The Zone system, as used in black and white photography, is a refinement of the old "Expose for the shadows, develop for the highlights" adage. It is more concerned with determining the correct film development than with the correct exposure. ("Correct" in this context is a subjective term)
Once you can "see" and understand what happens to film when it becomes developed, the Zone system becomes clear. Let's pretend that you take a picture of a black square next to a white square (black square on the left) on a sheet of film. Let's also pretend that you can see the film as it develops (pre-fixed and everything.)Place the film in the developer and what do you see? The black square is already there! Remember, this is a negative, the black square will show as clear and the white square will show as black when the negative is fully developed. After about a minute or so, you start to see a light gray square starting to form on the right side of the negative. After 2 or 3 minutes it will get darker and darker. The longer you have the film in the developer, the darker it will get until it appears pure black. (and hence will print pure white) What happened to our square on the left? Nothing. It was going to show as a clear square on the developed film (so it would print pure black). That's the way it started out and that's the way it ended up. The silver halides of the emulsion were not "triggered" by light on the left side so the developer did nothing. Increasing or decreasing development time can only change the highlights, never the shadows.
The degree of expansion and contraction may vary with different films. You should do your own testing to see what works for you.
I believe any good photo should have good, rich blacks in it, so I use that as a starting point. Not "shadow with some detail". Also it is easier.
It is only practical to use the Zone system with sheet film, each of which can be developed differently. However, one could use roll film on an overcast day, lacking in contrast, and use increased development to raise the contrast level for the entire roll of film, or the opposite on a brightly lit, high contrast day.
I do not usually bracket my exposures but I may bracket my development time. I may shoot 3 sheets of film with the same exposure, give one sheet normal development, one 25% less development and one 50% less development.
~Jim
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