Lens Bokeh Explained:

Bokeh, boke, blur, haze, haziness, dizziness . . . what? . . . :

Bokeh or Boke refers to how pleasing the out of focus areas of an image look to a viewer. If the out of focus area enhances the final image then the bokeh can be considered good. Bokeh does not mean “depth of field” nor does it have any relationship with vignettes like a lot of people try and tell you. Bokeh is something you can judge when a shallow depth of field is used in a photo but does not have any bearing on the depth of field itself. Bokeh is a Japanese word and means “blur” or “haze” and is pronounced similar to “bouquet” like a bouquet of flowers. You could consider sharpness and bokeh as lens qualities that exist at the opposite end of the “lens” spectrum. As sharpness is a quality you would look for at the point of focus of an image, bokeh is what you would look for in the areas out of focus.

Typically, on good fast professional lenses bokeh will be best. When I say fast lenses, I mean those with wide apertures, f/1.2, f/1.4, f/1.8, f/2, f/2.8 for example. The reason is twofold. Firstly, these high-end lenses, when used at their lowest aperture produce images with a very shallow depth of field. This means that your subject, when on a different focal plane to the rest of the image, will appear in focus while the rest will be out of focus. The further your subject is away from the background, the more out of focus the background will appear. Secondly, these professional grade lenses are constructed such that their aperture is controlled by a large number of diaphragm blades and the lenses themselves are corrected for spherical aberration. The larger the number of blades, the more round the aperture and as a result the more pleasing, spherical, smooth and blended the out of focus areas of an image will appear.

A lens with good bokeh can be determined by the smoothness at which the out of focus areas blend in with each other. Bad bokeh lenses on the other hand produce results where the out of focus areas still retain edges or at least retain more of their true form and texture. Bokeh is also dependent on the out of focus content. For example autumn leaves lightly lit by the sun or lights on a Christmas tree will give a different feel to the final image from a bokeh perspective. The leaves would have smoother more subtle bokeh whereas the lights, because they have greater contrast with their surroundings, will have a more pronounced and dramatic look in the image. However the ultimate goal is to ensure the out of focus areas of an image are pleasing and add depth and quality to the final image.

You can experiment with different bokeh shapes. Earlier I mentioned that the number of diaphragm blades determines the shape of your out of focus lights for example. The more blades, the more round the lights will appear. Nine bladed lenses produce round bokeh and in the case of Nikkor professional telephoto lenses the diaphragm blades are curved to more closely represent a circular shape. As you reduce the number of blades you start to get shapes. So 7 blades will give you 7-sided heptagons, 6 blades will give you hexagons, 5 blades give 5 sided pentagons and so on. But you are not simply limited to shape just because your lens has a certain number of diaphragm blades. You can for example create (artificially) different shapes by placing a shape in between your camera sensor and the out of focus background. Hearts, crosses, diamonds, Christmas trees (right) all sorts of shapes can be created simply by cutting the shape into a piece of black card that you place on the front of your lens.

In terms of lenses, diaphragm blades are the main consideration when determining the shape of the out of focus light discs. Personally I like rounded discs of light that are evenly lit from the centre to the edge of the disc. Bad bokeh lenses produce light discs that are brightest at the disc edge and darken towards the centre. The out of focus light discs tend to look more like donuts of light than discs on bad bokeh lenses. Good bokeh is a quality especially important for fast, wide-aperture lenses, telephoto zoom lenses and macro lenses. The reason is because they are used most often with a shallow depth of field and if your shallow depth-of-field images have poor bokeh then it will detract from the quality of the images you can produce with that lens.

If you want to test your lens bokeh qualities then all you have to is choose the lowest aperture on your lens and the highest focal length. For example on a 70-200 f/2.8 telephoto you would choose f/2.8 with 200mm focal length. The reason for choosing the max focal length is because it will compress your composition and will give you the most shallow depth of field. All you need then is a subject and a background. Your subject should be well lit, your background should have nice contrast with lights or textures that will be noticeable in the image when not in focus. Street lights or shop window lighting provide good backgrounds, essentially anything with light and dark is what you are after. Then you need to position your subject in relation to the background. The further your subject is from the background the more out of focus the background will appear. Also, the closer you are to your subject the better. With longer focal lengths you will give your image a more compressed look and this is better for your out of focus backgrounds. Shorter focal lengths on the other hand will have poorer quality out of focus areas as you have less depth of field and less compression.

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