What digital film resolution is needed to scan motion picture film: 4K, or higher?

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Technical Report
Digital Film Technology Weiterstadt
Digital film, Preservation Plans, Scanning

A white paper published by Digital Film Technology Weiterstadt, who make film scanning equipment. The paper is a rigourous investigation of the optical requirements for scanning film, based on foundation documents from Kodak about the resolution of film, and from the ITU (a standards body) on requirement for digital imaging.

The document argues for a resolution of 80 line pairs per mm, which translates for Super 35mm stock (which has an image 25mm wide) to 2000 line pairs, which can theoretically resolved by 4K scanning.

The paper relies almost entirely on measuring image quality using the Modulation Transfer Function (MTF), which is a standard approach across the image industry.

The key issue for scanning is aliasing: a high-frequency that is beyond the capability of the scanner, but affects the results by producing a visible artifact. The paper has a very important section showing how aliasing effects are related to the layout of the pixels in the scanner itself. If scanner pixels approach 100% of the photosensitive surface, aliasing is minimised. If the active area of the scanner surface is only 50%, aliased (unwanted) signals can be as strong as desired signals, to the point of potentially making the scanning worthless.

This is a paper for people who understand the basics of digital sampling and a bit of optics, and understand the MTF. It has an important result relating sensor design to final quality, and is recommended for anyone with a serious interest in film scanning technology.