Thanks David. What's fascinating is how much things have changed. In the sixties you pinned down ambient light with a hand-held meter and used a guide number to figure out the flash. If you were wrong you didn't know it until you developed your film or got your transparencies back from Kodak, so if you were smart, when you were doing commercial work in a complicated situation you over-shot, which could get expensive. Nowadays, with digital, you can meter from the camera, set a shutter speed and aperture, make a shot to check ambient light, and adjust shutter speed if you were off. Then, once you've got ambient in your sights you can depend on i-TTL to control the flash. That's true even if a couple of your flashes are out of sight. You make a shot with flash and look at the LCD screen to see if everything is okay. If not, you dial the flashes down or up right from the camera and shoot again. You have a kind of control nobody could even conceive of in the sixties.