Started using a canon 5D recently. Have a 430ex speedlite. Before, I have been using an Olympus dsrl, and when I use second curtain flash it only flashes once before shutter closes, as expected. With the canon I have read and now see that in this mode the flash kind of fires 2 times, one to measure and the second is actually the exposure flash. What I read is that its normal on these cameras to do this. So my question is, if the camera uses a "preflash " to measure why on the first curtain setup it only fires once? Does it extends the burst to measure and expose at the same time? And on second curtain mode, is there a way for the canon to just shoot the last flash? It messes some photos because people move thinking the first flash it actually the camera taking the final picture.