The job is simple:
- Find a plain white business card
- Pop-up the camera's flash and look for the two metal rods that hold up the flash
- Line up the business card so its centered over the rods and make a mark
- Use a pair of scissors and make about 1/2" cut where you marked
- Now slide the business card so that is snug with the rods sticking into the business card
This trick works well for longer focal lengths but really doesn't work well for shorter ones, so it'll be good for a portrait but not good for photographing something like an entire room. I also just checked to see if you can do the same thing with the 40D and sadly you can't, the flash's pop-up mechanism is slightly different on the 40D.
I learned about this from a photography website, sadly I don't remember which site it was. Despite its limitations I think it'll come in handy in a pinch, and hey all it requires is putting a business card in your camera bag.
Technical Specs (only for top picture): Canon Rebel XTi, 17-85@61, f/5.6, 1/80s, ISO 1600.