Originally posted by Skodadriver Is my reasoning correct that the slower the shutter speed at an aperture the more the flash duration has effect?
If the shutter speed indicated without flash is 1 or more seconds, the sync speed using flash is 1/180th sec and the duration of the flash is in x/1000ths of a sec, should the only parts of the image illuminated by the flash at this faster flash duration show in the image and any movement be too underexposed to show? I remember taking multiple images of my wife in the dark with flash on film, donkeys years ago.
No.
Flash duration is very short and only depends on flash power (but is very short nonetheless).
Slowing shutter speed will change the contribution of ambient light, influencing the areas of your picture that aren't illuminated by (or get little light from) the flash.
For instance, let's pretend you shoot a flower at 2m, with a background at 20m. If you shoot at 1/180s you'll probably see the flower and the background will be all black. Dragging the shutter will allow you to get (available) light from the background as well, but won't affect how the subject is "frozen".
Zoom was ok, so my belief is that TTL gave enough power to expose the scene correctly, but much less than it was needed to freeze the subject. Since ambient light was plentiful, at ISO400 f/8 1/180s you were probably already close to a correct exposure, so the flash contribution was minimal.
Your options:
1. stop down more, so you reduce available light and the flash will increase its power
2. use a faster shutter speeed, thus reducing available light, and forcing the flash into HSS mode (if it's supported): you'll reduce maximum power, but flash contribution will be proportionally higher, and you won't need max power anyway at 10m
3. use option 1. but in manual mode, and ensure the flash fires near full power.