Exposure level indicater w/ Flash not correct...

I notice even with the 580EX II flash attached to my 40D and a 2.8 lens the exposure level indicater shows underexposure. When I do take the shot the images comes out perfect. So does the flash suppose to let the camera know that it will compensate for the low lighting and the meter would be showing perfect exposure?

That doesn't help when you're using as fill flash. With fill flash, you set perfect exposure and the fill flash helps the shadows.
The amount of underexposure (according to the meter) tells the flash how much flash is needed to get correct exposure.

The exposure level indicator in your camera measures ambient light only. The flash doesn't really know how it will impact the exposure until a pre-flash is fired or during the actual exposure (depending on the mode, etc...).

e-k

kdfederer wrote in post #5709292
That doesn't help when you're using as fill flash. With fill flash, you set perfect exposure and the fill flash helps the shadows.
The amount of underexposure (according to the meter) tells the flash how much flash is needed to get correct exposure.

I'm not using it as fill but for low lighting conditions. In that case looks like I don't need to worry about what the sensor is show since the flash will compensate correct.