how does flash come into play with freezing action?

editing post for removal.

Say you're shooting inside.

At ISO 100, shutter speed 1/250 and aperture f/9, you're more than likely going to kill all ambient. Most small speed lights can have flash durations close to 1/15,000. That's the only thing that's lighting the scene. You could have your shutter speed at 1/60, and as long as you're killing all ambient, the flash will only creat the exposure, essentially giving it a "shutter speed" of 1/10,000, etc...

Using a flash now creates the ability to get two exposures. What ever you're getting from the ambient lighting with the shutter speed, and whatever you're getting from the flash exposure.

This can also be a problem, because if you're shooting longer exposures and getting a lot of ambient light while using a flash, you could be "ghosting" from the ambient expsoure and freezing from the flash exposure.

Unless you're still getting ambient, but not enough on the subject to cause ghosting.

Like if you expose fro the sky during sunset and use the flash to light an otherwise dark subject in the foreground.

tomdlgns wrote in post #6286098
ok, so if you use flash to freeze action, you need to make sure the camera doesn't capture ambient light?

Yes.

But if for some reason the ambient light is almost equal to the light from your flash, then make sure you have a fast enough shutter speed to also freeze movement.

This can happen if you are using your flash at lower power settings, or high ISO, or shooting outdoors.

You can leave the shutter open for 5 minutes in a dark room, but if you pop a flash just once, you can freeze just about anything you want.

My favorite situation for this is to have the subject of interest in a dark area, while the background is lit somewhat better. The ETTL flash lights up the subject and freezes him/her while the 1/2" -1/8" exposure blurs background creating a sense of movement.