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Bokeh

Introduction:
Bokeh comes from the Japanese word Boke meaning blur. It is used as a photographic term to describe the rendering of out-of-focus areas of images. Bokeh refers to the aesthetic qualities of this rendering and varies from lens to lens.
 
Bokeh:
Bokeh tends to come into play when you shoot with a fairly wide open aperture. This creates a lot of out-of-focus points of light in your image. Bokeh refers to how these points are rendered.

Bokeh has been referred to as good and bad. These terms are quite general, but it is assumed that lenses that produce smooth blurred out-of-focus areas have good Bokeh, while lenses that create harsh and noticeable out-of-focus points of light are referred to as producing a bad Bokeh. When you see bad Bokeh, you are essentially seeing large circles of confusion. When you see good Bokeh, these circles flow and blend into one another and are not visible.

You need to worry about Bokeh if you shoot a lot of portraits, macro, use a long range telephoto lens, or shoot with apertures wide open. In photography it is generally preferred to have in-focus foregrounds and out-of-focus backgrounds.

This is an example of good Bokeh. Notice how the background is very smooth and blends together. In an image with bad Bokeh you would be able to see obvious circles and the background would look quite harsh.

This is an example of when Bokeh starts to get worse and more noticeable. The out-of-focus background now has some apparent circles of confusion also called blur circles. These would get more pronounced and harsh with a lens with bad Bokeh.
 
Bokeh Types:
A point of light in focus will yield a point on the imaging plane. A point of light out-of-focus projects a cone of light onto your sensor (instead of a point). Since all lenses contain some sort of spherical aberration, these out-of-focus points are rendered differently from lens to lens.
 
Bad BokehThis is a very magnified example of bad Bokeh. This is how out of focus points of light will be rendered. The edge is very defined and when a background is filled with this type of Bokeh it is very harsh. Mirror lenses tend to produce this type of Bokeh. Here spherical aberration is working against the image.
Neutral BokehThis is neutral Bokeh. This is the result of an optically perfect lens with no spherical aberrations, which renders the light in a blur circle evenly all around. There is no such lens, but some come close. This is not very good for a pleasing and smooth background either, but it's better than the type above.
Good BokehThis is an example of good Bokeh. The out-of-focus point of light is rendered very gradually. When a background is filled with this type of Bokeh it is very soft and flows together quite well. Here spherical aberration is for the image.
Be aware that most modern lenses are designed to be as near perfection as possible and will often yield results that resemble neutral Bokeh. Good Bokeh is proof of lens imperfections though it will create better backgrounds. You can't have an optically perfect lens that gives good Bokeh...no such thing. Some lenses, especially macro and long telephoto are designed to produce images with good Bokeh since the occurrence of bad Bokeh can easily ruin these types of images. Keep in mind that just because a lens produces good Bokeh, does not mean it's a bad lens.
 
Think about Bokeh when you are shopping for a lens that will be used with the aperture wide open, especially in macro and long telephoto shots.

Cheers

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