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I just hate it when I get a good image spoiled by someone closes their eyes (and I rarely use flash). .....Tom
That is the much hated pre flashes that these modern DSLR and Speedlight's emit before making the exposure. The DSLR measure the light off the shutterblades the guesing how much light should the main flash contain,,, The sensor is too shiny to make a reading off,,,
In the good old film days we used to have proper TTL on the cameras,,,
Fast reacting people react to these pre flashes and manage to close their eyes quickly enough to avoid the main flash, impressive actually, especially young people are quick
Only way to make sure this is not an issue is to use full 'M' Manual or use the good old 'A' Automatic where the flash itself, a small sensor on the front of the flash, turns off the flash output when enough light has been received back from the scene, the larger Nikon speedlight's have this feature.
I use it also on my Leica M9 with a Nikon SB800 in A works impressively well!