Flicker or digital artifacts in the frame — caused by mains frequency, LED panels, or sensor noise. Most visible under artificial light or high ISO.
When shooting with artificial light — especially LED panels and fluorescent lights — something treacherous happens: the mains frequency (50 Hz in Europe, 60 Hz in the USA) modulates the brightness so quickly that the human eye doesn't see it, but the camera very much does. The result is a rhythmic flicker in the image, which becomes particularly visible in dark or mid-tone areas. The same problem arises with fast shutter speeds — anyone shooting at 1/100s or faster while the light is pulsing at 50 Hz is guaranteed to capture only half or quarter cycles.
The practical solution on set lies in shutter synchronization: at 50 Hz, you work with 1/50s (or multiples like 1/25s), at 60 Hz correspondingly 1/60s. This guarantees that each frame captures a complete light cycle. Modern cameras offer flicker-free modes that automatically select the ideal exposure time. If you're shooting high-speed — 60fps, 120fps — you need to adjust the shutter speed accordingly: at 60fps and a 60 Hz network, you use 1/120s (double the frame rate). But beware: some LED panels work with high-frequency pulsing (several kHz) to circumvent this exact problem — they are significantly more expensive but flicker-free regardless of your shutter speed.
The second problem is digital noise — not to be confused with flicker. This arises from poor sensor performance in low light or from over-amplification (high ISO). Here, only the following helps: better lighting, longer exposure (if possible), or accepting that you'll have to work with noise reduction in post-production — but this costs detail sharpness. In documentaries or found-footage scenes, noise can also be used deliberately as a stylistic device to suggest authenticity. This is rarely seen in high-end cinema, but it works atmospherically in thriller or horror projects.
Practical tip: Check the electrical system during location scouting. Old wiring, poor grounding, or dimmer circuits significantly exacerbate flicker problems. When in doubt: use an external stabilized power supply for the lights or switch to battery-powered LED panels. Flicker is difficult to fix in post-production — preventing it on set is much more efficient.