I'm having a bit of trouble using my Nikon SB700 flashgun (with a Better Beamer extender) to photograph hummingbirds. My main objectives are to have a fast shutter speed (e.g. 1/3200 sec) and acceptably low ISO for the exposures. When I look through the viewfinder, ISO is shown at 320 with flash attached and active. The flash fires fine, but when I check the actual images the ISO is 10,000. What am I doing wrong? I have ISO set to Auto ISO in the camera - is this the problem? I don't often use fill flash, so I'm a bit of a novice. Any advice you might have would be much appreciated.