Before roll film there was sheet film.
With sheet film a photographer could select particular films with particular emulsions individually for every shot they took. Applicable characteristics of a film's emulsion would include sensitivity/film speed, fineness of grain, color response/balancing, contrast, as well as the overall 'look' that different films might provide.
The main advantage of roll film loaded in a light-proof cartridge is the speed at which consecutive shots can be taken and the convenience of having multiple shots in a single, compact, light proof container. The main disadvantage of roll film is that it locks a photographer into a single emulsion for the entire roll.
Of course there are various techniques for unloading and later reloading roll film cartridges. But depending on the design of the camera involved it is, at best, a royal pain. At worst it is highly problematic and can almost certainly mean losing parts of some of the exposed images if not done correctly.
Prior to 1934 when Kodak introduced the single use cassette everyone had to load their own roll film onto spools or cassettes that came with the camera. Rumour has it that Oskar Barnack, who was the driving force behind the first mass marketed 135 camera - the first Leica introduced by Ernst Leitz in 1924, selected the 36 frame length because five feet was the maximum length he could wipe holding one end in his left hand and wiping it with his right hand without having to place it in a holder.
From A Brief History of Photography – Part 13: The Early Story of Leica, Short Version
Other features introduced in the Ur-Leica included a combined film advance and shutter cocking mechanism, which eliminated unintentional double exposures, and the accessory shoe, to hold the external viewfinder. The basic design of the accessory shoe has survived dimensionally unchanged to the modern day, evolving along the way to incorporate hot shoe functionality for electronic flash, etc. The camera was originally designed to hold a length of film incorporating 40 frames, but the film had to be loaded and unloaded in the dark. To address this limitation, Barnak then developed a reloadable film cassette that could be loaded and unloaded from the camera in broad daylight. The size of the cassette dictated that the film length be reduced to 36 frames per roll, the standard seen in 35mm film cassettes today. (Leica legend has it that the real reason for the 36 exposure roll length came from the length of film Barnak could hold in his outstretched arms – feel free to believe the version you prefer!)
This roll of film was made in England in 1941.
Preloaded 36 exposure rolls have been available in 135 format since Kodak introduced preloaded single use cassettes in 1934. That was what would fit in the cassette at the time. Shorter 'half rolls' were introduced with 18 frames that soon grew to 20. Up until about 1980 the two sizes available for most 135 film were 20 and 36 exposure rolls. Around 1980 the 24 exposure roll was introduced and gradually replaced the 20 exposure roll. It happened faster with negative film than with positive slide film. When I got into 35mm photography in the mid 1980s 24 frames were common for negative film, but slides film was still usually sold in 20 frame rolls. (Before that I had been shooting in the square 126 cartridge format and the tiny 110 cartridge film. The film was too small and the cheap plastic lenses on the dime store cameras that used 110 were awful.)
If you loaded the camera carefully and used the absolute minimum amount of leader you might get 39 frames out of a 36 shot roll. I routinely got 26 and occasionally 27 frames out of a standard 24 frame roll. With later thinner film substrates in became possible to fit enough length for around 42-45 frames into a standard 135 film cassette, depending on how tightly you were willing to roll it (which increased the risk of scratching the film as it was rolled in and out of the cartridge).
Ilford once produced a very thin polyester based 72 frame variety of B&W 135 film, but it curled horribly and wouldn't lay flat against the camera's back plate. It also required specialized developing equipment to handle the longer length.
Many high volume photographers bought their 135 film in bulk until the end of the film era and loaded it into reloadable cassettes themselves. Of course this had to be done in a completely dark environment or with a "daylight loader" that was filled with about 100 feet (about 800 frames worth, less the amount used by the leader for each roll) in the darkroom and then could be used to load 135 cassettes in daylight conditions.
Some of the people who preferred shorter film lengths:
- Insurance adjusters who could keep a separate roll of film for each claim number.
- Police forensic units who could use a separate roll for each case.
- Real estate agents who wanted a single property on each roll.
- Press photographers who wanted the flexibility of using different types of film for each assignment during the course of a workday.
- Camera stores that gave away promotional rolls of film with the purchase of a new camera or when a new film was introduced.
- Casual photographers who only shot a few frames at a time and didn't want to wait weeks or months until they'd used a longer roll before having the film developed.
8-10 shot 135 film was often labeled "Insurance agent" and 12 shot rolls were sometimes marketed as "reporter rolls" in commercial sales channels. Although a few retail sales channels carried 8-10-12 exposure rolls it was fairly rare (other than the occasional promotional 'sample' roll giveaway at large camera stores). Most of those very short rolls were sold through commercial supply companies.