Controlling Depth Of Flash

This is a photograph of the reflector and head of one of my Norman 800 WS flash units. The patch-together look is the result of some Rosco Soft Frost, and a scrap of mount board held in place with some pieces of Gaffer Tape. Ugly as it is, this adaptation solved a serious problem - Exposure variations when large groups are photographed in depth, such as this group of students forming a heart using their red-shirted bodies. Let's cut to the chase and view the final result.

For the moment, please ignore the fact that small shafts of afternoon sunlight is highlighting certain students. Beyond that, very VERY little post production was used to balance the exposure from the front to the back of the arrangement. Our retiring Director, standing at the heart's apex, received the same amount of light as those in the  very back row. My solution may not be mathematically precise, but the even front-to-back exposure speaks for itself.

The Mechanics: I standing on a walkway about 12 feet above the ground. The Norman 800 head is on a stand another six feet higher. It's pretty obvious that the Big Guy in front is much closer to the flash than those in the wayback. When I did the math, I was surprised by just how large the difference was.

Sometimes Math Comes In Handy. For those math doubters, Pythagoras got it right. Using the height of the flash above the ground and the distance of my subjects from the wall, I calculated the flash-to-subject distances for specific taped markers on the concrete inside my chalked heart.

Once I started filling in the heart, the tape was covered by my subject's feet. When seen from above, you get a feeling for the actual distances, and can now predict how much the light would fall-off  from front to back.

Photographing so large a group is often precision guesswork, and from the calculated distances I concluded that if I exposed for the apex (subject closest to me), the middle will be underexposed by one stop, and the most distant by two stops.

Let's put it another way: If I set my exposure for the most distance subjects (38 feet), the middle subjects (33 feet) would be over-exposed by one stop, and the front-most (24 feet) by two stops.

With this in mind, I believed that if I could cut the effective flash output using a one-stop diffuser and position its upper edge at the half way point of the reflector, I could halve the amount of light hitting the closer subjects. And if I could cover the lowest portion of the diffused reflector, I could reduce the output still further.

In this illustration you can see the effect I was trying to achieve with the Soft Frost and that small piece of mounting board. By tilting the flash head up slightly, I was able to get a full dose of light coming from the top half for my most distant subjects and  diminishing output for my subjects standing closer to the camera.

Not this is by no means a "speedlight" trick, and presupposes that one has access to a powerful flash with a well-shaped reflector bowl. If I had a 1600 WS unit instead of one with only 800, I probably could have reduced the shooting aperture by a stop and reduced the overexposure resulting from those stray bits of sunlight dappling my subjects. But 800 is what I had and what I used, and I was pleased with the results.