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 SB-800.
I was shooting a photo today with a Nikon D3 and an SB-800 flash manually set to 1/2-power. 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 SB-800 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 indeed fire at full power. At in-between shutter speeds such as 1/160 sec. and 1/200 sec., full power fires about 1/3-stop and 2/3-stop less than full power, respectively.
By comparison, an old Nikon SB-24 flash fires at about 1/3-stop less than full power when manually set to full power at 1/250 sec. With an ancient Vivtar 285 flash, you get what you set, always spot-on.
What’s happening?
It appears the culprit is long flash duration at full power. With an SB-800 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 SB-800 has the same flash duration claimed for both full power and half power. Even then, Nikon’s claim of 1/1050 sec. flash 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 to 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 SB-800 at full power (1/370 sec.) is three times longer than that claimed by Nikon (1/1050 sec.). Also, just to note, the new SB-900 comes with an even lower claimed full-power flash duration of 1/880 sec.
So the next time you need a full-power manual flash from a Nikon SB-800, use 1/125 sec. or slower.
