British Cinematographer: Ed Lachman On The EL Zone System

“I discovered, to my amazement, that IRE stands for the International Radio Engineers,” Lachman says. “They developed the system in 1895, before film even had sound. So as strange as it seems, we’re basing our exposure on radio signal voltage, not photographic exposure of light.” 

Why are we basing our exposures on sound waves? In the photographic medium there’s a discrepancy in IRE units which don’t exponentially increase the amount of light between exposures with stop-values as with light meters and stops on lenses. 

“The engineers who set IRE values didn’t think like we do in the photochemical world,” Lachman explains. “They based exposure on linear light values that don’t track exponentially in the world of photography.” 

The IRE system works on a linear scale but stops on a lens come from a geometric sequence based on of the square root of 2. A stop of 2.8 lets in twice as much light as 4. The IRE system doesn’t track this way. 

Note: One day someone will wake up and ask themselves why we’re basing home theater brightness on obsolete technology invented in 1897. Oh wait, they already did!


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