If you weighed a 10-lb bag of potatoes and found that it weighed only 5 lbs, would you be concerned?
If a store clerk said that a 10-lb bag of potatoes weighs 10 lbs only if you buy it in the morning, would you be confused?
Let’s talk about camera flashes, in this case, the Nikon SB800.
I was shooting a photo today with a D3 and an SB800 set to 1/2-power manual. Deciding to add more flash, I upped the flash to full power. Surprise! The flash exposure remained the same. What’s going on and where’s my light?
If you set an SB800 flash to fire at full-power manual, then you probably expect it to fire at full power. Well, it does and it doesn’t. It depends on the camera’s shutter speed.
At 1/250 sec, full-power fires at about 1/2 power (as checked with a Minolta flashmeter). Half-power still fires at the expected 1/2-power output.
At 1/125 sec, full-power does fire at full power. At in-between shutter speed settings such as 1/160 sec and 1/200 sec, full-power fires around 1/3-stop and 2/3-stop less than full power, respectively.
By comparison, an old SB24 fires at about 1/3-stop, or so, less than full power when set to full power at 1/250 sec. With an ancient Vivtar 285, you get what you set, always spot-on.
What’s happening?
It appears the culprit is long flash duration at full power. With an SB800 and a shutter speed of 1/250 sec, the camera starts and stops the flash within the 1/250 sec, which is too short for the flash to fire a full charge. But at 1/125 sec, or slower, the flash has time to fully discharge its capacitor.
Generally, flash duration gets shorter as flash power decreases. But, oddly enough, Nikon’s SB800 has the same flash duration claimed for both full power and half power. Even then, Nikon’s claim of 1/1050 sec duration for full power seems to be, uh, optimistic from a photographer’s point of view of usable light.
The industry standard is to measure flash duration to the point where the capacitor is 50% discharged (t.5), and not 100% discharged as a photographer might expect. A handful of manufacturers might quote a “t.1″, the time for the capacitor to discharge down to 10%. A “t.1″ is about three times longer than a “t.5″.
Just to point out, a measured flash duration of an SB800 at full power (1/370 sec) is three times longer than that claimed by Nikon (1/1050 sec). Also, just to note, the new SB900 comes with a lower claimed full-power flash duration of 1/880 sec.
So the next time you need a full-power manual flash from an SB800, use 1/125 sec or slower.
Tags: photo technique
