Canon 40D & 580EX11 how to do ettl

Can someone explain how to set up the camera and the flash to be able to shoot in ettl . I am not sure if I just have to set the flash to ettl and not the camera. Also, do I have to set the ISO on the flash or just in the camera ?
Indoors, do I set the camera to Program ? Outdoors to what ? I shoot weddings and have just switched from shooting in manual mode camera and flash to ettl. I appreciate your help, rollsman

On the flash, press the mode button until ETTL is displayed on the LCD display. If you can't get ETTL to show on the display, reset the flash custom function C.Fn-05 to 0.

There really isn't anything you need to do on the camera other than to verify that ETTL mode is set to Evaluative, which is the default mode, in the menu. And ISO is set on the camera. There is not provision for setting the ISO on the flash unless the flash is in External Manual mode and even then, the ISO should match the ISO set on the camera unless you specifically want them to be different.

In ETTL mode, the flash is basically in automatic mode with the camera determining the flash exposures for you. The camera itself can be set to P mode, if you wish, or to any of the other creative modes like Av, Tv or M.

rollsman wrote in post #5979154
Can someone explain how to set up the camera and the flash to be able to shoot in ettl . I am not sure if I just have to set the flash to ettl and not the camera. Also, do I have to set the ISO on the flash or just in the camera ?
Indoors, do I set the camera to Program ? Outdoors to what ? I shoot weddings and have just switched from shooting in manual mode camera and flash to ettl. I appreciate your help, rollsman

Here is a website devoted to the Canon flash and is a must read IMHO.
http://super.nova.org/​DPR/#Canon

Thank you Leo and others for the info, rollsman

Also can only ETTL with the flash pointed foward at 90 degrees.

If you try to bounce it won't work.

Keithaba wrote in post #6311395
Also can only ETTL with the flash pointed foward at 90 degrees.

If you try to bounce it won't work.

Say what? Can you clarify your post for us because as stated, it is not true.

is that not correct? I thought the ETTL only worked if the thing was pointed foward? Or else it's not going to measure correctly as it can't know how high the ceiling is, or what color the ceiling is.

Keithaba wrote in post #6311449
is that not correct? I thought the ETTL only worked if the thing was pointed foward? Or else it's not going to measure correctly as it can't know how high the ceiling is, or what color the ceiling is.

ETTL exposure is based on the preflash light that's reflected back to the camera so it really doesn't matter which way the head is pointed. You'll get the most efficient use of the flash light when it's pointed forward (although it doesn't necessarily mean that you get the best lighting) but using the flash in the bounce position will work, too.

ah shucks, I better trash the thousands of amazing bounced E-TTL shots I've taken in the past few months as apparently E-TTL wasn't working

Keithaba wrote in post #6311395
Also can only ETTL with the flash pointed foward at 90 degrees.

If you try to bounce it won't work.

Others have already covered how E-TTL works and WHY it still works when bouncing....

Keithaba wrote in post #6311449
is that not correct? I thought the ETTL only worked if the thing was pointed foward? Or else it's not going to measure correctly as it can't know how high the ceiling is, or what color the ceiling is.

What does not work when the flash is not merely pointed forward, is the max distance scale that appears, based upon the FL of the lens. It cannot estimate max distance when bounce is employed, it can calculate max distance in direct mode only.