to consider if your budget is less than a few hundred thousand dollars.
Faking it with software
There is software that produces fake blur. Real background blur depends on the distance to the background and individual items in the background can be at different distances. Software does not know the distance to each item in the back ground. Software produces just one even background blur. In some cases you can tell the blur is fake. In other cases the fake blur just looks wrong.
The secret to faking effects in Photoshop is to get the image right in the first place so there is little to do at the image editing stage. If you start with rubbish then the Photoshop user will use lots of techniques that rarely work perfectly and the overall result will look fake. Background blur is one of the easier things to create right in the camera. Leave Photoshop for removing skin imperfections from the model.
An understanding of boke (bokeh and some other spellings) is useful but you will rarely be in a position to choose between two lenses based on boke unless you are such a successful photographer than you can afford to buy both lenses.