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Re: Hot-Shoe Flashes (Canon 420EX) - is light controlled by output strength or duration?
In reply to sethmarshall,
6 months ago
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sethmarshall wrote:
This is a really simple (stupid) question. I've always thought flash output was a factor of strength in terms of electrical power delivered to the unit (more power, more flash) but the duration was the same. This assumption made because I perceive a visual intensity difference when altering it's output settings (but not a noticeable duration difference).
Recently I read something that suggests my hot-shoe flash output is a result of a longer flash duration. Perhaps the perceived intensity differences have to do with the interpretation of my eye and at faster durations my eye "sees" less intensity.
Can someone correctly answer this simple question?
If you plot a graph of light intensity over time, the total exposure is the area under that graph.
The brightness of a flash is NOT constant. It starts at zero, spikes very quickly to full brightness, and then drops fairly rapidly at first, followed by a relatively long "tail" where the brightness trails off to zero.
All hotshoe flashes that I am aware of have the same discharge curve regardless of the power setting. What they do is to cut off the flow of electricity into the flash tube almost instantaneously when the control electronics decides that enough light has been emitted. The result is that the flash duration is shorter for lower power, and longer for higher power. By reducing the flash power just a little you cut off that long, slow decay of the light output at the end, vastly improving the motion stopping ability of the flash. At very low power (like 1/64) the flash duration gets really, really fast, since the flash pulse is quenched very early in the discharge curve.
Take a look at this link for graphs of flash intensity over time for flashes set to different power levels:
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Regards,
Duncan C
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