UWA edge/corners, new to canon, even new to FF, need some advice.

I second the Samyang 14mm. You need to be full manual but it is stunning at any price.

I've put some handheld samples at 20mm with the 20/1.8, 17-40 and 12-24 here if you want to look. Taken on the 5D11 click for info and the lens model is in the file name

http://www.howardsheardphotography.com/Equipment-Samples/About-20mm/12887557_axKiV#930640938_8DuXD
a few more with the 20/1.8

http://www.howardsheardphotography.com/Equipment-Samples/Sigma-20mm-f18/12718066_HWMPf#915430555_Emmxi

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http://www.OCwildlife.com
There is no other photography like Wildlife

I have the same needs, and will probably spend the money on the 12-24canon. I have the 24-105 and think they would be the perfect set up together. The probalem that has kept me from the 20mm prime is sometimes it still isnt wide enough, and also the amount of lens changing going on.

Canon is making some new lenses this year. I am holding out for a new wide. And if it's anything like the 100f2.8IS L I'll jump for joy. If you can hold out a while, there may be some new lenses from canon, tamron and sigma. I do also like sigma color. So much less CA.
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http://www.OCwildlife.com
There is no other photography like Wildlife

Maybe not the sharpest lens compared to Nikon 14-24, but very nice rectilinear performance at 12-18 an close to your price range. Copies I have tested are sharper in corners than my 17-40 at 17mm. Little distortion compared to 17-40mm....and I tested 3 copies of the 17-40 before i kept mine.

The only downside was I had to test 3 copies of the Sigma, just like the Canon, to keep one.

Here are a few everyday samples, mostly at the 12mm range.
http://delsolar.org/nature/webs/SIGMA%2012-24/

Eduardo
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http://www.delsolar.org/nature

At 17mm, the left side seems softer than right. I tested two copies that had a soft and a sharp corner. In my case I sent those copies back. Canon will probably fix this lens, not sure about cost.
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http://www.delsolar.org/nature

There is also the upcoming Tokina 16-28 F2.8 for full frame cameras, it isn't cheap, the prices on ebay new out of Japan are like $1100 and god knows when this lens will be out in Canon mount. Otherwise, this looks to be a good option to consider.
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http://www.pixelmap.com