What Is Bokeh?

clipped from www.vanwalree.com

Usually, the chief concern of lens designers is the best possible image quality of the plane of sharp focus. The rendering of out-of-focus (OOF) image parts does not enjoy a large weight in the overall design compromise of a normal photographic lens. However, the OOF blur characteristics mattered to certain Japanese photographers who introduced the term "bokeh" to the photographic society to describe the aesthetic quality of the blur. In the absence of a single English word with the same meaning, there seems no reason not to adopt the Japanese term. The internet abounds with lens qualifications like "good bokeh" and "bad bokeh" but strictly speaking this use of the word should be discouraged. Owing to the subjective implications of some unquantifiable aesthetic value, it would be more appropriate to speak of pleasant or unpleasant bokeh, respectively.

Characterization of the blur disk

Since any image is represented by a large number of images of points, we may attempt to understand the whole by considering the blurring of a single point. An unsharply imaged point is associated with a circle of confusion, or a blur disk. This blur disk is characterized by

1. A size.
2. A shape.
3. The light distribution across the disk.

The size of the disk determines the "amount of blur". The shape of the blur patch does not need to be circular, in which case the designations "circle of confusion" or "blur disk" are misnomers. Nonetheless, for convenience the word disk will be freely used to mean a patch of arbitrary shape. Although the size and the shape of the disk are unmistakable blur characteristics, they do not touch the essence of bokeh as the Japanese intended the word. The distribution of light across the disk does [1]. However, the distinction is not always clear and what follows is intended as an overview of a variety of factors that influence the rendering of OOF image parts. Explanations of the underlying mechanisms will be brief and the reader is referred to other pages for elaborateness.

Amount of blur

It is well known that the amount of background or foreground blur is controlled, among other things, by the F-number. Fig. 1 shows a picture taken at a small and at a large aperture. The larger aperture comes with a more blurred background, but the question that needs to be answered to define the bokeh is not to what degree the background is blurred, but whether the blur is a pleasing one. In this case, the Japanese would probably speak of a neutral bokeh.

Figure 1. Gromit captured at f/22 (top) and at f/4 (bottom).

Shape of the blur patch

It is also well known that out-of-focus highlights (OOFH's) assume the shape of the lens aperture. At reduced apertures the shape of the blur disk is the same as that of the diaphragm opening. For instance, a six-sided diaphragm leads to hexagonal blur patches. Generally, the better an aperture approximates a round opening, the more pleasing the blur. However, when a lens is used at a large aperture, obliquely incident light is confronted with a narrower aperture than normally incident light. Consequently, the blur disk narrows from the image center towards the corner. This is known as the cat's eye effect, a result of optical vignetting. When there are many OOFH's scattered across the frame, the cat's eye effect yields the impression of a rotational background motion (Fig. 2).

Figure 2. Optical vignetting creates a sense of rotational motion of the background around the street sign. Photograph by Edo Engel.

There is more to this article that I felt is a lot more complex to understand and therefore out of the remit of this blog. You can access the rest of it by clicking on the link at the beginning of this post.
Sumit