Short ends most commonly occur either when the film left in the camera magazine is shorter than the amount expected to be required for the next take or at the end of a shooting day when all of the day's exposed footage needs to be sent to the film lab. When this occurs, the clapper loader will break the roll at the loop protruding from the magazine; the exposed footage is placed in a black bag inside a film can, sealed, and labeled for film processing, while the unused short end will be placed in another black bag in a film can which is then sealed and identified by length, type, which original roll it came from, and when it was unloaded.
The short end may either be used later in the production, or sold to a film dealer who will resell it to productions who are on a position to use it.
Identification is critical, as there will be no other way to know what exactly is inside the can. Short ends cans should also be sealed in either white or colored gaffer tape, depending on what (if any) color scheme is being used to identify film types; in any case, black gaffer tape should not be used, as it is considered the standard tape color used to seal exposed cans. Alternatively, in the event that the footage will be needed very shortly, the short end may not leave the magazine in the first place, but rather be freshly threaded through the magazine as if it were a new roll.
If the entire roll has remained unshot and is to be unloaded back into a can, this is technically not considered a short end, but rather a re-can. Additionally, if the unshot footage is going to be thrown away almost immediately, then this is considered waste footage. Short ends only exist if there is both enough unexposed film left in the magazine to warrant using it to start a new magazine load and also if there is either a magazine or a film can available to store it in the interim. Otherwise, the film may have to be wasted if there is nowhere it can be kept safely away from any light. Short ends may exist for a number of reasons, but usually they are either created at the end of a shooting day when the exposed film is unloaded to be sent with the day's rushes or at the end of a film shoot when there is still film left in the magazine on the movie camera. Additionally, particular film stock types might only have been used for a few scenes and thus have considerable short end lengths at the conclusion of their usage.
Short end minimum lengths vary depending on the needs of a production; professional productions usually will prefer to waste any quantities less than 100 to 200 feet rather than constantly interrupt shooting to reload so often, while lower budget or student productions may be eager to use any amounts greater than 20 or 30 feet. In any case, the minimum length needs to be substantial enough to allow for the magazine to be loaded, the camera to be laced, and something to be usably shot, which usually will require at least ten feet prior to the shot. The maximum length of a short end will be anything close the full length of a roll without counting as a re-can.
Short ends are considered distinct from fresh new rolls because they may have been subject to environmental conditions since leaving their originally sealed can and are no longer under factory warranty, and are generally identified distinctly to avoid any confusion in the matter. Assuming proper storage and fast turnaround time, they are considered perfectly usable. Older short ends or ones of uncertain provenance should be clip tested first to check their condition. Re-cans and short ends are often sold at discounts to lower budget productions by labs, larger production companies, or businesses which specialize in their sale.