I'd like some advice on setting up my (non-canon) wireless slave flash, using my pop-up flash as the trigger, on my Canon 40D. I'm using diffusers on both the pop up and the slave, and what I'm looking for is soft fill-in flash, in ambient light (such as a person at sunset) as opposed to flash being my main light source. Am fairly new to flash, and have avoided it mostly... however, with the diffusers I'm actually quite pleased with the softer results and am now looking to use it more. I'm trying to get my head around output / ratio calculations... But i think I'm overthinking it... So, bearing in mind that I lose about a stop on each diffuser. And that the internal light meter is reading for the pop-up flash only. How do I factor in the wireless slave and compensate for a good exposure? I'm also thinking I want to lower the power output on my slave to the lowest setting because none of my subjects are gonna be more than few feet away and I'm really looking for a soft fill-in. Not knowing what the hell all the numbers on the back on my flash mean, can anyone explain how to do that simply too. Thanks.