To get aperture you divide the guide number of your flash by the camera to subject distance. That's if the flash is on the camera. If the flash is off camera, and at a significantly different distance, what distance do you use: flash to subject or camera to subject? Or is there some other formula for this case? When using flash for fill is it better to use the flash on-camera or off-camera? How can you get two flash units to go off at the same time? Is it possible to use some sort of Y for the flash cable that connects to the PC? (Here PC means Photo Contact, not Personal Computer.) How far off the camera does a flash have to be to avoid red eye?