I should have been more clear & complete on how the flash works in Program mode. As I said, it depends upon the ambient light. With the flash turned on, the camera will determine a correct exposure for the ambient light within limitations, including a shutter speed of 1/60-1/250 and the available apertures on the lens, though my 30D seems not to open up wider than f/4, even with an f/2.8 lens. If it can do this, the flash becomes a "fill flash," rather than overpowering the ambient light. Thus, you'll get a normally-exposed picture with some "fill" in the shadows from the flash. If there isn't enough ambient light, the system will generally set the shutter speed to 1/60 (it may depend upon the camera) and the aperture to f/4 or the widest the lens has, if it's not as wide as f/4. Then, the flash will be the primary light. You can force the flash to be the primary light by using manual exposure and setting it to underexpose for ambient light. (E.g., set the shutter speed to 1/250 and use a smaller aperture than required for the ambient light.) The flash will still work with E-TTL--using manual exposure on the camera is not the same as setting the flash to manual mode.


For a LOT more, check this Photonotes article. Scroll down to the E-TTL section for more on how E-TTL works. The article is quite long and extensive--there are two more pages.