Monday, February 24, 2014

Day 43, Learning 43: Nikon SB800 flash duration

Okay, sticking with the Nikon flash theme.  Understanding how your flash creates or emits light can be helpful, especially working with high speed photography like photographing a droplet of water.  The little-known, little appreciated aspect of all flash units is that they adjust their power by adjusting their duration.  To say it a different way, at half power your flash doesn't emit a less powerful amount of light, it emits the same light for half the time.  And it's quantifiable.  For the Nikon SB800 (a wonderful unit that you can get used for half the price of its contemporaries), those times look like this:

Full power =  1/1,050th of a second
1/2 power  =  1/1,100th of a second
1/4 power  =  1/2,700th of a second
1/8 power  =  1/5,900th of a second
1/16 power = 1/10,090 of a second
1/32 power = 1/17,800 of a second
1/63 power = 1/32,300 of  a second
1/128 power = 1/41,600 of a second

Why is this helpful?  Well, first it's a nice conceptualization.  Working with your flash at full power, even if your shutter speed is 200, it's effect is the same as if it's at 1000 (for the exposure that's attributable to the flash; obviously is there's a lot of ambient light, that's being exposed at 200).  The other is that droplet of water I mentioned before needs to be caught at 1/17,800 of a second to be perfectly clear-- or 1/32nd power on your flash unit.  So to photograph the splash of water from an ice cube dropping into a cool bourbon, you need to set up your system so that the correct exposure occurs at 1/32.

Studio strobes, on the other hand, tend to be about 1/5,900th of a second (or the equivalent speed of 1/8th power).

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