Flash Key is when you want flash to be the main light. Flash fill is when you want flash to be the secondary light. To make Flash Key effective, you often, although not always, want the background darker than the subject. This is done by using EC, aka Exposure Compensation. EC simply is adjusting the shutter speed. There is no button or anything marked EC, it is just a name to describe a variety of ways to change the shutter speed from what the camera meter says is the best exposure. Here is an example: You meter the scene and are in AP mode. You The content you are trying to access is only available to members. Sorry.