which ultrawide lens to buy? I have the 17-55mm IS USM

In wide angle shooting every mm is a pretty dramatic difference, the percentages goes larger and larger...

I have used the Tokina 12-24 and the Sigma 10-20. I found the Tokina front heavy.

The suggested sigma 8-16 would be the widest you could go.

The Tokina 11-16 is thought highly of... ( http://www.photozone.de/canon-eos/666-tokina1116f28eos )

I would say the only lenses in this area you probably shouldn't consider are the Tamrons, while I have not used them the data I have seen didn't look to great.

Also consider what part of the 17-55 you use the most. If you mostly shoot at 17 then having a 10-20 or a 10-22 would maybe become the lens you use most of all...

For me this is the case... I just moved to FF and am likely to use the 17-40 L + 50 mm for almost everything. (IE I have no desire to own a 24-105 or 24-70).
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