Bokeh for beginners

(From EOS Magazine - March 1998)

There's a new word entering the language
of photography - and its meaning is unclear!

It is relatively easy to decide whether or not a lens offers good sharpness.
But how do you define its performance with out-of-focus images ? And does it matter ?
Well, it does to Japanese photographers, and they have a word for it - 'bokeh'.

Bokeh is an approximation of the sound of a Japanese word - there is no equivalent word in English.
It refers to the quality of the out-of-focus areas of an image.
You probably thought that one out-of-focus image was much like another, but it is not. The appearance of the
out-of-focus image can be very distinctive, and can add to or detract from the effectiveness of the image.
Bokeh is big in Japan, where some photo magazines include the results in lens test alongside other performance
criteria. The problem is that bokeh is - at the moment - purely subjective. No one has yet come up with a reliable
method of measuring the effect, and translating this into a quality index. It is also very difficult to describe in words.
German lenses are praised for good bokeh, while many Japanese lenses, even from major manufacturers, are not
well regarded. Canon lenses seem to be an exception, and Canon uses good bokeh as a selling point in some of its
Japanese adverts. In particular, the EF 35mm f2 and EF 100mm f2.8 have good reputation in this respect.
It is unlikely that bokeh will ever replace sharpness as the main selling point of a lens, but certainly influences
our impression of lens performance.

The theory behind bokeh

Above: To understand bokeh, you need to know how a lens forms an image. In

simple term, rays of light from a point on an in-focus subject pass through the

lens and come together at a point on the film surface. This is repeated for all the

other points at the same distance from the lens.

Above: Rays of light from parts of the scene which are not in focus come

together at a point in front of or behind the film. When this out-of-focus beam hits

the film, it is a small circle (white area on diagram), rather than a point of light.

That, at least, is the theory. In practice, no lens is perfect

and the out-of-focus area is more likely to be

an ellipse

than a circle (top shape). Alternatively, it

may take on some of the characteristics of the lens aperture and be a

polygon - possibly with an irregular shape (bottom shape):

Also, the tonal range across the shape may not be even. It can fade towards the edge

(below left), or the centre (below centre), or from one side to the other (below right):

The causes of these variations are the usual lens aberrations - coma, spherical
aberration and chromatic aberration. These aberrations vary within a single lens
element, and are usually greater towards the edges, so bokeh may also vary
across the image. In extreme cases, a thin out-of-focus line might even become
two parallel lines.

"Good" bokeh seems to come from lenses which produce circular, smooth-toned
out-of-focus shapes across the film. There is nothing you can do to alter the
bokeh of an existing lens, though you may find it changes at different aperture
settings.