Rhythmic strobing or shimmer in image—from shutter/refresh-rate mismatch with artificial light, or intentional. Glitch aesthetic or deliberate visual signature.
You know the problem: you're shooting under artificial light, the camera is running at 24fps, the lights are flickering at a 50Hz mains frequency — and suddenly the entire image is dancing. This is the flicker effect, and it's one of the most persistent problems on set if you're not careful. The frequencies get out of sync, creating a rhythmic overmodulation, and the viewer sees an annoying blinking that ruins the entire scene.
The effect arises from a simple mathematical relationship: your camera exposes each frame for a specific fraction of a second — typically 1/50s at 25fps or 1/48s at 24fps. When your artificial light cycles at 50Hz, individual frames hit different phases of the light cycle. One frame catches the peak, the next catches the trough — the result: visible flicker. In Europe, the problem is acute because the power runs at 50Hz; in North America, at 60Hz, it's less dramatic but still relevant. LED lights often exacerbate the problem because they pulse faster than traditional halogen lamps.
On set, you combat this with several strategies: adjusting the shutter angle (some cameras allow fine adjustment of the shutter), synchronizing the frame rate to the mains frequency — i.e., 25fps in Europe, 30fps in the US — or switching directly to HSS-capable LED panels that run at a high frequency and bypass the problem. Some DoPs deliberately work with dimmers to shift the flicker frequency. In the edit, the effect is difficult to repair later; prevention is everything.
On the other hand: some directors and cinematographers deliberately use the flicker effect — as a visual statement for instability, psychological stress, or for a specific aesthetic disturbance. This is then not a source of error, but a design element. Think of found-footage horror or surveillance camera sequences, where a gentle, subtle flicker creates just the right sense of unease. Here, you then work with frame rate desync or intentional frequency mismatch.