Pre-expose film stock to controlled light before principal photography — flattens contrast, lifts shadows. Classic analog tool for sophisticated tonality.
You expose the film to controlled, diffuse light before the actual shot—that's preflashing. It sounds simple, but it radically affects the image: the entire film receives a minimal base exposure before the camera even starts rolling. The result: shadows open up by themselves, contrast becomes flatter, and tonal gradations become smoother. You're not fighting the film emulsion—you're deliberately utilizing its curve.
On set, it works like this: you set up a simple light source—classically a large, diffuse surface or even just a white wall with even illumination. The entire roll is moved in front of it or remains still while you expose briefly with an open aperture. The amount is crucial: too much preflashing and your image becomes too soapy and contrastless; too little and the effect dissipates. When working with negative material, apertures in the range of 2.8 to 4 and exposure times of fractions of a second are often used. With reversal film (especially high-speed material), the result can be significantly more pronounced.
The classic application: portrait films of the 1980s and 90s. If you wanted elegant, matted skin tones without extreme contrasts—preflashing was your weapon. The technique also paid off in black and white work: the film retained detail in the darkest areas while the highlights didn't blow out. This is not a digital trick, not a LUT, not color curves in the edit—this is pure film chemistry.
Today, you either need original film stock for this or you simulate the effect through flashing in the DI—but true preflashing remains an analog affair. The challenge lies in control and reproducibility. Each roll needs the same dose, otherwise, you'll get fluctuations in the image across your scenes. With modern digital cameras, you try to simulate this through increased sensor gain and monochrome grades—but the subtlety of the grain and the organic opening of shadow tones remain characteristic of true film preflashing.