# flash to stop action?



## Dmitri (Feb 5, 2010)

I had believed that you could use flash to stop action, regardless of shutter speed. For example, you could set the shutter to 1/6 to over-expose the background, while the subject (in the flash) would be caught as if 200 shutter speed. I finally tried it today with a friends dog and it was no different than 1/6 without the flash (in terms of motion blur).

Am I doing something wrong or did I misunderstand how flash could stop motion?


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## the Virginian (Feb 5, 2010)

If your ambient exposure is longer than the flash's (1/1000 and faster), it continues to expose the shot. This technique is used to show blurred movement with an abrupt freezing at the end of the shot such as with moving model trains, but won't work with subjects that need to be stopped immediately.


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## Garbz (Feb 5, 2010)

Need to understand where your light sources come from. You're over exposing the background with continuous visible light. If this light is hitting the subject then the subject will show motion blur. This is called dragging the shutter and is used to convey movement by freezing a moment of action AND showing that there was movement at the same time.

If you want to freeze the action you need to ensure your subject is lit only by the momentary light (flash) and not exposed by the continuous light source lighting the background.

The other issue if the background is lit by a continuous source and the foreground is lit by momentary source, when you drag the shutter your subject will have a dark outline since it prevents the background light getting to the camera.

Lots of fun playing with this stuff. If you want to play with it more, and don't have a dog, then simply play with shooting stationary objects while moving the camera when pressing the button.


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## kajiki (Feb 5, 2010)

easy answer is underexpose the background, but get correct exposure for the flash. That way the subject stands out from the background. QV Martin Parr.

A Metz 45 CT at full output burns for 1/300th of a second. So at say quarter power it burns for 1/1200th sec.


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## Big Mike (Feb 5, 2010)

You need to remember/realize that every shot taken with a flash, is actually two exposures.  One being the ambient and one from the flash.

In most cases, if you are using the flash, it's probably somewhat dark, which means that the ambient exposure is probably going to be fairly weak.  The flash exposure simply overpowers the ambient and you end up with a sharp image.

Now if there is more ambient light (or you use a longer shutter speed etc.), then your ambient exposure will be stronger and it will be harder for the flash exposure to overpower it in the image.  This is where you might see 'ghosting' or some motion blur with a somewhat sharp subject (from the flash).

If there is a whole lot of ambient light (and you expose for it) then the flash exposure isn't going to overpower it at all.  The two will likely overlay perfectly, the flash just adding fill. 

So with that in mind, you need to choose your level of ambient exposure, especially your shutter speed, carefully when shooting a moving subject with flash.  If there is enough ambient exposure and a slow enough shutter speed....you will get blur.
This can sometimes be a neat effect, but you'll probably want to ensure that you are using 2nd curtain sync.

As mentioned, you can easily freeze a lot of motion with flash...and the darker it is, the better (less ambient to blur).


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## Dmitri (Feb 5, 2010)

Thanks all. I didnt realize the ambient light would also affect it, tho it does make sense. Thanks again!


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