The word ‘boke’ (ボケ味) originates from the Japanese language and is used to describe the blur in out of focus areas of an image. According to Wikipedia, a Western journalist changed the word into Bohkeh, in order to improve the pronounciation.
Bokeh indicates how the blur appears on parts of the picture beyond the focus plane. Bokeh is beautiful, when blurred parts on a photograph are of a fuzzy blur. The bokeh is not pretty, when you for instance see double lines at transitions from dark to light.Bokeh quality cannot be quantified well and that is why we show practice examples of Bokeh in our lens reviews.

The properties of the lens (focal length, aperture, number of aperture blades, but also aberrations of the lens) affect the bokeh, but also the shape, light intensity and color of the subject, the degree of blur (how far outside the zone of depth of field) and the lighting conditions (high/low contrast) make a difference. Harsh lighting and a troubled background may provide a less beautiful Bokeh. Below is a picture of a bottle and a jar with two glasses in the background. On its right is a detail of this picture, in which we show the Bokeh of a lens in our Lens reviews.

Circular Bokeh

Bright telephoto lenses with an aperture of 2.8, or preferably even brighter, are often known for their beautiful Bokeh. The larger the sensor, the easier it is to get a nice Bokeh. Therefore, you have a better background blur when you take a picture with a lens on a camera with a full-format sensor compared to a recording made with the same lens at the same aperture and the same focal length, but on a camera with an APS-C sensor.

Diaphragm Bokeh

If you continue to diaphragm, the shape of the aperture becomes visible in the Bokeh. Far from all lenses have a perfectly round aperture. There are lenses with 3 to 9 aperture blades. The Bokeh can take on different shapes depending on the lens (pentagon, hexagon, etc.)

Bokeh & Vignetting

With lenses where vignetting occurs at the edges, you can also see the shape of the Bokeh change: Due to the light, the Bokeh changes from a perfect circle to a sort of rugby ball.

Color Bokeh

With bright lenses (2.0 or greater) color dispersion may occur, causing green and purple stains in the blur. This effect can be very annoying, but it is also an excellent tool for manual focusing: red and blue (magenta) are before the focus point and green is behind the focus point.

Diffraction Star Bokeh

Very bright point light sources in the out of focus areas can turn into diffraction stars, which create a very distracting Bokeh.

imitate Bokeh in Photoshop

Fine examples of Bokeh can be found in magazines in sports photography or fashion photography. The art of getting a nice background blur is to use a telephoto lens with an open aperture and narrow depth of field. There are several very expensive lenses, such as Canon EF 85mm f/1.2L II or the Micro-Nikkor 105mm f/2.8, which are famous for their bokeh.

But how do you get the background blur to be as nice if you do not have a full frame SLR with such an expensive telephoto lens, but for example a compact camera instead where even at aperture 4, the shots are almost in focus from front to back? With the Bokeh filter of Alien Skin, for example.