ON APRIL 8th Rialto Pictures and Studiocanal will premiere “The Graduate” (1967) in a “new digital print” in honour of the film’s 50th anniversary. A 2016 version of “Dr Strangelove” (1964) boasts a “restored 4K digital transfer”. “Citizen Kane” (1941) “dazzles anew” in a “superb 75th-anniversary high-definition” digital restoration. Most film buffs understand these terms to be vaguely synonymous with improvement. But what does the process of “restoration” and “remastering” involve? And is it necessary, or a ruse to sell old movies in new packaging?

Until the 1990s, movies were made exclusively with analogue cameras and photosensitive film. These produce an image as light streams through the lens and turns microscopic crystals into silver forms—which can then be developed into a permanent (and even colourful) picture using chemicals in a darkroom. The resulting frame is highly detailed, but also susceptible to flaws. Temperature changes, dirt or rough handling can introduce stains or a grainy texture. Digital cinematography avoids these problems: an image-sensor chip converts the scene into millions of pixels, each of which is a miniature square with a numerically coded brightness and colour. Most modern movies are made and distributed this way. It allows directors to immediately review takes, editors to enhance them on computers and studios to send them to cinemas without shipping hefty reels around the world. Some purists demur, since analogue film can still produce a higher resolution.

Viewers aren’t as picky, and almost all consume video digitally. Whether they are streaming “Casablanca” (1942) or watching “The Godfather” (1972) on Blu-ray (a more capacious format than DVDs), they are served a scan of the original 35mm film. The studio has converted each physical image into pixels. A full restoration and remastering of a film, however, goes a step further. The film roll is cleaned, to remove dust. Technicians then work frame by frame to restore the film, removing interferences (such as noise, scratches and other signs of ageing), enhancing colours and sharpening outlines. Special effects and CGI may be added. The audio will be overhauled at this stage, too, and perhaps arranged to introduce surround sound. The process is laborious, usually taking upwards of a year to complete.

Such painstaking adjustments are easy to miss without looking at a side-by-side comparison. Fans tend to focus instead on tweaks to the action, as some directors cannot resist tinkering with the story as well as the image. George Lucas, who pioneered the use of digital cameras in the “Star Wars” prequels at the beginning of the 21st century, upset fans by adding new scenes and editing dialogue to the original trilogy when it was remastered in 1997. DVDs of Ridley Scott’s “Blade Runner” (1982) boast of a “futuristic vision perfected”, partly because of the improved special effects, but also thanks to an alternative ending. There are other risks: though reels of film decay and are easy to lose, they make a noble attempt at permanence—well-stored film can last a very long time; digital bits tends to decay. And heavy-handed remastering risks losing some of the qualities that made these films so special in the first place.