Bokeh is a buzz word that describes the out of focus
portions of an image. If you want to
impress your peers, point to a photo with a lot of out of focus areas and say
"Wow, the bokeh in that photo is amazing." Most likely, your peers will nod in
agreement.
The definition of bokeh is subjective. To a large degree, you know good bokeh
when you see it. I describe bad bokeh as
an image where the out of focus areas are distracting. Instead of a smooth and gradual move to blur,
the bad bokeh image creates patterns in the blurred area.
Look at the image of
the Northern Flicker and Harris Sparrow above.
The Flicker is in focus. The nearby
Sparrow isn't. The creamy green out of focus area behind the
Flicker is good bokeh.
A super telephoto lens has an extremely shallow depth of focus. It is so shallow that I often have to decide
what portion of my subject should be in focus because I can't make the
whole subject sharp. In this photo of a female Red-winged
Blackbird, I used the shallow depth of
field to highlight the bird and to create a mood. There were so many plants in front of the
Blackbird, that it would not have been recognizable with a normal lens. With a shallow depth of focus, the stem in from
create a blur that lowers the overall contrast.
This photo of a Swamp Sparrow illustrates bad bokeh. The sparrow is sharp and crisp in spite of
the tangle of branches around it. The
background is out of focus. Portions of
the left side of the background have a weird pattern. When you look at a large version of this
image, the background patterns are a flaw. Bokeh so bad that there is a name for it - nisen bokeh. This is when an out of focus line is broken into several light and dark out of focus lines.
Good and bad bokeh is usually considered to be an attribute of the lens. Canon uses extra blades in the iris of some of their lens to make the aperature more circular. This prevents the pattern of the blades from showing up in the out of focus highlights. In this case, the same lens produced good and bad bokeh on the same day.
What went wrong? I'm not an expert but at least two factors contributed. First, the bad bokeh on the left side of the image is out of focus branches between me and the sparrow. Just like aperture blades, they imprinted their shape on the out of focus highlights. Second, heat waves in the air caused distortion of the highlights. Look at the out of focus specular highlight a little above the sparrows tail. If you enlarge it, you will notice that it isn't a smooth blur but there is extra detail in it.
I could have reduced or even prevented the bad bokeh in the sparrow image by moving my camera to a place without branches beteween the camera and sparrow. That works in theroy but the reality was that the sparrow wasn't posing - I had to follow it. I do try to do most of my nature photography in the early morning before the heat waves become a problem.
