It has taken longer to get back to this than I expected. A week of teaching photography at John C Campbell Folk school and moving to Arkansas for the summer has diverted my attention. So...back to the topic of LIGHT!
The thought that light and earthquakes have something in common came to mind recently when two earthquakes occurred within a few weeks of each other. One was a magnitude 8 and the other a magnitude 7 but it was noted that the magnitude 8 was hundreds of times more powerful than the magnitude 7 quake. That started me thinking about how the power of light and earthquakes are measured and how both are non-linear. When graphed, their lines are not straight but curved and in that way they have something in common.
In photography we measure light with a light meter and convert the measurement into "f" stop and shutter speed combinations for use on the camera to control exposure. If we hold shutter speed and ISO constant, then as we measure brighter and dimmer light we will change the "f" stop to control exposure.
The most common "f" stop numbers are f2.8, f4, f5.6, f8, f11, f16, and f.22. I think of these as "whole" f stops. Older lenses and some new lenses have these numbers marked on them. Moving from one whole f stop to another either doubles or halves the amount of light that will go through the lens for any given length of time.
So what does all this mean to me for practical use as a photographer? Well...it helps me visualize how fill flash works and I use that in portrait, event and close-up nature photography.
To make sense of this lets start by imaging a photo situation. It is a little controlled so that I can reduce the number of variables and highlight how light works. You are photographing the side of a building where the sun is shinning through a tree. In some places the sun is blocked by the trunk of the tree, in others the direct sunlight shines through. There are all levels of light in between where light is blocked by one or more leaves. I might look something like this:
You set up your camera so the lens plane is parallel to the wall of the church. That minimizes the effect of the Inverse Square Law because the camera and your flash are essentially equal distance to all areas of the subject.
Set the camera up in spot metering mode, set ISO 100 with a shutter speed of 1/125 and don't change those settings for the rest of this exercise. When you spot meter the wall you find areas that meter at all f stops from f2.8 to f22.
For purposes of this exercise we are going to assign a value of 1 to the light intensity in the areas that meter at f2.8. Since the intensity of light doubles as you move from one f stop to the next, the area that meters f4 has a value of 2, the area that meters f5.6 has a value of 4, and so on to f22 which has a value of 64.
If we graph those values it would look like this:
This non-linear relationship is what makes fill flash possible!
So...imagine that you fire a flash and the flash adds 4 units of light to all areas on the wall. How does that effect the exposure?
That is how fill flash works! It adds visible brightness to shadowed areas with little or no effect to the highlight areas. Fill flash adds light to the highlight areas, it is just that it is so little that it has no visual effect.
You now have the two biggest things to think about anytime you use flash. The Inverse Square Law defines how light behaves as it moves out from its source and the non-linear nature of how we see light gives us ways to manage light on a scene. If you have a good grasp of both of these concepts you are well on your way to using flash creatively and effectively in your photography.
Next topic is a little information on the "character" of light and a start to how you can control the character of light in your images.
No comments:
Post a Comment