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So I've already done some homework on flashes, the 430EX and the 580EX but a couple of things are unclear to me...

Can the 430EX III-RT be used without also firing the built-in flash on my T6i? Or does my built-in flash also have to fire? I know it at least has to be up, but is there a setting where I can at least have it up but not firing? I don't want the light from the built-in flash to interfere with my external...

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Yes, it's possible to use the T6i's pop-up flash to trigger a compatible off-camera flash (like the EX 430 III-RT). There's a setting for exactly that in the Custom Wireless settings under Flash Control, IIRC. The pop-up flash will still fire in order to trigger the slave flash, but it will do so before the shutter opens, so no light from the popular flash will affect the photo.

  • At least part of the "trigger flash" is visible while the shutter is open. I first noticed this while taking high ISO Macro shots. I could see a reflection of the trigger flash. If you were to take a photo while pointing at a mirror, you would see that the trigger flash is very visible in the mirror. It is a low power flash so it is not normally noticeable in the final image. – Mike Sowsun Aug 2 '17 at 13:07
  • @MikeSowsun What you are describing is the residual light from the flash tube after it has fired (prior to the shutter opening) as the gas inside it is cooling back down while the shutter is open. – Michael C Aug 3 '17 at 6:32
  • @MikeSowsun I've never noticed that, but I don't doubt that it's possible Ito see it in macro shots. On the other hand, there's no need to use optical triggering for macro shots -- owe a flash sync cord instead. – Caleb Aug 3 '17 at 11:13
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If you don't have any additional triggering gear to get the 430EXIII-RT off-camera, then, yes, you have to fire the T6i's pop-up flash to trigger it in optical TTL slave mode. Without firing the pop-up flash, there's no way to tell the off-camera flash when to fire. However, at regular subject distances, you can tell the pop-up flash not to contribute light to the scene.

Your other option is to get an on-camera master -RT unit to use on the T6i's hotshoe. The ST-E3-RT has no flash head on it, but is a radio master in Canon's -RT system. And there are 3rd party units that can do the same thing (e.g., Yongnuo YN-E3-RT, Phottix Laso transmitter, etc.)

You can also add-on radio triggers if you don't want to use Canon's -RT system.

See also: Is there a Canon equivalent of the Nikon CLS?

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