Carl Zeiss Explains Bokeh

The subject of Bokeh is a fascinating one and a lens with good bokeh is prized by photographers. Bokeh is a Japanese term used to describe the quality of background blur produced by a lens.

Note that Bokeh is not just about background blur but more specifically about the quality of that blur.

Yes, background blur can be good or bad. It’s good if it’s smooth and does not distract from your main subject. It’s bad if it’s those bright circles of light (popularized by graphic artists) and that distracts from your main subject.

The famed lens manufacturer, Carl Zeiss, has published a document (by H. H. Nasse) on Bokeh and it is well worth a read. It’s a bit very technical in nature and read like a translated text. If you slosh through it, you may learn something. He lost me somewhere on Page 3, my eyes glazed over and it was all downhill from there on. There are lots of beautiful graphs and equations for those of you who like them.

Fast forward to Page 40 for the “rules about bokeh” (which we reproduce here):

  • Some caution is advised when making judgments about the bokeh depending on the lens correction, because bokeh is extremely variable.
  • The correction balancing has an especially strong influence on the blurriness of the rendition at small deviations from the focal point. If there is a lot of blurriness, it usually becomes more and more negligible.
  • The aperture has a strong influence; even closing the aperture a small amount can cause very visible changes to the nature of the blurriness. Slower prime lenses generally have smaller spherical aberration by nature. So it is no wonder that their bokeh is praised for its appeal.
  • The spherical aberration of a lens also changes depending on the imaging scale. Bokeh characteristics therefore depend on the focusing distance as well.

If it’s all clear to you, please summarize the takeaways from this article. Speaking real s-l-o-w-l-y will be much appreciated.

Download the article [1.8MB]

Related Links: