We spend a lot of time concentrating on the quality of the sharp areas in our photographs, and of course it’s always good when our subject is pin sharp just where we want it to be. But out of focus areas can often be very important too – and in some types of image, an out of focus background can be just as important as the subject of the photo itself. Think of a flower against an out of focus blur of other flowers, or a portrait of a person against a pleasingly blurred backdrop – in cases like these, the background can make or break the photograph.
The aesthetic quality and character of the blur in out of focus areas of an image is known as bokeh. In out of focus areas light spreads or diffracts into round discs, which are rather wonderfully known as circles of confusion. These circles will take on the shape of the lens aperture, so depending on the number and shape of the aperture blades this may vary from a perfect circle to a polygon. In general, a perfectly round aperture is thought to produce the most pleasing bokeh.
It’s easy to recognise a photo taken with a mirror lens, because the internal structure of these lenses results in out of focus highlights having a characteristic ring doughnut shape.
The quality of the bokeh will also be affected by how well the lens is corrected for spherical aberration. Depending on this, the circles of confusion may be uniformly illuminated discs, or discs which are brighter at their centres or at their edges.
Much has been written about what is good bokeh or bad bokeh. In the end though, I would say that out of focus areas that add to the beauty of your image are good bokeh, while areas that are unpleasing or distracting are bad. But, like so many questions of what makes an image artistically good or bad, the quality of bokeh is a subjective thing!