Sorry I'm very late to this topic but there are only a few lenses made for 35mm SLRs (or their digital equivalents) that can be used on medium format cameras and no ready-made adapters exist for them. A good example of one such lens is the Yashica 100 f4 ML Bellows lens which could be used with up to a 6x7 camera. The only adapters that I know exist for the Contax 645 are the Hasselblad one and the Zork for the Mamiya 645 lenses. I have recently bought the Zork adapter (it's very expensive at £350+) so that I can use the stunning Mamiya 145mm Soft Focus lens and the Bellows N (as the C645 Bellows is impossibly expensive and hard to find). I have to agree with gillou that you will be out of luck with the Canon lenses. Sorry I can't give you better news.