Monday, December 8, 2008

Flash Duration

Canon does not publish flash duration in their manuals. Nikon does and lists 1/1050 of a second for a full power flash. There are a number of independent testers out there with data from canon flashes but this one I think is signifigant because although the peak is short the trail off at full power makes the effective duration 1/200 of a second. Nikons Published data claims 1/1100 at 1/2 and from there on out the duration halves at each decrease in power.
Why is this important? Because the less work the flash has to do the better it will freeze action. The closer you get your ambient exposure to correct, the less work the flash has to do and thus the shorter the time of illumintion and bettering the ability to freeze action.
Conclusion is that if you set your lens to say 2.8 and the shutter to 1/15 and with the appropriate ISO achieve about 1/2 stop underexposure the flash will fire at 1/4 power or less. This means that if your subject is waving or even clapping the hands will be completly frozen. However if you stop down to F11 and try this there will be motion. Of course decreasing the shutter speed to achieve the same ambient exposure will not help because aperture is what determines the effect of the flash.

No comments: