I have owned the renowned Tokina 11-16mm f/2.8 for Nikon since earlier this year. It started my love affair with ultra wide lenses. The Tokina is razor sharp, even wide open, and sports a 2.8 max aperture allowing for some creativity in low light. I have really enjoyed it and it has become one of my three lenses that comes with me everywhere!
Sigma introduced the widest lens for crop sensor cameras earlier this year...the Sigma 8-16mm f/4.5-5.6 lens. On a Nikon crop sensor, this gives a 12mm field of view (in 35mm equivalent) which means I can now go even wider! Needless to say, I recently purchased the Sigma 8-16mm lens to test head to head against the Tokina. I'll be taking a trip to Alaska in May 2011, and I'm going to need the best ultra wide I can get for that trip!
I did a variety of tests on the Sigma and the Tokina and various apertures. The biggest difference many may be interested in is the 8mm to 11mm difference. It doesn't SOUND like that big of a difference, does it? To test it out, I took the following two photos on a tripod with the same settings at 11mm on the Tokina and 8mm on the Sigma. The first image shown below is the Tokina at 11mm, f/4.5, 1/640. Below that you will see the Sigma at 8mm, f/4.5, 1/640. Both of these photos are straight out of the camera (SOOC). No editing has been done.
Tokina 11-16mm @ 11mm, f/4.5, 1/640
One thing to keep in mind is that, at shooting at f/4.5 this is "wide open" for the Sigma lens, but f/4.5 is actually stopped down for the Tokina (since it has a max aperture of f/2.8). In reality, you would expect the Tokina to perform even better than the Sigma. As you can see, though, the Sigma has much better color and contrast SOOC than the Tokina.
The Sigma also has a shorter minimum focus distance than the Tokina, allowing for some creativity! The Sigma has a minimum focus distance of 24cm (keep in mind, that distance is from the sensor and NOT from the front of the lens), and the Tokina has a minimum focus distance of 30cm. That doesn't sound like a lot, but I have to say after trying out both lenses myself, there is most definitely a noticeable difference.
To demonstrate the close focusing ability...my dog Marley at 8mm. Her nose was very close to the front glass!
The Tokina is always touted for its sharpness, but I found the Sigma to be equally sharp or sharper (after I received my second copy). As with all lenses, there are bad copies out there. The first Sigma 8-16mm lens I received from B&H backfocused and was pretty soft. In addition, it suffered from some pretty severe CA. After returning that lens to B&H and receiving a new copy, I've been thrilled with the lens. My second copy was MUCH better! It is sharp and has very little CA. I didn't test CA specifically between the Tokina and Sigma, but I have read that the Sigma tends to have less CA than the Tokina, as well. However, during my time of owning the Tokina, CA never proved to be an issue with it, so I wouldn't discount the Tokina for that reason.
I know I've raved about the Sigma, but I have to say...the Tokina 11-16mm f/2.8 is nothing to sneeze at! It's a fantastic lens, as well. If I had unlimited funds, I would have kept the Tokina AND the Sigma, but unfortunately I had to sell the Tokina to fund the Sigma. The Tokina beats the Sigma with its 2.8 maximum aperture. It works incredibly well in low light, and it is pretty darn sharp even wide open. If you plan to shoot in low light a good bit, I would absolutely recommend the Tokina.
All in all, the Tokina and Sigma are two wonderful ultra wide angle lenses. I chose to keep the Sigma 8-16 and sell the Tokina 11-16, and I absolutely adore my Sigma!
At the time of this writing, the Tokina 11-16mm f/2.8 is priced at $599 and the Sigma 8-16mm f/4.5-5.6 is priced at $699.
If you have any questions about the lens, feel free to leave a comment and I will respond!