I attended a wedding last weekend from a colleague. I was trying to shoot with my 1DM3 with a SpeedLite 550 attached. In some cases the images came out highly overexposed, at an unrecoverable level. I tried several ways like Program, Tv, Av and M, in all modes I got bad images. Finally I gave up in the flash and used ISO 3200 with noiseNinja in post processing. But I don't like that too much as I loose sharpness. I know: photography is a different word for compromise. What is your rule of thumb for successful flash photography for a Canon man (and I know that Nikon has better flash system, but I have too much glass) Thanks for any good tip or link to tutorials.