Sunday, December 16, 2007

High-Definition Video - Film to high-definition transfer

Most major motion pictures are shot in negative film. Film negative is a very high resolving medium, with resolution measured in cycles/mm. One cycle is also called one line pair which consists of one black line and one white line. In simple terms, one cycle is equivalent to 2 pixels, one black and one white. Film by itself can commonly resolve from 50 c/mm to 400 c/mm (100 pixels/mm to 800 pixels/mm) depending on emulsion stock. However, since the image on film is formed by exposing it through a lens and this lens also has its own resolution limits, the final resolution on the photographed negative is always less than each component's individual resolution.

Depending on the year and format a movie was filmed in, the exposed image can vary greatly in size. Sizes range from as big as 24 mm × 36 mm for VistaVision/Technirama 8 perforation cameras (same as 35 mm still photo film) going down through 18 mm × 24 mm for Silent Films or Full Frame 4 perforations cameras to as small as 9 mm × 21 mm in Academy Sound Aperture cameras modified for the Techniscope 2 perforation format. Movies are also produced using other film gauges, including 70 mm films (22 mm × 48 mm) or the rarely used 55 mm and CINERAMA.

The four major film formats provide pixel resolutions (calculated from pixels per millimeter) roughly as follows:

  • Academy Sound (Sound movies before 1955): 15 mm × 21 mm (1.375) = 2160 × 2970
  • Academy camera US Widescreen: 11 mm × 21 mm (1.85) = 1605 × 2970
  • Current Anamorphic Panavision ("Scope"): 17.5 mm × 21 mm (2.39) = 2485 × 2970
  • Super-35 for Anamorphic prints: 10 mm × 24 mm (2.39) = 1420 × 3390

In the process of making prints for exhibition, this negative is copied onto other film (negative → interpositive → internegative → print) causing the resolution to be decimated with each emulsion copying step and when the image passes through a lens (for example, on a projector). In many case, the resolution can be reduced down to 1/6th of the original negative's resolution (or worse). Note that resolution values for 70 mm film are higher than those listed above.

Typical high-definition home video uses the following resolutions:

  • 1280 × 720
  • 1920 × 1080

Usually when studios master movies for home video release they use assets in high resolution and then master them to 1920 × 1080 and/or 1280 × 720. For standard definition applications (e.g., DVD or SDTV), they are also anamorphically compressed and mastered to 720 × 576 (PAL) and 720 × 480 (NTSC).

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