I'm currently doing something similar with a Nikon and opted to use an opto-isolator to keep the camera's electronics away from my DIY.
I'm using a TLP504A which is a dual opto-isolator so it can do the focus as well. I'd take a look at this site: www.doc-diy.net :: camera remote release pinout list which shows the pinouts for the remote for various cameras.
I would have thought you'd need to activate the focus as well, a few hundred milliseconds ahead of the shutter.
John.