Day for Night

Day for night is a set of cinematic techniques used to simulate a night scene while filming in daylight. It is often employed when it is too difficult or expensive to actually shoot during nighttime. Because both and digital image sensors lack the sensitivity of the human eye in low light conditions, night scenes recorded in natural light, with or without moonlight, may be underexposed to the point where little or nothing is visible. This problem can be avoided by using daylight to substitute for darkness. When shooting day for night, the scene is typically underexposed in-camera or darkened during post-production, with a blue tint added. Additional effects are often used to heighten the impression of night.

As film stocks and video cameras have improved in light sensitivity, shooting day for night has become less common in recent years.

During the silent era of film, release prints were often tinted blue during night scenes to enhance the illusion. Although moonlight is not actually blue, it appears bluish to the human eye due to the Purkinje effect. To give a bluer appearance to scenes filmed in color, some techniques use 3200K tungsten-balanced rather than 5000K daylight-balanced film stock. The tungsten balance renders artificial lighting (street lights, headlights, lit windows, etc.) as white and unlit areas as "moonlight blue." With professional video cameras, color temperature adjustments are made to achieve the same effect. With digital post production now nearly universal, the color temperature adjustment is usually made in camera, to preserve the "white" artificial lights, but scene darkening is left to post production for finer control of the effect.

Underexposing the shot can add to the illusion of darkness or moonlight. It is typical to underexpose by about two f-stops. A neutral density filter is often used to achieve this darkening, so that the camera aperture remains unchanged.

Daytime sky can be darkened to simulate night. With black and white film, a red lens filter will turn a blue sky black. Infrared film is occasionally used for long shots, but it renders green foliage as white. Yellow or orange filters (Wratten 8 or 15) can be substituted for closer shots, to preserve the performers' flesh tones. With color film or video, a graduated neutral density filter can achieve a similar effect, as can a polarizing filter. Using either of these filters can limit camera movement during a shot, as the axis of a graduated filter must match the horizon, and the effect of the polarizing filter changes as the axis of the camera lens moves relative to the sun. If the scene is backlit by the sun for a "moonlight" rim light effect, faces and other foreground details may be too dark to see properly. Partially filling shadows with reflectors or a 5000K (daylight-balanced) key light can compensate for this. Even so, shadow areas are still slightly under-lit, to match the higher contrast of the overall scene.

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Wikipedia