Stray external light entering camera or viewfinder — through seals, gaps, or unsealed ports. Kills contrast and color fidelity instantly.
You know the drill: The sun is in an awkward position, you're shooting an interior scene, and suddenly the monitor shows you blotches, washed-out areas, or a color cast you never intended. That's a light leak — external light penetrating the camera itself through tiny cracks, poor seals, or open spaces, hitting the sensor directly. Not the lens. That's the crucial difference from lens flare or unwanted reflections: a light leak destroys from the inside.
On set, this usually happens at the interfaces around the camera body — between the viewfinder and the housing, along the ground glass, with poorly fitting masks, or when you're working without a follow focus and the focus membrane isn't properly sealed. With older cameras or in intense daylight, especially when shooting against the sun, it becomes critical. The consequences are immediately visible: contrast flattens, blacks turn gray, colors lose saturation — and all of this uncontrollably across multiple frames. It's almost impossible to fix in post-production because it's not a transition, but a systematic error in the recording itself.
Practically, you avoid it like this: Seal all visible gaps — with black gaffer tape, camera cloth, or simply with your hand. Particularly critical areas: the space between the follow focus gear and the focus membrane, the seam between the camera body and the ground glass (when shooting without a viewfinder), and the back of your monitor if it's not seated correctly on the camera. For exterior shoots with bright light: Use a sunshade or matte box to control stray light. Always test before long shots — stand in front of the sun yourself and see if light is entering from the side.
A light leak differs from lens flare in that it cannot be used artistically — it's an error, a loss of control. Some cinematographers also confuse it with overexposure, but a light leak is much more insidious: the exposure might be correct, but the internal light source makes the entire image milky. Pay attention to this, especially during longer takes or when your DoP doesn't have an eye on the monitor.