Paul C. Buff, Inc. Technical Forum

I have a alienbee B800, 320ws
Hears my delemma when I set the strobe a full power the shot comes out the dimest as I move the selector down in power to say 1/32 the subject is the brightest and would make for a good picture. All were take at the same camera settings (1/160sec, f8, iso200). I fired the strobe with an external flash facing the subject at the same distance.

My question is: Are the slider settings backward for my unit in that the real full power is at 1/32 not "Full"? Also at "Full" setting the strobe make a loud popping sound when it discharges thus I'm assuming this is the max energy being discharged.

Thanks for your help
Ken

I think this is the key. If it is an external speedlite set to TTL, then it is firing a evaluation preflash just before the actual exposure. Preflashes are a known amount of light that the camera uses to adjust the flash output. When slaving a studio light, the studio light fires when it sees any flash and it does not know the difference between a preflash and an exposure flash. Similarly, the camera does not know how much of that light is from its own pre-flash vs. that of a slaved light.

So when the preflash fires, the slave flash fires, and the camera says "thats a lot of light, I need to adjust down for that". The camera tells the flash to fire just a little, and the studio light is recycling from the flash it just emitted, so it fires at a power lower than it is set to, resulting in under exposure.

As you dial power down on the studio light, two things happen. First, less light is fired from the studio light during the preflash, so the camera does not try to adjust down as much. Second, the studio flash is at a lower power, and can recycle porportionally faster and will trigger again on the exposure flash. The lower you go, the less the camera adjusts, and the more closely the two flashpulses from the studio light are in power.

If you are going to use an external speedlite, set it to "M". This will prevent the preflash from firing, resulting in one flash from the speedlite and studio light. As you adjust power on the studio light, you will see a correlated reduction in exposure. You can manually set the speedlite to a certain power to act as fill, or dial it way down so that it does not impact the image at all.