Buying a super wide angle lens for my 5D Mark lll

On the subject of wide angle lenses, I have been using the Sigma 12-24mm mk2 for some time now and for the enthusiastic amateur I consider it good value for money.

The attached pic was shot hand held, f13 for max DOF at ISO 1600. (In hind-sight I am sure I could have gone for a much lower f stop because of the intrinsic DOF from such a wide angle lens.)