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The other day, while I was out riding my old Schwinn, I had a work-related "Ah-hah!" moment despite the fact that the B&H Employee Handbook explicitly prohibits doing company business on personal time. But when one enjoys what one does for a living, it invariably spills into one's personal life, even when riding an old bike along a tree-lined canal trail.
The "Ah-hah!" moment referred to above was the result of a conversation I had a few days earlier with a friend who's a well-versed, experienced photographer, but had never heard of the term "bokeh." I learned about it myself about 15 years ago from a magazine article lent to me by Jeff Hirsch, a good friend and former employer. Bokeh is a transliteration of a Japanese term that describes the image qualities of the areas of a photograph that are not in focus. And while you might not be conscious of its very existence, the subtle aspects of bokeh can cause you to like or not like a photograph—and not even realize why.
My epiphany occurred while glancing down at the pavement passing beneath me. I realized that the shadows cast by the bright, overhead sunlight were projected as soft, rounded outlines of the trees and branches dancing back and forth above me. And like the outline of out-of-focus highlights in photographs that display "good bokeh," the leaves and branches were rendered in the same soft, rounded, "natural-looking" manner.
The basic tenet of bokeh is that the human eye sees the world through a circular opening (the pupil), which in turn projects the image it sees to the rear of our eyeballs, which are also round. As a result, the portions of the scene we are not focusing on take on a softly rounded quality, which becomes increasingly softer and rounder farther from the central point of focus.
What struck me while I peddled along was how the canopy of foliage above me was being rendered on the blacktop "photographically" in real time, and in the real world. And as you might have expected, I stopped my bike and started taking pictures.
Quality of glass aside, one of the primary factors of creating a lens that displays "good bokeh" (or "bad bokeh") is the circular (or less than) quality of the opening formed by the diaphragm blades when they stop down to form the physical f-stop opening. Back in the day, good bokeh was primarily found in lenses from Leica, Zeiss and a handful of other manufacturers (usually from Germany) who took the time, effort and expense of incorporating up to twice as many aperture blades in the diaphragm design of their lenses, compared to the fast rising, consumer-centric manufacturers from the Far East. While f/8 is f/8 regardless of how pure their diaphragms' circle is, the resulting imagery of "comparable" lenses—exposure index aside—can often be noticeably less than comparable.
Note: The terms "bokeh" and '"selective focus" are often used interchangeably, which is incorrect. Selective focus is a term that describes the range of focus within a photograph, i.e., narrow depth-of-field or greater depth-of-field. Bokeh, on the other hand, describes the quality of the areas of the image that aren't in focus, which is determined by the design characteristics of the aperture blades as well as the quality of the glass. Therefore, while you can control selective focus by choosing a wider or smaller aperture, the bokeh qualities of any given lens are determined by a combination of its optical quality and the design and/or number of aperture blades that form the lens's f-stops.
The real differences between "good bokeh" and "bad bokeh" optics are most obvious when one examines the out-of-focus specular highlights of a photograph. In photographs taken with lenses containing fewer diaphragm blades, specular highlights are often rendered as octagons or hexagons. The image is directly affected by the optical limitations inherent in trying to create a perfect circle with only six or eight straight-edged blades in the diaphragm.
To circumvent this issue, and avoid the increased expense and precision required to add additional diaphragm blades, many manufacturers have adopted the stratagem of using not more, but curved diaphragm blades, which at the end of the day, seems to work well in practice.
So, the next time you read a press release for a new lens that touts, among other things, "curved" diaphragm blades for "natural-looking imagery," you'll know what they're referring to is what enlightened shooters have long referred to as bokeh. And good bokeh, at that.