Rhythmic brightness fluctuation when camera shutter syncs poorly with mains frequency — ruins fluorescent and LED footage. Match 50Hz power to 50fps, 60Hz to 60fps.
Film flicker
You know the problem: you're shooting under artificial light, watch the footage later, and see an annoying rhythmic flicker across the entire image — especially visible in close-ups of monitors or under certain LED installations. This is film flicker, and it arises from a fundamental discrepancy between your camera's frame rate and the frequency of the power grid supplying the lighting.
The mechanism is actually simple: artificial light — whether halogen, HMI, or modern LED — pulsates with the mains frequency. In Europe, this is 50 Hz, in North America 60 Hz. This pulsation is invisible to the naked eye because it's too fast. Your camera, however, operates at a fixed frame rate — say 25fps or 24fps. If this rate is not harmonically synchronized with the mains frequency, each frame captures a different phase of this brightness pulsation. The result: flicker in the image, sometimes subtle, sometimes catastrophic. The classic rule of thumb: with a 50 Hz mains, you should shoot at 25fps, 50fps, or 100fps — never 24fps or 60fps. With a 60 Hz mains, it's the reverse.
In practice, you'll notice this immediately when working in digital editing. Especially when copying screen content or when shooting under older LED installation lights — new high-frequency LEDs flicker less. As a DoP, you have several levers: First, adjust the frame rate — this is the safest method if production planning allows. Second, change the camera shutter angle — some cameras give you leeway to shift the phase. Third, use gels and diffusion to smooth out the LED frequency, or switch to battery-powered lights. Fourth, in post-production: with some NLE systems, you can calculate it out as an artifact, but this is time-consuming and often not entirely clean.
Practical advice: during the lighting check with the camera assistant, do a short test shot — several seconds under the planned lighting conditions, watch it on the camera's monitor. If it flickers, immediately change the frame rate or lighting setup. When in doubt: shoot at 50fps (with a 50 Hz mains) — this significantly minimizes flicker and also gives you options for slow-motion in post.