ISO and FLASH and SHUTTER SPEED and MODE

I'll be shooting an indoor, poorly lit swim meet in an hour.

HELP!!!

Does ISO matter when using my 580ex?

When I do portraits, I use AV mode so as not to darken the background.

I know a fast shutter speed will darken the background. I don't care about that.

Question 1. Does the flash, and the flash alone, freeze the action?

Question 2. What effect on motion blur does 1000 shutter speed have over 250 when used in conjunction with flash?

Question 3. As far as the swimmers are concerned, those shots are coming out great. The divers on the other hand are blurred. Is that mostly my technique in possibly not holding the camera steady?

Question 4. If the flash is responsible for freezing the action, what is high speed sync used for?

I've read the articles on flash, etc, but sorry to say I still don't get it. Any help anyone can give me is greatly appreciated.

mark

1. The only time flash alone freezes action is if it is the only light source for the shot. If ambient lighting is strong enough to register in the shot, then it will cause motion blur unless a shutter speed sufficiently high enough to free the action is used. If that's greater the 1/250, then HSS will be required.

2. Obviously, the higher the shutter speed, the more the motion is frozen. If a flash is used but the ambient lighting is strong enough, the flash will freeze motion and leave a sharp image on the picture. However, the image from the ambient lighting will also show as a blur on the picture. If 2nd curtain is not used, then the blur will be in front of the "frozen" flash image and will look unnatural. That is the reason people use 2nd curtain when using flash to freeze action and ambient lighting to leave a blurred trail behind the frozen image.

3. See above.

4. See above.

ISO will affect your flash range much the same as changing the aperture. Doubling the ISO (1 stop) will increase your flash range by a factor of 1.4x, just like opening up the aperture one stop. Of course, changing ISO won't change your depth of field, so it's a way to manage your flash range while managing DOF.

So increasing your ISO may be necessary to get sufficient flash range, depending on your aperture.

Play with it a bit and watch the distance scale on the back of the flash unit. You'll see the effect.

Ah, I think I might be starting to get it. If only these indoor pools were brightly lit, or if swim season were during the warmer months and could be held outside.