Basic flash question

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raven4ns
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p.1 #1 · Basic flash question

I am just getting into outdoor flash usage and have a question. How or why is a flash able to freeze a moving subject like a flower which is being pushed around by a breeze? Most cameras have a shutter speed of approx 200 with a flash and if the ISO is say 200 or 400 and perhaps an aperture of f11 or f16 I don't understand why it can freeze a subject that has some movement. Thank you.

Tim

Jul 19, 2012 at 12:12 AM

hugowolf
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p.1 #2 · Basic flash question

raven4ns wrote:
I am just getting into outdoor flash usage and have a question. How or why is a flash able to freeze a moving subject like a flower which is being pushed around by a breeze? Most cameras have a shutter speed of approx 200 with a flash and if the ISO is say 200 or 400 and perhaps an aperture of f11 or f16 I don't understand why it can freeze a subject that has some movement.
Tim
It can’t if there is enough ambient light such that the shutter speed determines the exposure duration. However, if there is enough difference between the ambient light and the flash light, then the flash will stop motion.

The duration of the flash burst at full power is quite lengthy: somewhere in the region of 1/500s or longer, but at less than full power the flash duration is very short. At half power the duration is about 1/1000s, and at 1/4 power about 1/2000s; approximately halving for each one stop drop in power.

Most studio type flash units work on a different principle and in most cases their duration increases at lower outputs. There are exceptions to this, Photogenic Solair and Buff Einstein studio lights, amongst others, use similar systems to shoe mount type flash units.

Brian A

Jul 19, 2012 at 12:30 AM

BrianO
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p.1 #3 · Basic flash question

raven4ns wrote: ...How or why is a flash able to freeze a moving subject like a flower which is being pushed around by a breeze?

As Hugo said, it all depends on the ambient light level.

If one were shooting in total darkness, the shutter could be set to "Bulb" and left open for hours, but the flower would only be exposed for the short duration of the flash and would thus appear frozen in time.

If the ambient light is bright enough to add to the exposure, then you can get "ghosting" -- there is a primary frozen image exposed by the flash, with a blurred image of the moving flower exposed by the ambient.

If the ambient is so bright that even at the lowest ISO setting and with a small aperture set that it generates unwanted exposure levels, the next step is to add a neutral density filter to darken the incoming image, shade the subject, wait for dimmer light, or some combination of the above.

Jul 19, 2012 at 05:12 AM

raven4ns
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p.1 #4 · Basic flash question

Thank you for your input. I don't really understand what you said so I will have to try it and see what works.

Tim

Jul 22, 2012 at 04:22 PM

BrianO
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p.1 #5 · Basic flash question

raven4ns wrote: ...I don't really understand what you said...

I'll expand it a bit.

As you said, 1/200 second may not be fast enough to freeze a flower that's moving around in the wind. (It may be fast enough, if it's not moving a lot, or if you're shooting from a distance that makes any given flower fill only part of the frame.)

If the flower is only lit by flash, though, the light from the flash can freeze it, since the burst of light from the flash is only around 1/1000 second...or even shorter.

So, no matter how long the shutter is open -- whether it's 1/200, 1/60, a full minute, or even longer -- the flower will only be exposed for that 1/1000 second.

That's in total darkness, though. If there's any ambient light in addition to the flash, then the flower will exposed for however long the shutter is open.

If there's only a little ambient light, and if you have a small aperture and a fast shutter speed, then the ambient light may not be at a recordable level, so only the flash would count.

If there's a lot of light, though, then it'll count, and you'll have a blurred flower.

The final option is to use a special flash mode in cameras and with flash units that have it, and that's FP (focal plane mode) or HSS (High Speed Sync mode) flash. In HSS, you can use faster-than-sync-speed shutter speeds, because the Speedlite is pulsed very quickly to act like continuous light during the time the shutter slit is moving across the film/sensor.

HSS mode flash isn't as powerful as normal flash, but at the close range used for most flower photography that isn't an issue.

Jul 22, 2012 at 11:07 PM

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