How does flash freeze action in the exposure and have blur too?
I am new in flash... like so many
I am reading about how setting flash up for different scenarios. One thing that does not make sense is freezing action with the flash while blurring can occur in the same exposure.
I watched a great video on how the shutter curtain opens and closes on Pocket Wizard, starting around 2:20 in the video;
Russ' blogger post offers an example of when you can freeze foreground subject yet get blurring in the background:
"Now, decrease the shutter speed to 1/10th sec and shoot again. The background will be brighter, but the subject will be the same brightness as before. Also, the subject will be sharp, but the background may also show some motion blur, because 1/10th sec is too slow to hold the camera perfectly still (unless you use a tripod). Since the flash was primary on the subject; ie, the ambient was overpowered by the flash, and the flash duration is normally faster than 1/1000 sec, the subject will be sharp with no motion blur."
How does this work, how by adding flash can you freeze motion on a subject or part of the exposure yet have blur to?
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#1. "RE: How does flash freeze action in the exposure and have blur too?" | In response to Reply # 0blw Nikonian since 18th Jun 2004Fri 25-Dec-09 08:59 AM
> One thing that does not make sense is freezing action with the flash while blurring can occur in the same exposure.
You have two exposures going on in flash. One is for the ambient light, the other is for the flash. They can do different things, even in the same frame.
For purpose of illustration, consider shooting in an entirely dark room, but with flash. We will manually pre-focus, so that the target, a hampster running in its wheel, is in focus. When we release the shutter, the ambient exposure is, essentially, nil. If there's no flash, there's no other light - so you get a black frame. No blurring, no anything else. The exposure is irrelevant, since there is no light. Now let's reshoot, but this time the flash is on. The flash pulse is 1/1000th or so, so the hampster will be frozen in mid-stride. And as long as the flash is powerful enough to light the scene, it's well exposed. Of some importance is that the camera picks an exposure of 1/60th, f/2.8, ISO 200. This occurs because the lens is wide open, the ISO is fixed, and the shutter speed is clamped to 1/60th maximum by default when the flash is enabled. Without the flash, this would be drastically underexposed, so the scene as a whole does not contribute anything of significance to the resulting image. Since it's completely dark without the flash, there's really NO contribution.
OK, now let's turn the flash off, but let's also light a candle. Clearly you can see now, but the exposure is probably going to be something like 1 sec, f/2.8, ISO 200. If we release the shutter now, what happens? We get the whole scene probably blurry, although it will be reasonably exposed, since even with VR nobody can hand-hold a 1-sec exposure rock steady. If we are shooting with a moderately long lens - say a 100mm macro - the blurring will be fairly substantial.
Finally, let's turn the flash back on, but now set the flash mode to slow sync. The scene is still lit with candlelight, so the exposure remains 1 sec, f/2.8, ISO 200. Slow sync allows the shutter speed to go as low as necessary, even if the flash is going to contribute. When we release the shutter this time, it's still going to be open for a full second, and the ambient exposure will be just as above. There will be different, random movement, but the magnitude will be approximately the same. However, this time the flash also goes off. The hampster will be frozen in mid-stride again - the flash is still the same duration - but what you'll see in the frame is a sharp, frozen image of the hampster superimposed on the blurry version of the hampster. (In fact, you'll have two kinds of blur: camera motion, and subject motion.) What you're getting in this case is a slice of the scene in 1/1000th, superimposed on the scene in 1 sec.
The reason that flash usually "freezes motion" is that the way flash is used most often is like the first flash case: the subject is drastically underexposed without the flash, so little or no ambient contribution, and the flash does the whole job, freezing it.
Brian... a bicoastal Nikonian and Team Member
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