Have a look at this picture I came across when browsing photos about Japan: Raining in Ginza, it is what is called a bokeh which is a photographic term referring to distorted out-of-focus areas in an image produced by a camera lens. The Japanese term bokeh stems from ピントが暈ける (pinto ga bokeru - out of focus) where ピント is an imported word from the Dutch: 'brandpunt'.
The 2nd project I have set myself is to try a couple of these photos when we'll be staying in Tokyo. I asked around at Usenet and got the following answer from Floyd L. Davidson:
Position yourself where everything in the near foreground is dark. Set the camera to the widest f/stop, focus it just slightly closer than the nearest object that is well enough light to be distinguishable in the image.
Then expose at a relatively fast shutter speed and look at the histogram it produces. Change the shutter speed to position the brightest parts of the image almost to the right side of your histogram. Ideally that would be with the aperture wide open, but if you run out of shutter speed range it may be necessary to use an ND filter, or stop the lense down slightly.
The problem with stopping down the lense is loss of what everyone has been saying is "great bokeh". With the aperture wide open it produces circular out of focus highlights, but if the lense is stopped down they will be the shape of your aperture. If you do have a great lense it won't be bad (if you have a 9 bladed shutter it might even be great!), but it won't be the same as the example image.
The focus point, and the relative distances to highlight objects, will affect how the image looks. Hence you might want to look for a location where all objects are about the same distance (relatively, and keeping in mind that focal length of the lense will affect how relative that is) if you want the out of focus circles to look all similar.
In any case, you'll have a very hard time judging what they look like through the viewfinder. It probably won't be all that easy to tell looking at an LCD display either! So take several exposures, each with the focus at slightly different points. And then switch focal lengths and do it again!
Additional information of Ray Fischer summarizes it well:
Skip the light meter and use manual exposure. The nice thing about digital cameras is that you can try many different versions without spending money.
At ISO400 I'd start at f2.8 and, say a 30th of a second. Don't reduce the aperture by much if you want the blur.
