So, what is it? In all of its overly technical glory, the Inverse Square Law– as it applies to photography– is an equation that relates the intensity of a light source to the illumination it produces at any given distance. While the Inverse Square Law comes into play more often with strobes, it is absolutely a concept that applies to every light source, and therefore affects every photographer. Some photographers put themselves in the “natural light” category, while others work their magic with a firm grasp of off-camera flash. We capture light in a box and use it to tell a story. It is the defining element of what we do. It waited for me to feel secure in my knowledge and execution of studio lighting and off-camera flash. It waited for me to get comfortable in my new skin a professional photographer. No– the Inverse Square Law is much too cunning for that. It didn’t jump out and attack me right away. Even when I ditched my briefcase for a camera bag and embarked on a new career, I felt pretty secure in the knowledge that confusing math had no place in the world of photography.Īnd then the Inverse Square Law reared its ugly head. I practiced law for fourteen years, where any math I needed was either pretty easy or done on a calculator. I chose a double major in college (Journalism/English) that required no math. I needed a tutor in high school for both geometry and physics. The average of the values of m is 3.05, indicating that as the magnet gets farther away, the exponent m is about -3, indicating that for this type of magnet only, the force from its magnet field varies inversely by the cube of distance.I don’t know about you, but I was never much of a math student. Distance is shown in mm exponent is "m".Ī really important point: the compass could detect magnet field at 356 mm, over seven inches away. The table below shows the relationshio between magnetic force and distance, as measured by the compass needle deflection method. You measure compass needle deflection at different distances, then find the exponent for the relationship using trig formulas provided.Īfter doing this test a few times and taking the average, the results are shown in the table below. I then decided to measure magnetic field using an ordinary compass. My previous tests with the scale were inconclusive the distance I could detect with my device only went to 18 mm. One key hint I learned: my measuring/testing system started to register a magnetic force at about 20 mm, the magnetic field extends much further. In fact, the only equation that really fit was a fifth order polynomial. Excel offers an equation that agrees with the plot, but it was not a simple inverse square or inverse cube relationship. I was impressed by the nice plot that resulted, but the more I looked at it the more confused I got. ![]() The magnet is attracted to thecast steel surface of my table saw.Īll other components are non-magnetic, brass, aluminum, wood.įorce is measured by the scale at 0.8 mm intervals over the full range the magnet is attracted to the steel, and recorded in a table.Īfter adjusting data due to geometry of the balance beam, I plotted these data using Microsoft Excel. ![]() The magnet is attached at the end of a threaded brass rod, 32 threads per inch. I then would analyze data, plot a graph, and come up with an equation. My initial method was to build a device that could measure magnetic force at various distances using a precise scale.
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