Originally posted by pathdoc that every zone is also worth a stop of exposure (which greatly simplified the understanding of the whole thing for me).
Yep! One stop per step and 18% gray at Zone V are the keys as far as metering and placing exposure go. Keeping all pertinent zones in the linear response range on the negative is where film choice, exposure index, and processing come in.
I find it helpful to consider that the Zone System is a perceptual system and was conceived working backward from a monochrome print where the darkest tones are determined by the silver-based emulsion (or ink as the case may be) and the lightest tones are determined by the reflectivity of the paper base. There is no detail at dMax for silver/ink or at paper white. Assuming linear response and dynamic range of 9 stops, an 11-step exposure wedge may be used to visualize the sequence. Since there is a mathematical relationship between tones, Zone V falls to a middle gray (18% in printer's parlance). The middle gray corresponds to the calibration point of most light meters. Note that the 9 stops of dynamic range is perceptual and somewhat arbitrary. Zones I and IX are assumed to have detail, but no texture. Unless reduced texture is desired (e.g. high key work), critical areas of the subject should be
placed within zones II to IIX. Note the emphasis on the word "placed". Appropriate exposure and processing places the the subject's tones within the limits of the photographic system.
I think you can see why questions regarding meter accuracy often result in snarky comments from knowledgeable users on this site. I try not to be snarky, but give a standard response that the metered exposure will return a centered histogram (middle gray) for an exposure on an evenly-lit blank wall. Appropriate exposure, OTOH, will reproduce a reasonable range of texture for the subject and will account for the dynamic range within the frame.
More than you needed, eh?
Steve