Zone System, simplified - Farzad's 5-stop approach
Excerpts from The Simplified Zone System Book

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4.6. Subject Brightness Range (SBR)
Another term that needs to be understood to analyze a complex subject is the Subject Brightness Range (SBR). This subject analysis will eventually help us to find our subject’s desired exposure.
Brightness range is simply the measure of the number of EVs or STOPs in a scene, including its darkest and brightest tones. If you are confused, let’s try again. Measuring SBR is very much like measuring the height of a subject from the lowest to its highest point. The subject can be a ladder or a file cabinet. To measure the height of a ladder or a file cabinet in feet, we could count the number of 1 foot regions from the lowest to the highest point (please see Diagram 4.6.1). To measure the Subject Brightness Range in stops, we can use a spotmeter that has markings in EVs. As I mentioned before, each EV can be treated as 1 stop on a camera. Now, let’s examine the snow-covered cottage that we used in the previous chapter. To refresh your memory, the darkest tone of the subject (the door) had a normal exposure of 1/125 @ f-11. The brightest tone of the subject (the snow), had a normal exposure of 1/125 @ f-22.
To measure the Subject Brightness Range (SBR) of our snow-covered cottage in the previous chapter, simply count the number of EVs from darkest to the brightest. In this case, it is 3 stops (f-11, f-16, f-22). This is the number of stops that must be recorded on film.
PLEASE NOTE: As is illustrated in the height of the ladder (Diagram 4.6.1), the spotmeter or your camera measures the regions or divisions and not the markings (remember, the number of markings is always one more than the number of regions.) This means each EV indicated on your spotmeter is equivalent to a “yardstick” or in this case an “EV stick” and not like the notches (markings) on your rulers or measuring tapes (Diagrams 4.6.4 and 4.6.5.) Please look at these diagrams and make sure that you understand this important concept.
The range of the cottage’s brightness, as was explained, is 3 stops (f-11, f-16, f-22). Now let’s use a spotmeter to find the Subject Brightness Range of the same snow-covered cottage. By pointing the spotmeter at the snow, we get an EV of 17. By pointing the spotmeter at the door, we get an EV reading of 15. Counting the EVs, provide us with an SBR of 3 (15, 16, 17) stops.
The sequential EV numbering of a spotmeter makes it a very convenient tool to measure the Brightness Range of a Subject. For example, if a subject has a lowlight reading of 10 EVs and a highlight reading of 16 EVs, the SBR for this subject is 7 stops (10, 11, 12, 13, 14, 15, 16).
In our Tone Train example, each EV (stop) is represented by one tank car. The Subject Brightness Range (SBR) will be represented by the number of tank cars of a loaded train. For example, if the SBR of a subject is 6 stops, it will have 6 loaded tank cars; if it is 3, then it will have only 3 cars. As before, the outer tone of the car will represent the tone of the subject. The Tone Train illustration will be extensively used to illustrate the relationship between a subject and its corresponding image.

Copyright 1999, cpgbooks, The confused photographer's guide to photographic exposure and the Simplified Zone System by Bahman Farzad