Inverse Square Law

The inverse square law describes how light weakens as it travels from its source. The strength of the light drops with the square of the distance, so a subject twice as far from a lamp receives one quarter of the light, and a subject three times as far receives one ninth. Photographers use the rule to predict exposure and to control how bright a background appears relative to the subject.

Light spreads out as it travels. At twice the distance, the same amount of light covers an area four times as large, so each point in that area receives a quarter of the energy. The spread follows simple geometry: area grows with the square of distance, which is where the law gets its name.

About two stops. A quarter of the light equals two halvings, and each halving is one stop. So moving a flash from one meter to two meters costs two stops, and moving it from one meter to four meters costs four stops. Compensate by raising the flash power, opening the aperture, or increasing the ISO.

Technically yes, but the distance is so large that the effect is invisible. The sun is about 150 million kilometers away, so moving a subject a few meters closer or farther changes the light by a meaningless fraction. In practice, sunlight keeps the same strength everywhere in a scene, and the law only becomes a working concern with flashes, strobes, lamps, and windows.

When a light sits close to the subject, the background is many times farther from the source in relative terms, so it receives only a small fraction of the light. A white wall can render gray or nearly black. Photographers use this to shoot low-key portraits in small rooms without any black backdrop: bring the light in close and let the falloff do the work.

A flash placed close to a group lights the front row much more strongly than the back row, because the relative distance between rows is large. Moving the light farther away shrinks that relative difference, so all rows receive nearly the same amount of light. The trade-off is less total light, which may require more power or a higher ISO.

Large sources like softboxes and umbrellas follow the law less strictly at close range, because they behave more like a wall of light than a point. The classic quarter-light rule assumes a small source. In practice, falloff from a big modifier close to the subject is gentler than the math predicts, then approaches the normal rule as the distance grows.

Yes, and in the photographer's favor. A light moved closer becomes larger relative to the subject, which softens shadow edges, while the faster falloff darkens the background. Moving a light away does the reverse: harder shadows and a brighter background. Distance is one of the strongest controls in lighting because it changes strength, softness, and background brightness at once.

Distance evens light out. Because falloff is steep near the source and gentle far from it, placing a light farther back means objects at different depths receive nearly the same amount. Product photographers and group shooters pull lights back for this reason. The cost is total output, so bigger distance often means more flash power.

Yes. The inverse square law is a property of light itself, not of the source type. LED panels, tungsten lamps, candles, and windows all fall off the same way a flash does. The difference is only that continuous light shows the effect live, so the falloff can be judged by eye or with a meter before taking the photo.