Bokeh is the term used to desribe the quality of background blur in a photo,
i.e. how pleasing the blur looks. Since the softness of the background blur is usually more important than how the foreground items are blurred, bokeh usually relates to the background blur.
The bokeh of a lens is influenced by numerous factors, including
- the lens design,
- the actual aperture used,
- the number of blades in the aperture mechanism,
- focusing distance,
- the distance of the out-of-focus objects,
- and in the case of the Nikon DC lenses, by how certain abberations are selectively introduced.
Some people will say that bokeh is subjective, but that is a point I don't entirely agree with.
When you see good bokeh, you'll recognize it. Same with bad bokeh.
To illustrate how good bokeh appears, and what bad bokek looks like, I set up this simple shot.
It isn't art, so don't critique the photography - I wanted those random out-of-focus items in the background.
I used two lenses here, comparing the Nikon 28-105mm f3.5-4.5 with one of the very best lenses that Nikon has .. the 105mm f2 DC. The 105mm DC lens has superb bokeh, so it should be quite apparent in comparison photographs why I didn't like the 28-105mm lens in this respect.
Here's the set-up shot .. my daughter in our backyard, with some back-lighting and arbitrary
out-of-focus clutter in the background. The general photo is reduced in size, but the other two are 100% crops, shot with low sharpening on the D2H, both lenses at f5.6
As you should be able to see from these crops, the 28-105mm zoom has a harsh bokeh ..
the out-of-focus areas look 'wiry', wheras the 105mm DC lens has a very smooth looking out of focus background.
The 100% crop from the 105mm f2 DC ..
.. and the 100% crop from the 28-105mm zoom at 105mm setting ..
(The part of the railing that is visible, seems to be clearer in the one shot, but that was because she moved slightly and I had to refocus on her eyes. I did try to keep things as consistent as possible, but little kids aren't inanimate. The photos should still serve as a comparison.)
Anyway ... this was one of the reasons I sold the 28-105 mm lens.
The bokeh was crap. And yes, the client might not notice, but *I* do.
It may not be fair comparing a lower-priced general-purpose zoom like this to a lens that is highly specialised for precisely this aspect - smoothness of out of focus areas - but in the end I wanted optical quality, and in this aspect the zoom was a disappointment.
I sold it mainly because of this.
(The slowness of focusing compared to the AF-S zooms was another factor.)
As I said, most people think bokeh is subjective. I don't believe it is.
The difference is obvious - The one image looks good, the other doesn't.
No subjectivity there.
Here's a further page where I tried to clarify the difference in foreground and background blur,
and how it relates to bokeh.
Links to websites explaining more about bokeh:
Van Walree's webpages on optics
Flickr: Bokeh - silky & smooth - Hello Doodle's blog - a brave attempt at demystifying bokeh
http://www.pathcom.com/~vhchan/bokeh.html
http://www.bokeh.de/en/index.html
http://zuiko.sls.bc.ca/swright/archives/1997/msg02265.html
http://homepages.ihug.com.au/~parsog/Guy/bokeh.html
http://www.trenholm.org/hmmerk/ATVB.pdf
http://www.luminous-landscape.com/essays/bokeh.shtml
Please feel free to e-mail me with comments & questions.
Neil van Niekerk