“Bokeh describes the appearance, or “feel,” of out-of-focus areas. Bokeh is not how far something is out-of-focus, bokeh is the character of whatever blur is there.
Unfortunately good bokeh doesn’t happen automatically in lens design. Perfect lenses render out-of-focus points of light as circles with sharp edges. Ideal bokeh would render each of these points as blurs, not hard-edged circles.
The word “bokeh” comes from the Japanese word “boke” (pronounced bo-keh) which literally means fuzziness or dizziness.
A technically perfect lens has no spherical aberration. Therefore a perfect lens focuses all points of light as cones of light behind the lens. The image is in focus if the film is exactly where the cone reaches its finest point. The better the lens, the tinier this point gets.
There are no perfect lenses, so one usually does not see these perfect discs.
Real lenses have some degree of spherical aberration. This means that in practice, even though all the light coming through the lens from a point on the subject may meet at a nice, tiny point on the film, that the light distribution within the cone itself may be uneven.”
Which is what I love. the variety and feel of different types of bokeh you can achieve with different lenses. Continuing our love of the imperfect, here is a bit of our variety today: