I have been eyeing the Tokina 16-28mm lens. The photozone tests show it is very good against the Canon 16-35mm L.
There have been reports of poor quality control, I expect that they are very difficult to assemble properly.
They are worth looking at. Good deal price wise.
The Tokina AF 16-28mm f/2.8 AT-X Pro SD FX is a high-performance ultra-wide zoom lens that gives the Canon EF 16-35mm f/2.8 USM L II a run for the money - but it's not a lens without flaws. Its primary weakness is corner softness at f/2.8 but that's not unheard of in this class anyway. However, the center quality is great and the borders are generally sharp as well. The corners start to catch up at f/4 and they're very good from f/5.6 onward. Vignetting is, of course, visible at f/2.8, specifically at 16mm but the issue is better controlled than average. Lateral CAs, an old Tokina disease actually, are modest and not overly field-relevant when stopped down a little bit. Typical for such lens it shows some barrel distortions but they're, again, comparatively moderate even at the very wide end of the range.
Technically the Tokina is superior to the current Canon EF zoom lenses in basically all the analysed image aspects!
Unfortunately there may be a hair spoiling the (optical) soup here - quality control. As mentioned we purchased three lens samples for testing, two in Nikon and one in Canon mount, and all three showed some centering issues. The initial Nikon variant was so poor that we had to cancel the testing procedure. We'd like to urge the manufacturers to take lens centering (alignment) more seriously - just a good or possibly even great base-design is simply not enough without proper manufacturing. We are pretty sure that consumers, especially in the mid-to-high end market, would be happily willing to pay a little more for better quality control. Especially ultra-wide and standard lenses show more outliers than desirable - not only among Tokinas but across the manufacturers (e.g. recently we tested 3 (three!) Sigma 8-16mm f/4.5-5.6 in Nikon mount without success). We are probably seeing negative outliers in excess of 25%(!) in this segment which is, frankly, embarrassing and unacceptable! That all said we'd like to end this review with some more positive aspects. The (outer) build quality of the Tokina is on a very high level. The lens body is only based on tightly assembled, high quality plastics rather than the "duraluminium" finish used in previous AT-X lenses. However, the quality is still up to pro standards with the exception of the missing weather sealing. Tokina has improved the AF quite a bit - it doesn't really operate "silently" as promised but it's both fast and accurate in phase-detection AF mode. Some users may not like the huge, bulb-like front element which prohibits the use of front filters. However, it seems as if only this design approach solves the performance issues that are usually associated with ultra-wide angle lenses. So setting aside potential sample variations the Tokina is definitely worth a deeper look!