You Can't Light Light:

by ralph

Not long ago I found myself joining a bunch of other relatives, taking snapshots at a confirmation party for my grandson. Since we often lined up to shoot the same groups it's a safe bet that most of my shots looked like those taken by anyone else -- with one exception. During the candle lighting ceremony I made sure that the candles would show up quite brightly - something that most people didn't do. It seems that most beginners use flash as an act of faith, hoping it will magically brighten the scene with all its nice, ambient lighting. T'aint so of course.

To clarify any fuzzy thinking about this situation lets imagine a subject consisting of a face lit up by many candles on a birthday cake, in a very dark room. The candles bathe the person in a cheery but dim glow. If this is a desirable picture you only need to put the camera on a tripod and shoot a long exposure which utilizes the marvelous ability of film to gather and accumulate light. Now further imagine that the whole side of the room was a window and direct sunlight was trying to come in. It was stopped by a full length black drape, of course, when the room was dark. Now the drape is suddenly opened and raw sunshine spills onto the cake and subject. Blinding, hard edged light takes over and now the warm glow is gone and the flames on the candle are barely visible. That's what happens when you take a flash shot. Surprisingly the brightness of even a medium size (camera top) flash can exceed the sun, very briefly, for up to 15-20 feet. At the risk of beating the obvious to death - you cant light light. Sure you can light objects but light added to the ambient light will only wash it away.

When I did those flash shots I took notice of the rather limited light in the room and guessed that even if I went down to 1/15 sec at the f8 lens opening that I was using for flash, I would pick up very little room lighting. So I took the candle shots at 1/15sec.(still handheld) which recorded bright flames and some glow, but left the overall flash exposure unchanged at f8. A few years earlier I used the same approach when taking a shot of two little twin sisters in front of a crackling fire by a living room fireplace. It was so dim that I used a 1 sec. exposure which showed really bright flames. In both cases there was no blurring of the rest of the subject because the flash lights most of the scene and is faster than most shutters.

Controlling the exposure this way usually has to be done on "manual", but there are a few newer cameras that have a "Background" button that lowers the speed to include more of the ambient lighting. Auto or Programmed modes only work to control flash exposure when they automatically switch to "dedicated" mode, in those particular cameras with their own dedicated flash units or units connected by dedicated cords. That's why you can't use Auto or Program mode when you plug into a studio type multi flash setup. Sure the camera has sensitive electronics to read the exposure, but having done that it has to drive mechanical elements (f stop and shutter speed) to set it. The flash has long come and gone by that time. In the fully dedicated setup the meter continuously reads the exposure, off the film, during the course of the flash, and when the film is "well cooked" the remainder of the flash is "squelched" a process that takes less than 1/50,000 sec. All of this is done long before any mechanical elements can begin to respond. This requires the rather complex multiple contacts of the dedicated hookup. One contact has to trigger the flash while others have to feed information to read exposure, drain off the "squelched" energy, provide a ready light, and often provide a zoom connection, and of course, none of this can be hooked up to an external, studio flash type setup.

Then there's the thing about the Inverse Square law. As you probably learned in High School. "The intensity of light decrea
s inversely as the square of the distance from its (point) source". Translated to English that means if you double the distance from a flash source the light drops to 1/4 (2 stops) -- double it again and you're down to 1/16 (4 stops). Just a bit further, when you hit a 5 or 6 stop loss, you've covered the range from a light highlight to an inky black void (at least on slide films). That means, for any given exposure and distance setup, the light will fall off to blackness somewhere beyond 4 times that distance. The tiny built-in pop up flashes hit the limit pretty quick. They need pretty fast film to get beyond ten feet. That's why most pros get a laugh from the wedding guests who try to shoot a large group (at 20 or more feet) with such pea shooters. It takes a combination of fast film and bigger pro flash to get that effective lighting distance up to 100 ft. and after that it still falls off to blackness around 500 ft.

I was truly impressed by the optimism of several unsavvy photographers once when I saw them shooting night flash shots of Niagara Falls from the Canadian side, across Niagara Gorge. That's double dumb. Not only was the flash pathetically inadequate, but, let's say they had a flashbulb the size of a tour bus, they would wash away the lovely colored flood lighting that created the picture in the first place. -- Again, you can't light light.

So if light falls off as the square of distance why aren't distant objects darker?? That's an easy one. The apparent size -- that is the size of the image on a piece of film or on the retina of your eye falls off in exactly the same ratio so that the intensity of light stays constant.

Here's a thought to chew on. We generally assume that atmospheric particulates (haze or fog) merely scatter light about, but in fact they actually absorb it. Did you ever notice how inky black it can be when driving in fog -- until you suddenly emerge into a gauzy pool of light near well lit intersections. It seems that bright light can be completely soaked up beyond a critical distance. I used to use a pretty standardized exposure on night skyline scenes of cities but when I shot long, across-the-bay tele shots even on a fairly clear night the shots were quite underexposed. Obviously, even "clear" air absorbed over a stop of light and at a great enough distance would absorb all of it and the skyline would fade to black.

Now here's a shocker. We're all familiar with the concept of "visibility" in daytime - the distance to the point at which all details of a subject are lost in haze. That means that the actual light from the subject has also "faded to black" even in broad daylight. We just don't realize that's what happened because it was replaced by an equal amount of scatter light supplied by the sun shining on those same particulates along the line of sight.

"Softness" and "hardness" are qualities of light that we all have to learn to recognize and use. Softness is defined by the width of the light source, for a given distance from the subject. Obviously the wider the light source the more spread out (softer) the shadow is and the less bright and shiny the highlight will be. This is desirable for many subjects from portraits to any complex shapes macro or nearby, that might be chopped up by deep shadows, or poorly rendered by harsh contrast. The hardest light comes from tiny sources like the sun or a small flash. Although it's an "easy" light to come by, and you sometimes have to tame it, it does have some uses like skimming across a surface to create a texture, or clearly delineating a distant scenic. Some photographers even go against the grain and use it for portraits creating a pretty distinctive style. Putting a diffuser on a light source does not do much to soften it (it just makes it dimmer) because it doesn't increase its width. On the other hand, even a small flash unit will be a medium soft light when you're within a few inches of the subject.

Since a photo
is a graphic record of light it seems that light itself should be the subject of our most intense interest. The above is just a little food for thought.