Most, if not all Litho film is as thin as roll film. The process cameras normally used to expose this film to make negatives for burning printing plates universally have a vacuum back for holding the film flat. These cameras are usually built into a wall between the darkroom and the room where the artwork is put behind glass on an easel for exposure. One loads the film under a red safelight in the darkroom on the vacuum back which swings open from the camera like a door on a refrigerator. There are no guides to slide the film under, rather there are marks on the flat back to position the film, the vacuum holds it flat.