What does the camera flash have to do with preventing blur?

Question

What does the camera flash have to do with preventing blur?

Answer 1

It allows you to use a sufficient shutter speed that would normally cause underexposed picture.

Answer 2

Astrobuf has it right; the other two are incorrect.  Here's why.

When you use a flash on a camera, there are effectively two exposures going on, 1 from the flash and another exposure from the ambient light.  If the ambient light is dim enough, then the effect it has on exposure is effectively zero, which then means that the exposure is determined only by the aperture of the lens and the duration of the flash itself.

A modern electronic flash (unlike an old flashbulb) can flash from 1/1000 of a second to as short as 1/100,000 of a second.  The duration of the flash determines how much power the flash is putting out.  In any case, as you can see, the short flash duration (in dim light) acts like superfast shutter speed, particularly if your subject is closer to the camera.

You can use this to capture drops of water falling from a faucet, for example.

Now, remember how I said in the beginning that there are two exposures going on?  Well, if the ambient light is bright enough, then it too can cause an exposure as well.  This can result in a ghosting effect in a moving subject--one image that is sharp with a ghostly image around it.

And why the other answers are wrong:  cameras have a synchronization speed, where the flash matches up to where the shutter is completely open over the sensor/film.  On an SLR, this is mechanically based and relatively slow.  In bright enough light, the sync speed can actually be slower than what is a correct exposure and this can lead not only to blur from the ambient light, but also overexposure.

On a digital point and shoot, which typically does not have a mechanical shutter, the sync speed can be fairly high, so this helps to reduce the effects of ambient light.

So the bottom line:  camera flash duration can determine the overall exposure by acting as a superfast shutter, if the light is dim enough.

Answer 3

It forces a higher shutter speed, so it is less likely that you or the subject wil move fast enough to become blurred.

Answer 4

A  flash is only lit for a millisecond or so. Since allmost all of the light used to make the exposure comes from the flash, it effectivly "stops" motion much like a very fast shutter would and thus limits blur.

Astrobuf