The inverse square law states that when the distance from a light source to the subject is increased; the intensity of light falls off as the square of the distance from the source. Any light source that spreads its light in all directions obeys this law.
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| The Inverse Square Law of Light |
According to the law, the power of the light will be inversely proportional to the square of the distance.
The only light source that does not seem to obey the inverse square law on earth is the sun. reason being the distance from sun to earth is so great that, when we move something on earth it becomes trivial. Thus we also need to understand an important consequence of the inverse square law; that is intensity variation with a change in distance is far more pronounced close to the source. For example, moving your subject back one foot when they are 50 feet from the light source will result in an insubstantial change in intensity, certainly nothing that would require an adjustment to your camera settings. On the other hand, moving your subject back one foot when the subject is two feet from the source will result in more than an f-stop loss in intensity, a significant difference.
The inverse-square law generally applies when some force or energy is radiated outward radially from a point source. Since the surface area of a sphere (which is 4pr2) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source.
Put simply, the inverse square law means that as you double the distance from the light source to your subject; you actually quarter the light intensity. The light measured at 2 metres from a light source will be 1/22 or 1/4 the intensity at 1 metre. The light measured at 4 metres from the same source will be 1/42 or 1/16th the intensity at 1 metre.
In photography a difference of one stop means a halving or doubling of light, 1/4 the amount of light is 2 stops down; 1/16th of the light is 4 stops down. Therefore, a light meter reading f/16 at 1 metre, for example, would read f/8 at 2 metres and would read f/4 at 4 metres. It is important to understand this law, as it is one of the main ways in which light intensity can be controlled in the studio.
In the next article we will discuss about The Inverse Square Law And Practical Photography
The only light source that does not seem to obey the inverse square law on earth is the sun. reason being the distance from sun to earth is so great that, when we move something on earth it becomes trivial. Thus we also need to understand an important consequence of the inverse square law; that is intensity variation with a change in distance is far more pronounced close to the source. For example, moving your subject back one foot when they are 50 feet from the light source will result in an insubstantial change in intensity, certainly nothing that would require an adjustment to your camera settings. On the other hand, moving your subject back one foot when the subject is two feet from the source will result in more than an f-stop loss in intensity, a significant difference.
The inverse-square law generally applies when some force or energy is radiated outward radially from a point source. Since the surface area of a sphere (which is 4pr2) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source.
Put simply, the inverse square law means that as you double the distance from the light source to your subject; you actually quarter the light intensity. The light measured at 2 metres from a light source will be 1/22 or 1/4 the intensity at 1 metre. The light measured at 4 metres from the same source will be 1/42 or 1/16th the intensity at 1 metre.
In photography a difference of one stop means a halving or doubling of light, 1/4 the amount of light is 2 stops down; 1/16th of the light is 4 stops down. Therefore, a light meter reading f/16 at 1 metre, for example, would read f/8 at 2 metres and would read f/4 at 4 metres. It is important to understand this law, as it is one of the main ways in which light intensity can be controlled in the studio.
In the next article we will discuss about The Inverse Square Law And Practical Photography
Related Reading
- An Introduction to Light
- Wavelengths and Colours
- What Happens When Light Falls On a Surface
- Quality Of Light
- Shadows
