Artificial or natural haze in the frame — breaks light rays, reduces contrast range, creates depth and mystery. Dense fog (mood), subtle mist (wet atmosphere) — deploy both differently.
Fog/mist acts on set like an invisible veil between the camera and the subject—it absorbs light, scatters it diffusely in all directions, and thus destroys the clarity of edges and contours. This makes it one of the most powerful tools for emotional image design, as long as you understand what it technically does. Dense fog reduces the dynamic range by 2–4 stops, depending on how close the fog machine was to the set and how long it ran. This either forces you to use significantly higher light intensities or to accept a flatter, softer image—both can be dramatically intended.
Practically, you distinguish three scenarios: First, artificially generated stage or theatrical fog (theater effect, immediately controllable); second, hazers (finer, longer-lasting, for subtle volumetric lighting effects); and third, natural fog on location—this is unpredictable but authentic. Indoors, you mostly use machines; outdoors, you need patience, weather knowledge, or, if necessary, travel to areas with morning natural fog. Many cinematographers underestimate that fog is not evenly distributed—it layers, sinks, is destroyed by ventilation systems and doors. You must expect constant re-activation.
From a lighting perspective: Fog needs volumetric light to become visible—without directional radiation (HMI, Fresnel), it remains invisible or appears flat. Side light reveals the fog layers most dramatically; backlight creates mystical halos. Play with color temperature: Warm fog appears intimately dangerous, cold fog more uncanny or sci-fi. Also consider your camera's sensitivity—digital sensors tend to get noisier faster in dark, fog-laden scenes, while film negative is more forgiving.
Editing and grading note: Fog is difficult to add authentically later (only believable with extreme effort in VFX), but can be significantly softened in the DI. It's better to shoot with slightly too much fog—post can regulate it. During the shoot, ensure that important detail information (eyes, gestures) doesn't get completely lost in the fog, unless that is dramatically desired. A touch of fog per scene is often enough for mood without destroying readability.