Wide angle lens dor 40d?

[BIG4D]

I will be attending a landscape photography course in March and I think I need a new lens.
I currently own a Canon 40D with the 28-135 lens.
I have been doing a lot of reading about the "crop factor" of this camera. I find that it is 1.6, which make wide angle photography a little more difficult.

My question is what lens would you recommend for wide angle landscape photography?

Are there any brands of "aftermarket" lenses to stay away from?

[inkista]

Zeiss ZE Distagon 21mm. Just kidding.

The most commonly recommended ultra-wide Canon lens is the EF-S 10-22, and no, you don't need to stay away from the 3rd party lenses, here. Tamron, Sigma, and Tokina all make good lower-priced alternatives to the 10-22. I would check out discussions about specific lenses, though, to find out about things like copy variance. And you may want to avoid crop-body only lenses if you plan on upgrading to a 5D sometime within the next year or so.

The things to keep in mind with ultrawides are that a few things tend to be unavoidable at extremely wide angles: vignetting (light falloff in the corners), CA (chromatic aberration), distortion, and corner softness are kind of inherent in the design. It's a rare lens that doesn't exhibit most of these "flaws" at <12mm.

Commonly recommended ultrawide from the third parties would include the Tokina 11-16 or 12-24, the Sigma 10-20 or 12-24, and the Tamron 10-24.

There is also another alternative to using a wider lens, which is panoramic stitching, but that's a whole 'nother ball of wax. If you're interested in it, I'd recommend looking at Max Lyons's website. The nice upside to pano stitching is that you also drastically increase your resolution.

[BIG4D]

Thanks inkista.
Good information. I think I have decided on the Tokina AT-X 12-24. It is getting really good reviews.

[Aegea]

I'm considering that lens too, and I'd be interested to hear your opinion after you've had a chance to use it.