Bokeh

Bokeh is Japanese (boke, ボケ味). "blur" or "haze" is how it translates. In photography, bokeh is defined as the aesthetic quality of the blur produced in the out-of-focus parts of an image produced by a lens; "the way the lens renders out-of-focus points of light".

Bokeh should not be discounted as it is often a large part of the image.

Good bokeh is low in contrast creating a smooth blend of the out-of-focus parts of an image and the out of focus highlights do not have a halo around them. Bad bokeh is busy and distracting, taking away from the overall image.

Do not be concerned with onion-shaped bokeh highlights because this is caused by the cylindrical nature of a lens and is not indicative of good or bad bokeh. This is like looking through a paper towel roll, and as the roll is moved from one side to another then the image seen through it is seen as onion-shaped.

The shape of the diaphragm can have a negative affect on the bokeh highlights. For example, a diaphragm that forms a heptagon will have highlights shaped as little heptagons.

Bokeh is produced by circles of confusion.