What is the Zone System?

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.


From the above chart, it can be seen that over or under development does not affect the blacks but will lighten or darken the whites. If my spot meter indicates that normal development will cause my highest whites to fall on Zone VII (light gray), I will over develop to push them up to Zone IX, if I want them to be brighter. If my whites were going to fall on Zone X or higher (pure white, no texture), I would underdevelop to pull them back down to zone VIII or IX, whichever I desired.
There are many different ways to achieve N+1 or N+2 or N-1 or N-2 etc. I prefer to just alter the development time.I use Plus-X or Tri-X films. These may not be wholly accurate but they work fairly well for me:
N-1 = 25% decrease in development time
N-2 = 50% decrease in development time (never go beyond a 50% decrease) I actually do about a 40% decrease.
N+1 = 25% increase in development time
N+2 = 50 % increase in development time.
Many books call for about a 20% factor for Plus-X and 30% for Tri-X but I split the difference and use that 25% for both. Low speed films should be about 15% factors.

The degree of expansion and contraction may vary with different films. You should do your own testing to see what works for you.

Use of the Spotmeter

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.

Pentax spotmeter dial with homemade zone scale.

1. I aim my spotmeter at the darkest shadow or area that I want to print as pure black. Let's say the needle points to 6 on the viewer scale.
2. I turn the dial to match 6 with I (one) on my homemade scale.
3. I can now read the correct exposure choices off the top of the dial. ( 1/4 sec @ f-16, 1/8 sec @ f-11, etc.)
If I were not to process my own film this would be the end.
However, (4.) I now aim my spotmeter at the brightest white and the needle points to 12. (It is an overcast day.) This corresponds to Zone VII (light gray) on my home made scale and I want it to print lighter than that so I mark +2 on my film holder. (To be overdeveloped and pushed from Zone VII (light gray) to Zone IX (white with some texture)
The whole process takes about 10 seconds.
Note:This does not mean that I take a reading off the darkest shadow and set the exposure to that. If you take a meter reading off the darkest shadow, you have to close down 4 stops to get the correct exposure. My home made scale takes care of that for me.


Consider the above pictures. Both were taken with the same exposure. The film on the left was developed normally; the film on the right received 50% less development time. Underdevelopment brought the cloud values down to Zones 7, 8 and 9 and didn't affect the blacks or dark grays.. This was done because my spotmeter indicated that the white values in the clouds would fall on Zones 9, 10, and 11 (pure white with no features) as in the image on the left. Giving the film less exposure to darken the clouds would have also darkened the castle walls causing them to print black.

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.


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J.G. Dainis 1999
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