Light is Cumulative
Light is Cumulative
In several of my workshops I mention that, photographically, light is cumulative. I spend several minutes explaining the concept; how for as long as the shutter is open your camera is gathering and recording all the light that passes through the lens. I'd like to think that I explain it well enough and in accessible enough terms that most everyone gets it. But I think that very few people realize what a powerful thing it can be to fully understand this concept.
Look at this image.
Except for resizing to be more appropriate for this newsletter, this is straight out of the camera. Eventually I will do a little processing on the original copy of this image. I'll straighten it, cut out a bit of the foreground and even tweak the light levels a bit.
For now though, this is the perfect image to help illustrate the concept of cumulative light.
- The light that was recorded. Note how everything that is stationary and lit is nice and sharp and more or less well exposed. Note also how everything that was moving while the image was being made shows up as streaks of light.
- The light that wasn't recorded. This is perhaps an even more powerful concept. The camera was mounted on a tabletop tripod and was only a few inches off the ground. This exposure was about 30 seconds at f11. During the course of the exposure, probably 8-10 people walked right in front of the camera. Most didn't even notice I was taking a picture, a couple of teens didn't care, a couple even waved (thinking that would be captured in the image) and one woman noticed when she was halfway across the field of view. After moving out of the camera's field of view, she stopped to apologize and I briefly explained that she wouldn't even show up in the image. I didn't have to look at the image to know this but the interaction was what inspired me to write today's post.
So how could so many people walk right in front of the camera and not one of them show up in the final image? One of the keys is there was no light shining on any of the people. None stopped and lingered in my frame; they all kept up a pretty steady pace as they crossed. It took the average person about 3-4 seconds to cross the frame. That means, at worst, they each blocked the camera from "seeing" 3-4 seconds worth of light. (That's the reason some parts of the image are overexposed; I had to estimate how many people would walk in front of the camera and overexpose accordingly.)
Imagine painting on a wall with spray paint. As you wave the paint in front of the wall, as long as you don't pause the paint will apply evenly. The same process works in reverse. Let's say you have a nice wide spray of paint and while it's spraying you wave something small such as a wooden spoon between the paint can and the wall. As long as you move it at a steady pace and do not let it linger in any one spot, it blocks a consistent amount of paint from reaching each point. The result is that you still get an even coat.
