This may be a silly question... When using off camera flash (I have the vivitar 285hv), how can you determine ambient lights and flash levels? For example, suppose I wanted a photo with a person in the foreground, but showing the background. I know two lights (one for person, one for background) -- obviously you can meter the ambient light with the camera, but how would you know what the flash will do? is it just trial and error every time? Like "Lets try it at 1/16. Too dark, ok how about 1/4?"