We were talking about light falloff... This info will be helpful for anyone who takes flash pictures or uses flash bursts or bulbs or existing night light sources shooting trains.
The Inverse Square Law. What the heck is that you say? It's actually very simple and it is also very important if you are lighting any type of subject. The theory behind the inverse square law states that light, from its source which is traveling out in all directions, becomes less effective (dimmer) as it gets farther away from its source, because it has a greater area to cover. Look at it this way: if my flash is ten feet from the depot, and I get a meter reading of F8 at the depot, then if I move my flash twice as far away from the depot (I can't move the depot - and come on you guys, lets not get into depot moving here!), then the same light then has twice as far to travel, so it spreads over a larger area. The light remained the same and the distance to the subject doubled, so the exposure was reduced by the square of the distance. The light now has to cover an area twice the size in diameter, which is FOUR times the AREA. The meter reading at the depot is now F4. There is a good example that I can think of to describe this if you can picture it visually while reading this. Or even draw out this description. My example is a drawing with the light as a dot on one end, and the light rays extending out away from it in a triangle shape or angle. The two light beam lines get farther apart the farther they go away from the light dot that we drew. And this is three dimensional, like a 'tube' of light that gets wider the farther it travels from the source. Lets pick any spot to put a subject at and draw a circle around the light rays like a dinner plate standing on end, viewed from an angle so we can see some of the face of it. Lets say we have a meter reading of F11 at this point. Now extend out twice as far (twice the distance) away from the light, extending out and down the light rays to a point twice as far away from the light source dot as the first circle we drew. If we draw another circle around the light rays at this point, then it is twice the diameter as the first circle which is back there twice as close to the light. Now draw a cross in the big circle to divide it into four quarters, just like a pie. Each of these quarters is receiving the same amount of light as the first circle that is twice as close to the light source. Because the light falloff equals the square of the distance, that means that in this example, this whole second circle (all four parts together) that is twice as far from the light source, going to meter at F5.6. And half of the circle area (two pieces of the pie) would meter at F8 in this example, (however that is not practical to do without getting off track here). Going back up to the top of this post, if I meter the depot flash exposure at F8 with my flash ten feet from the depot, then I move with my flash twenty feet from the depot, I know without metering that my meter reading on the depot will now be F4 because I am now twice the distance, so I lost two stops. The light fell off at the square of the distance. Once you know this and you have a light meter, you can light just about anything with confidence. If you do night bulb shots and you know that a clear number 25 flashbulb is about F16 at ten feet (100 ISO with an old Kodak Brownie 5 inch reflector), and you know about the inverse square law, then you will also know that when you back up to 30 feet (three times as far) to blow your bulb, that your exposure will then be about F5.6 (three times as much light coming into your lens as F16). Or use more bulbs just like more studio strobe light pops to increase your exposure reading while your shutter is open - another topic. And keep in mind that any ambient light will effect your shot some too depending on how long your shutter is open - pick an aperture to control the ambient light, not the flash exposure - that is irrelevant in this situation). Gee, we could get into color temperature of mixed light sources and controlling this color with daylight balanced strobe sources.... but not right now. Most important from all of this, is that you will know how far away to stand when you fire your flash or bulbs! All of this knowledge helps, but it won't do you any good unless you go out and try this yourself and see the results with your own equipment firsthand. AND MAKE NOTES. Get out and shoot! Dave Cohen Photographer [EMAIL PROTECTED] --> SPORRS: Serious Photographers of Railroad Related Subjects
