What the graph means.

Ok, last week I posted a photo of a graph, but left you hanging as to what it means. Here’s the lowdown on the mystery. In the class last night we examined the Inverse Square Law of light. This law says that the intensity of law deminishes on a logarithmic scale rather than a linear one. In English this means that light falls off faster the closer you are to the light source.

Are you confused? Let me explain the experiment we did that helped us understand the principle. The first thing I did was to setup a strobe on a stand. Then I put a piece of duct tape on the floor 1ft from the strobe. I further put a piece of tape on the floor every 5 ft after that until the final piece was 30ft from the 1ft mark. I then proceeded to meter the light at 1 ft, 5ft, 10ft, etc. A student then wrote the f-stops that I metered on a grease board. We then graphed it with feet on the horizontal scale and f-stops on the vertical scale. The result is the graph from last week.

What you see in the crude graph is that light falls off nearly 5 f-stops over the first 5 feet and only 1/2 stop over the last 5 feet! This means that in the first 5 feet away from the light source you nearly exhaust your entire dynamic range of a digital camera. Pretty amazing stuff, if you are a photographer. If you are not a photographer, frankly I’m surprised you’ve continued to read this far. 🙂

What this information tells us is where to place our lights in relationship to our subjects and backgrounds in order to get the results that we want.

Thanks for reading!! Next week I’ll have something more entertaining for non-photographers.

-Ken