Lighting Ratio Calculator

A lighting ratio expresses the brightness relationship between the two sides of a modelled subject, not between two lamps. The key light strikes only one side while fill and ambient spill fall on both, so the lit side receives key plus fill plus ambient and the shadow side receives fill plus ambient alone. Dividing the first by the second gives the ratio. Because exposure is logarithmic, the same number is usually quoted in stops: a factor of 2 is one stop, 4 is two, 8 is three, the conversion being the base-two logarithm. This calculator first reduces each source to its effective illuminance using the inverse square law, E equals I divided by distance squared, which is why a lamp at 2.5 m delivers only 64 percent of what it delivers at 2 m. Working values are well established. Around 1:1 to 1.5:1 the face reads flat, used for identity and passport work; 2:1 is the soft beauty and fashion standard; 3:1 is the conventional corporate portrait; 4:1 gives the two-stop dramatic look associated with Rembrandt lighting; 8:1 or higher moves into film noir territory where shadow detail falls toward the sensor noise floor. Several limitations matter. The inverse square law strictly describes a point source, so large softboxes close to the subject fall off more slowly than predicted. The model assumes uniformly reflective skin and ignores specular highlights, bounce and flare. Contrast in lux is also not contrast in the final image, since the camera tone curve compresses highlights and lifts shadows first.

Measuring rather than judging light followed the invention of reliable photoelectric metering. Weston Electrical Instrument Corporation released the Weston 617 exposure meter in 1932, and the General Electric and Norwood meters that followed made it routine to read incident light in footcandles at the subject. That turned lighting from a craft tradition into something quantifiable, and studio manuals from the 1930s began specifying key-to-fill relationships as numbers. The underlying inverse square law is far older, stated for illumination by Johannes Kepler in Ad Vitellionem Paralipomena in 1604 and formalised photometrically by Pierre Bouguer in 1729 and Johann Heinrich Lambert in his Photometria of 1760. The three-point convention of key, fill and backlight was consolidated by Hollywood studio cinematography in the 1930s and 1940s, where high-key comedy typically ran near 2:1 while low-key crime films pushed to 8:1 and beyond. The American Society of Cinematographers, founded in 1919, has published these practices in the American Cinematographer Manual since 1935. The term Rembrandt lighting refers to the Dutch painter Rembrandt van Rijn, whose seventeenth century self-portraits show the characteristic lit triangle on the shadow cheek; the label was applied retrospectively by portrait photographers. Digital capture changed the arithmetic rather than the vocabulary, since sensors with 13 or more stops of dynamic range hold detail at ratios that went solid black on colour negative stock.