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I just started experimenting with flashes and was surprised by something, hope someone can explain me what us happening.

I have canon 7d with built in flash extended, set to 1/32 power. Also, a flash (triggering optically) shining on subject from the left. The camera is set to manual, and the picture is somewhat correctly lighted. I add a canon 430ex flash on the right, and set in my camera that external flash should flash. If I point this flash at the subject, the picture gets very dark. If I put the flash behind camera pointing away, the exposure is fine.

How does this happen if my camera is in manual mode?

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When using an external flash as a slave, the built in flash can be set to only trigger the slave and not fire with the shutter timing. The slave flash has an IR sensor that receives the signal sent by the master flash before the shutter opens and follows the instruction to fire once the shutter is open (part of the instruction is exactly how long to wait before firing).

If you're using it as a slave flash, and is being triggered by the built in flash and the slave is in front of the camera pointing towards the subject, it's quite possible it's not getting triggered at all (due to not receiving the command from the built-in flash).

If you turned the flash towards the camera, but away from the subject, it's more likely that the light was hitting the slave and setting it off.

Have you noticed that the external flash is actually firing or are you just looking through the view finder and only recognizing the built in flash going off or are you using Live View?

To correct this use the swivel feature of the 430EX to allow you to turn the bottom half of the flash with the receiving unit towards your camera and still point the flash head at your subject. You may also need to use a small reflector in front of the built-in flash to bounce the signal towards the 430EX if it is placed at an angle outside the cone of light emitted by the built-in flash.

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    With the flash behind the camera pointing away, how likely is it the built-in flash could trigger it then? Maybe less likely than in the scenario when the flash is pointed at the subject. It all depends on how the room reflects the light from the built-in flash. – Michael C Jan 29 '14 at 1:23
  • Of course, but I don't even know if I'm right or if your edited/added content will fix the issue. – BBking Jan 29 '14 at 3:13
  • It can actually be set either way: To only flash to command the slave or to also flash during exposure. – Michael C Jan 29 '14 at 4:39
  • With the 430 behind the camera it still fired, so it noticed a reflection of the IR-signal. I guess I'll have to repeat my experiment and look around better, and keep notes. – user607139 Jan 29 '14 at 7:27
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The amount and angle of light in the area where you took the picture and camera settings are probably the cause of getting a darker image than you expected. Try it again in other areas with a different subject and angle; if you get similar results, then there's something wrong with the device itself or the settings.

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