Nikon 12 - 24
mm f/4, Tokina 12 - 24 mm f/4, Sigma 10 - 20 mm f/4 - 5.6 and Tamron
11 - 18 mm f/4.5 - 5.6.
Click an image to get it from Adorama. Also try Amazon. It helps me if you get yours at either one.
Guide Image (Tokina at 24mm)
What sort of a nut worries about bokeh with slow ultrawide lenses which always get almost everything in focus anyway? I don't, but since I'm sure someone would ask I decided to test this before I sent the lenses back.
This is a simple test. I focused each lens on a post at arm's length, about 26" (66 cm). I shot wide open at each end of the zoom range. I then blowed up a small crop at 100% as you see below. I added no sharpening or anything to the crops below. Is "blowed up" a verb? Blowed up is popular phrase in rural America for a recreational activity involving demolishing disused motor vehicles.
The sun and clouds are always moving. They were moving across my subjects. Ignore differences in lighting.
Widest Setting, Wide Open Aperture
Overall: It's tough to see anything because at these ultrashort focal lengths. Almost everything is in focus.
Quality of bokeh: The Tokina is the best. The Tamron is OK. The Nikon and Sigma are pretty bad.
Degree to which background can be thrown out of focus: The Nikon and Tokina are best. The Sigma and Tamron have a harder time throwing the background out of focus due to smaller maximum apertures and shorter minimum focal lengths.
Longest Setting, Wide Open Aperture
Overall: At the longest end we almost can see some defocus effects. Remember these are 100% crops from a high resolution D200. If you printed the entire image this size it would be over three feet (one meter) wide.
Quality of bokeh: The Nikon and Tokina are quite good. The Tamron is bad and the Sigma is awful.
Degree to which background can be thrown out of focus: The Nikon and Tokina are equally good. Their one-stop faster aperture and longer focal lengths, combined with pleasant bokeh, make it easy for them. The Sigma and Tamron are equally awful. Their slower f/stop, shorter focal lengths and awful bokeh are a three-way path to disaster.
Who cares? You have to work very hard to make an image that shows any bokeh with these lenses. Any portraits made this wide and this close are going to look scary.
If you do care you'll notice that the two best lenses at the wide end are the worst at the long end.
It's a draw.