Using diffuser with 580EX flash

I am relatively new to the flash. I got a diffuser for 580EX flash but somehow it doesn't work properly: 580EX always meter the light using its own beam emitter and metering sensor instead of firing a pre-flash, so it doesn't know the existence of diffuser hat, thus getting underexposure. How could I make 580EX using preflash to meter the light?

Do I need a diffuser for it at all? it already got a reflection pad.

Thanks

The camera is metering ambient light, whether you have the diffuser on or off.

What mode are you shooting? Shoot manual and your flash in ETTL.

If it is in ETTL it will use a pre-flash to determine the amount of power for the exposure whether you have a diffuser or not. Keep in mind you can not see the preflash, it happens too quickly before the real flash. You might be getting under exposure due to the flash just simply not being able to deliver enough power to get the proper exposure, which could be due to a diffuser. There are other reasons for underexposure in ETTL as well. Many things fool the system like highly reflective items in the field of view or brightly colored objects. This is where you might need to dial in a flash exposure compensation (FEC) to counteract it, or switch to manual mode flash and dial it in that way.

Canon EX flash units and ETTL are NOT noted for over exposing a scene. And it's not difficult to fool ETTL into thinking it has enough light. A simple bright spot in the scene will do that.

If you are not shooting into a mirror or a window or some other highly reflective surface bump the FEC to +1 or so.

I also find that if there is a white sign or wall in the photo, my 580 will underexpose.

The 580ex doesn't have any sort of beam emitter to measure light. The time between the preflash and the flash is just so quick that it is unusual to see two flashes. The flash is not evaluating the light from the preflash. Rather, it is the camera. Even if you have a 580exII that has a built in sensor, the light the flash is measuring is what it sees in the picture-taking light, not a beam emitted separately.

I'm not a big fan of the little clip on diffusers, but it has nothing to do with whether or not the light is being metered correctly. The flash is metered exactly the same way regardless of whether or not a diffuser is in place.