I have just bought 2 speed lights so in total I have:- Metz mecablitz 24AF-1 Digital 2 x Yongnou YN 560 IV I am using the Metz on camera to trigger the 2 x Yongnou on S1. I am shooting my EM10 in manual ISO800, 125s with the 14-42 kit @ F5.6 and shooting something 8-10 ft in front of me. I have a small bag defusing the Metz and the Yongnou. This is what I do not get - If I just fire the Metz the subject is nicely exposed. If I add the Yongnou's sometimes its darker than the Metz on its own and other times it is overexposed. This should not be a challenge for any flash and all are firing when asked, so why is it underexposed with more light on the subject??????????????