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September 2009 | Margaret Brown
Photographers often use the Japanese term, ‘bokeh' to describe the quality of the out-of-focus areas in an image produced by a camera lens using a shallow depth of field. Though it's difficult to quantify, some lenses produce more attractive bokeh than others.
Bokeh is most important for lenses that are used for portraiture and close-up or macro photography. It's also relevant for long telephoto lenses that are used for photographing wildlife because they are often used at wide lens apertures.
An example of attractive bokeh. Out-of-focus areas appear soft and natural-looking and don't distract the viewer from the main subject.
The shape of the lens iris (which controls the aperture) will influence the subjective quality of bokeh and also some of its objective parameters. Irises that close to a circle will produce more pleasing bokeh than polygonal irises. Mirror lenses typically produce ‘donut-shaped' highlights because part of the optical system consists of a ring of reflecting material that bends the light patch, thereby enabling shorter lenses to be manufactured for very long focal lengths.
A typical mirror lens shot showing the donut-shaped rings around bright highlights.
This is an excerpt from Mastering Digital Photography Pocket Guide 2nd Edition.
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