Only saw this today or I would have answered sooner!
Yes, you'll need a wide angle adapter. On my full frame body, I start to get some vignetting at around 19mm, using a 17-35mm lens and with 2 filter slots and a polariser ring attached. If the polariser is on as well, it becomes evident at around 20-21mm.
I would imagine this will not be the case with a cropped sensor until you reach a shorter focal length, but I wouldnt be surprised if there is still some vignetting at 12mm, depending on the number of slots you put on the front.
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