I'm shooting a Canon 7D. I have the canon flash 580ex and 580exii. Since I have had the newer flash I have used it almost exclusively. I don't remember having this problem with 5D or 30D, but now I have it with the 7D and 5DmkII. The flash misfires. I'll be shooting along with it connected to a Quantum Turbo battery and taking pictures and then several pictures will be taken without the flash firing and then it may fire one or two in between the misfires. I have bought new cables. Used the other Quantum Turbo with same results. Today I used the 7D with my 580ex not the exii and it worked fine. Question: Is there some setting on the 580ex ii that I need to change (didn't see anything in the book) or is my flash needing repair or replacement? Thanks for any help.