I have a Canon T2i and I have the Canon EF 70-300mm IS f/4-5.6 attached with a Canon Speedlite 270 EX external flash. My question is that when I attach a $10 plastic diffuser to the Speedlite, the shutter speed shifts from 1/100 down to 1/2 second in Aperture Priority mode @ f/5.6. In order to compensate, I need to set the Exposure Compensation to -5.0. When I do this, the exposure and color are great and natural, but this doesn't make sense to me as to why just adding a diffuser affects the shutter speed so much. I realize that the camera has a flash compensation setting, but I could not find a good setting for this that kept the exposure and colors to look good. I'll admit that I don't understand what the flash compensation actually does differently. could someone explain what is going on inside the camera to so wildly over expose the photo with the diffuser? or what I could be doing wrong? Thanks in advance.