Overview
A film holder is a accessory that holds one or more pieces of photographic film, for insertion into a camera or optical scanning device such as a dedicated film scanner or a flatbed scanner with film scanning capabilities. The widest use of the term refers to a device that holds sheet film for use in large format cameras, but it can also refer to various interchangeable devices in medium format or even 135 film camera systems.
Sheet film holders
The most common instance of film holder is the sheet film holder. Also referred to as a dark slide or double dark slide, they are flat devices, slightly larger than the films they hold, which commonly hold one sheet of film on each side. The plate holder, which is a very similar device, holds glass plates instead of sheet film. A dark slide, from which the device derives its alternate name, is simply a dark cover that slides into a recess in the holder to protect the film (or plate) from exposure to light. Many dark slides have differently colored bands or handles on each side, one usually light and the other dark, so the photographer can distinguish between exposed and unexposed film.
Traditionally, sheet film and glass plate holders have been made out of wood. Wooden holders, properly treated, can last a very long time, and apart from possible warpage, many very old specimens are still in service. Some companies continue to make wood models today, particularly for more uncommon film sizes, and as many are mostly handmade, they can be quite expensive. The majority of new sheet film holders are now made out of plastic.
From Wikipedia (CC BY-SA 4.0).