Swirly
Bokeh
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.
Drawing 1
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.
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.
Drawing 3
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.
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)