20D external flash, how to?

[kriptap]

I have just got my 20D and will be using it in an underwater housing. Is there a setting in the camera that tell it to use an external strobe?

Thanks

[tim]

There are custom functions that control the flash, but I don't understand what you mean. How will the camera trigger the external strobe (not sure what that is either).

[kriptap]

It would be just like you adding an external flash, would you have to tell the camera you were using it or would it just work when installed?

[robertwgross]

I can't address the underwater housing. However, there are only a few ways to trigger an external flash of any kind.

1. You can attach the external flash directly on the hot shoe, or indirectly using the correct off camera shoe cable. Canon does this.
2. You can do the Canon wireless flash system, which uses short range near-IR to get from a master (e.g. STE2) on the hot shoe to any slave (e.g. 420EX) nearby. I have no idea whether that would work underwater or what the range might be. I would not expect much.
3. If you throw aside E-TTL or E-TTL II, then you can do a "straight sync cable" from the PC sync port on the camera to a strobe that will take that type of cable input. I don't think that Canon goes here with its Speedlites.
4. If you used the built-in flash on the camera, then you might be able to optically trigger an optical slave strobe. Keep in mind about the pre-flash before the main flash.

---Bob Gross---