Bokeh
Boken is the way in which out of focus light is rendered by a lens. Good bokeh is a smooth and natural looking fuzziness. Bad bokeh tends to create unpleasant artifacts.
A point of light that is in focus should appear as a point. A point of light that is out of focus shows up as a disk. A lens with good bokeh will render a uniform disk, both in colour and intensity. A lens with bad bokeh will render the disk with varying levels of intensity, often the edge of the disk will appear brighter than the middle.
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[edit] Good Bokeh |
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Click on the image for a larger version
Notice the softness of the out of focus pedals. They appear smooth and naturally blurry. The effect is pleasing.
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[edit] Bad Bokeh |
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Click on the image for a larger version.
In this photo, notice the stem above and to the right of the flower. It looks uneven, almost as if a double image has been rendered. It is an unnatural look and is (a bit) distracting.
The Good News About Bad Bokeh
Bokeh is not an effect that is consciously noticed by many viewers, and compared to focus, exposure, composition and depth of field it's overall effect on an image is minor. The bufferfly picture above is a perfectly nice shot of a bug on a flower, and the poor bokeh does not really detract from that central image.