Thanks all. I just bought a 320Ex from B&S and it works great. The seller told me about the function called "shutter speed sync mode in AV" and I will be able to set it to 1/200. Looks impressive as I can make the aperture f5.6, shutter 200, and ISO 800 which is very good. If I did not use flash light, I think the ISO will be 12800.