Measurement of light sensitivity in film stock or digital sensors — reveals curve behavior, dynamic range, and color shifts. Essential for grading strategy and LUT design.
Sensitometry
You need sensitometry to understand your material — not to describe it, but to work with it. It's about how film stock or digital sensors react to light. Specifically: you expose test strips or digital grayscale charts with defined light values and measure what comes out. The result is a curve — the so-called sensitometry curve or gamma curve. It shows you how each light step from the darkest to the brightest is rendered in the final shot.
On set, you need this to know what your material is capable of. Film stock A has a different dynamic range than stock B, digital camera C reacts differently to clipping than camera D. Some films and sensors crush highlights brutally, others hold them open. Some have color casts in the shadows, others in the midtones. Sensitometry gives you the map — without it, you're calibrating blind. You'll be shooting, but you won't know how reproducible the look will be or how much latitude you have in grading.
Practically, it works like this: you shoot under standardized conditions — constant light intensity, color temperature, exposure series. Then you analyze the data: density measurements for film (optical densitometry), or digital histograms and waveform analyses for digital sensors. You quickly notice where the sensor starts to noise, where it clips, how linear the curve is in the working range. Some cameras have an S-shaped tone curve response (increased contrast), others are superlinear in log mode.
This is also the basis for LUT development and color grading. Before the colorist writes a LUT, they must know the sensitometry of the captured material — otherwise, they might clip highlights that are still relevant, or push shadows that only contain noise. With two to three well-documented sensitometry tests per project — different lighting situations, different ISO settings, perhaps multiple camera bodies — you're on the safe side. The test costs time and material, but pays off in reproducible image quality and saves you grading surprises in the color suite.