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In this Wikipedia article:

Unlike sheet film, in which each negative can be individually developed, an entire roll must be given the same development, so that N+ and N− development are normally unavailable.

Why is that? Don't we put each negative in the neg carrier and develop this individual negative at a time? I don't understand the above statement

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    \$\begingroup\$ I think the confusion here is between "enlargement" and "development". Here they are talking about film development where you have the whole roll of film on a reel inside a tank where you pour in the chemicals that turn the undeveloped film into the negative film you can put in the enlarger. Once the film has been developed, then you put the film in an enlarger's negative carrier to project the image onto photosensitive paper -- then develop that paper into a positive image. You could not normally cut the film before developing it. \$\endgroup\$ Dec 12, 2012 at 21:18

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You seem to be confusing "development" and "printing". Certainly, you normally print the negatives one at a time. This is about "souping" them.

Normally (for manual/home development) an entire roll of film is loaded into a spiral film holder, which is placed into the development tank (often more than one roll at a time) before any chemistry is added. The entire roll (or rolls) of film is treated as a unit during the development process—there may or may not be a water pre-soak (depending on the process), then the water is drained out and the developer added and agitated. When the development time is up, the developer is emptied out of the drum and stop bath added to halt development. It is only at this point that any images can be seen on the film without ruining the film immediately. The stop bath is then dumped, and the fixer added to remove any remaining unprecipitated silver and associated compounds, and after a sufficient period of time, that is dumped and the film is cleared (to remove the fixer), washed and dried.

With sheet film, you can handle each of the negatives individually during this process. You still can't see the latent images, but you know where they are: one image per sheet. They can be developed by hand in a shallow tray with a sort of slosh-and-shuffle technique, or they can be fixed to frames for a dip-and-dunk technique. Since each sheet is handled individually, you can develop each for a different length of time to develop the ideal contrast (according to the notes you remembered to write to yourself when you took the picture).

There are ways to separate negatives on a roll before development, but that almost always means losing one or more images. Basically, you have to cut the roll into several parts in perfect darkness, and while you can get pretty good at counting sprocket holes by feel, that doesn't make up for minor registration errors, differences from camera to camera, and how much leader you loaded before beginning to wind on in the camera. With 120/220 film, you don't even have the sprocket holes to go by.

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    \$\begingroup\$ This gets particularly interesting with slide film, where the exposure margins are smaller. Working with roll slide film you would commonly do fixed-step exposure series, and hope that one of the images is just right. With sheet slide film, you can take two similar "correct" exposures according to your light meter and develop one of them. When you have seen the result, you can push/pull develop the second one differently to get it just right. \$\endgroup\$
    – bhell
    Dec 12, 2012 at 21:51
  • \$\begingroup\$ I recall doing "snip tests" with chrome roll film as well. It involves sacrificing the first two or three images on the roll (unless standard development works, in which case there's only the one you likely cut in half to mourn). But that's roll-wise, so the whole roll needed to be a similar shot for it to work (not at all unusual on a commercial shoot). \$\endgroup\$
    – user2719
    Dec 13, 2012 at 15:19
  • \$\begingroup\$ When doing clip tests, we would "sacrifice" our "grey-card" shots at the head of "possible problem" situations. Failing that, we could squeeze some on the tail of some rolls. \$\endgroup\$
    – Stan
    May 22, 2016 at 0:30

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