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07-22-2013, 07:41 PM   #1
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Principles of light? (Stop Action)

I have a question that is related to how light works, and how it affects stop action photography.... this could fall under several categories such as technique or a few others but I put it here because it applies here as well.

Here goes... if I shoot a moving object with my camera...and for the sake of this discussion in both situations the moving object is moving at a constant speed...in the same external light conditions.

If I turn the flash on I can effectively 'freeze' the action with a much slower shutter speed than if I rely on natural light as is. If I want to achieve the same 'freeze' without the flash I will need to use much faster shutter speeds and compensate the other variables (ISO etc).

I would basically like to understand this principle a little bit better... not that 'it just happens that way' but 'why does it happen that way?' that a flash can stop action with much slower shutter speeds.

07-22-2013, 08:18 PM - 1 Like   #2
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Freeze!

Hello Alamo,
I'll take a try at answering this, although if I'm wrong, it will surely be corrected in later posts!
A flash generates light for a much shorter length of time than even the fastest shutter; 1/10,000s to 1/40,000s. If you shoot in near-darkness, all you'll record is the object as the flash "froze" it.
But, the shutter was open much longer, typically 1/180s for X-sync, or longer. So, the object continued to move after (or before, more on that later) the flash caught it. Why don't we see this additional movement? Because it wasn't lit. We might see a faint blur, that's all.
But suppose there IS ambient light, not enough for a 1/8000s shutter speed, but some light. Well, the flash STILL freezes the portion of subject travel that it illuminates, but there will be a blur either behind (rear or trailing curtain sync) or in front of (front curtain sync, the more common flash setting) the 'frozen' part.
You've probably seen the photos of racing cars, clearly outlined with a blur behind it? Flash, with trailing curtain sync, or a panning shot with a shutter speed JUST fast enough to catch the car, but slow enough to blur the background.
But the shots of a bullet leaving the muzzle, stopped mid-flight? No blur, because they were shot in (usually) complete darkness. There WOULD be a blur, but it's too dark to see it. Hope this answers your questions!
Ron
07-23-2013, 09:27 AM   #3
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I'll also take a try.... think of your camera's sensor as a bucket that collects light. Your aperture is like a hose valve - open it up for more light flow, close it down for less light flow. With continuous light, your camera's shutter determines how long the light flows to the sensor. However electronic flash is not continuous light - in fact it is extremely short in duration. However, it is also very bright. So in a very short time, the sensor sees as much light as the relatively much longer shutter is open to continuous light.

Because of the flash's extreme amount of light and extremely short duration, it effectively becomes the shutter speed.
07-23-2013, 02:33 PM   #4
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Ok here is an attempt to cover it all

You have with a camera , three control variables, ISO or the sensitivity of the capturing medium, shutter speed or the length of time that the medium is exposed, and aperture or the amount of light the lens lets through. You have only one external variable, the total light hitting the subject which is reflected back to the camera. You have stated that this external variable is fixed.

As a result, if you want to have the action frozen you need to capture the maximum light in the shortest time. You can achieve this many ways. Without flash, this of course quirks high shutter speeds. It is not that the subject is frozen, just that the movement over the duration the shutter is open, is so little that the object appears frozen. If you enlarge it sufficiently you will see movement, but that is a different topic.

To get the shutter speed high enough, you either need to have a more sensitive recording medium, (higher ISO ) or a larger aperture to let in enough light.

The other approach is with flash, which has a very short duration. Here you need to have a different set of settings and variables, you need to have the sensitivity of the camera or the aperture such that for a shutter speed below the sync speed, the ambient light does not contribute a lot of light to the overall image. Then the flash can illuminate the subject and due to the short duration, freeze the image in the same manner as the shutter speed. If you do not manage the ambient light properly (i.e. the ambient light is sufficient to light the subject, or almost light it without flash)!use of the flash can lead to severe ghosting with an outline of a sharp image at the moment of the flash going off

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