Glenn's random stream...

What is “bokeh”?

I am going to take a wild guess and say that it is not what most people reading this article think it is. “Bokeh” has become one of the most misused terms in the lexicon of photography over the last few years.

First let me tell you what it is not. Bokeh is not a fancy, interchangeable word for “depth of field”. “Depth of field” is that part of an image which is in “acceptable focus” and is determined by aperture size, focal length and subject distance. If you have only a small part of your image in focus and the rest blurred, this is “shallow depth of field”. This is nothing to do with “bokeh”. If you have almost all of your image in focus, then you have “large depth of field”. This isn't anything to do with “bokeh” either.

OK, so what is “bokeh” then? “Bokeh” is a term coined to describe the aesthetic quality of the blur in out of focus portions of an image or sometimes, more specifically, the way a lens renders out of focus highlights in an image. It's a qualitative term, not a quantitative one. You can have “good bokeh” or “bad bokeh” but it is nonsensical to talk of “lots of bokeh” or “not much bokeh”.

How about as quick example; OK, suppose I set up two identical dSLRs, one with a Canon 50mm f/1.8 on it and one with a Canon 50mm f/1.4 on it. I set them both up to take a photo of the same subject at the same distance and at the same aperture, f/2.0. I will get identical depth of field from both lenses. The portion of the image that is in focus will be absolutely the same. However, the “bokeh” from the two lenses will be different. The "bokeh" the 50mm f/1.4 will be much nicer. Why?

“Bokeh” is a characteristic of a lens. There's nothing that you can do in terms of twiddling dials and switches on your camera to affect “bokeh”. One of the biggest factors is the number of blades which make up the aperture diaphragm; in general more blades = “better bokeh”. Other lens construction factors also come into play. In the above example the 50mm f/1.8 has a five blade aperture diaphragm whilst the 50mm f/1.4 has an 8 blade.

So what constitutes “good bokeh” as opposed to “bad bokeh”. Well, since it's an aesthetic consideration that can be very much in the eye of the beholder. However, most people seem to prefer smooth, creamy out of focus areas with diffuse highlights as opposed to harsh transitions.

So... is "bokeh" what you thought it was?