Bokeh is a term in photography that refers to using out of focus parts of the picture for artistic and esthetic purpose. The source of the word is in Japanese. In some lenses, under certain conditions, out of focus background looks as if it was swirling around the center of the photograph.
Here is an explanation of the way this effect is created.
The beam of light coming from a single point and passes through a converging lens converges in a focal point, as shown in drawing 1. Since the shape of the lens is circular, any cross section of the beam parallel to the lens plane is circular. A point that is out of focus will therefore look like a circle.
In a camera, not the entire beam that passes through the front lens reaches the photographic plane, but only the part of it that passes through the aperture, as shown in drawing 2.
An out of focus point or small circular object, will show as a spot shaped like the aperture as seen in photographs 1 and 2.
Photograph 1 Photograph 2
Polygonal aperture (approximately) Round aperture
(Kathleen Frank) (Ajit Anthony Perm)
Depending on the optical and geometric design of the lens, it is possible that when the aperture is wide open, it is not entirely covered by the beam coming from the front lens, as shown in drawing 3. In such case, the beam passing through the aperture is bounded by the aperture on one side and by the front lens on the other side. The shape of its cross section will be the intersection of two circles. Such condition is only possible for points that are not near the lens axis.
When such condition exists, out of focus points around the center of the picture will look like circles while points out of the center will look flatten radially. As seen in photograph 3.
In a photograph taken with a lens under such condition which has out of focus background that consists of small objects like leafs of a tree, the flattened shape they get makes a sense of whirl or a vortex.
)posted under the name Kookieman)