Why do 35mm film rolls come in 24 or 36 exposures? I've seen some rollls of 12 exposures, but majority is 24/36. Is there some historical significance to this? I would think putting a roll of 42 or 50 exposures in a roll would be much better on the environment since there'll be less packaging used per roll. And each roll would last longer in the field too, especially in locations which film is hard to obtain.