Why does a slave flash make the photo darker when it fires at higher power?
Asked 2/15/2011
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I'm using a Panasonic Lumix G1 in manual exposure mode with the built-in flash enabled. Example settings were around f/6, 1/30s, ISO 200, with flash compensation set low. I triggered a Nissin Di466 wirelessly in slave mode and noticed this behavior:
- With the slave off, the photo is normally exposed.
- With the slave at lower power, the photo gets brighter.
- With the slave at higher power, the photo becomes very dark and warm/reddish.
I later found that the slave mode I chose was responding to the camera's pre-flash instead of ignoring it. Why would that make the final image darker instead of just leaving the exposure the same as the built-in flash alone?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
6
Your slave flash is not fully recharging before firing at the appropriate time and you're seeing the ambient light.
Here's how it works; when your slave is set to a lower output it's actually firing twice because it's 'seeing' the 'pre-flash' from your camera and firing on the 'pre-flash' and the actual flash from your camera.
When you crank up the juice on your slave it's firing on the 'pre-flash' and it doesn't have enough time to fully recharge before it's being asked to fire again (at the appropriate time).
So, the only thing that's left is the existing light in the room, likely an incandescent bulb which is casting a red/yellow cast on your image.
EDIT: As a very unscientific test, you could darken the room completely, slow your shutter speed to about 3 seconds and as soon as you hear the 'shutter' open fire the flash manually (at the higher setting). If that produces a brighter exposure then you know the flash wasn't firing when it should have.
EDIT 2: After some looking around, I'm under the impression that a flash mounted on the hot-shoe will disable the pre-flash. If you have a spare that you can crank down to it's minimum that may do the trick.
Originally by user3203. Source · Licensed CC BY-SA 4.0
user3203
15y ago
0
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Your slave is likely firing on the camera’s pre-flash, not just the main flash.
What happens is:
- The camera emits a pre-flash to meter the scene.
- The slave sees that pre-flash and fires.
- The camera meters that extra light and decides it needs much less flash power for the real exposure.
- When the actual exposure happens, the built-in flash fires weakly, and the slave may either fire again or fail to recharge in time if it was set to high power.
At low slave power, it can often fire on both flashes, so the image gets brighter. At high slave power, it may dump most of its energy on the pre-flash and not be ready for the main flash, while the camera has already reduced its own flash output. The result is an underexposed frame lit mostly by ambient room light, which often looks warm/red under incandescent lighting.
So the dark frame is caused by pre-flash metering being fooled plus the slave not being ready for the real exposure. Use a slave mode that ignores pre-flash, or use fully manual flash settings.
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