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| p.1 #3 · Total noob with flash photography, need help with vignetting issue |
...since she was against a flat wall I didn't feel like I needed such a great depth of field so I tinkered around until the meter said that 1/800 sec at f9 was correct and half her face was missing from a dark black shadow... The half that wasn't in the shadow looks perfect, but the other half is pitch black.
You've just learned your first lesson about flash photography: sync speed.
At slower shutter speeds the first curtain of the shutter opens all the way, then the flash fires, then the second curtain closes.
But at faster speeds the shutter blades can't move fast enough, so the system pulls a clever trick; the second curtain starts to close before the first curtain is all the way open, creating a slit that moves across the film or sensor.
With continuous light, like the sun or incandescent lights, the light doesn't change over time, so the moving slit exposes the whole image to the same amount of light as it moves.
With flash, though, the burst of light is so short that only the part of the sensor that is currently exposed will get the flash exposure, and then the flash dies out and the rest of the sensor only gets exposed to the room light.
So, when using flash you need to keep the shutter speed at or below the flash sync speed. For most DSLRs that's 1/200 to 1/250. At 1/800, only part of your picture got flash exposure, and the rest was dark.
The second issue is metering. Unless you were using a flash meter, the numbers you were given were for the existing light in the room. When you then added flash to the mix you overexposed the image.
You can buy a flash meter, or you can use manual power adjustments on the flash units and start with a low power setting, check the camera's histogram, and increase power until you get the proper level.
Shutter speeds at or below sync speed have no effect on the amount of flash exposure; only changes of aperture affect that. Shutter speed will affect the amount of continuous light recorded, and so we usually (in a studio-type setting) use the fastest shutter speed we can in order to minimize the effect of ambient so we can use placement of our flash and use of flash modifiers (soft boxes, beauty dishes, etc.) to maximum effect.
[Edit] I see from your profile that you have a Sekonic flash meter. Set the shutter speed of your camera to maximum sync speed as stated in your manual, and set the shutter speed on the meter to the same speed. Then adjust the power of the strobes until test firings match your desired aperture for the amount of depth of field you want.