Manual Flash Question

[BrianEE93]

Please help me understand what is going on here. I did a little experiment today. I have my Sigma flash shooting straight up at a very reduced power in manual to trigger my slaves. I have two slaves in umbrellas that are set at full power and I just move them to get the exposure I want. I left everything in place and just changed my shutter speed and aperature. I started at 1/250th f/8 and dropped by shutter speed by 1/3 and pinched down my aperature by 1/3 stop each consecutive picture. By the time I got to 1/60th f/16 the pictures were 3-1/3 stops underexposed. The starting picture was a little underexposed but not much. I would think a picture at 1/250th f/8 is the same exposure as 1/60th f/16!? What am I missing?

[Jon]

The first thing you're missing is that shutter speed doesn't affect flash exposure as long as it's slow enough for the flash to sync. So just by stopping down to f/16 you decreased the flash exposure by 2 stops. Increasing the shutter speed to 1/60 sec. doesn't affect this at all. If the starting picture was "a little underexposed, but not much", you could be looking at 1/3 to 2/3 stops, and if your 3-1/3 stops under was based on eyeballing the degree of underexposure, the combination could add up to what you're seeing. Examples?

[BrianEE93]

The first thing you're missing is that shutter speed doesn't affect flash exposure as long as it's slow enough for the flash to sync. So just by stopping down to f/16 you decreased the flash exposure by 2 stops. Increasing the shutter speed to 1/60 sec. doesn't affect this at all. If the starting picture was "a little underexposed, but not much", you could be looking at 1/3 to 2/3 stops, and if your 3-1/3 stops under was based on eyeballing the degree of underexposure, the combination could add up to what you're seeing. Examples?

The light bulb just turned on! Or was it a flash bulb?:D I am so use to shooting with continuous light(the sun) that I forget about short duration strobe light. I would assume that the slaves are in sync because even with 1/200th and 1/160th the flash was proportionally darker(because of the increased f-stop)

[Jon]

Right. I'd assume that too.