I can tell you that the sd-cf adapters DO NOT work in the Nikon D1, D1h, or D1x bodies. I can also tell you that they DO WORK in the Canon 1d, D30, and 30D bodies. I have direct experience with all 6 of these bodies (I own Nikon, and my wife owns the Canons). Since the 40d is even newer technology than the 30d, I'm pretty confident that they will work just fine. Good luck and let me know.