I am considering buying a Nikon-fit wide-angle zoom and have seen a Tokina at-x pro 11-16mm F 2.8 on a well-known auction site. The ad is rather vague and doesn't mention the fitting but when I've queried which fitting it is the seller has replied saying it is 'for both Nikon and Canon (aps-c). Is this actually correct - I was under the impression that both Nikon and Canon had different fittings?