Zone system research

The zone system begins with a zone chart. Zone 1 is pure black; zone 9 is pure white; zone 5 can be described subjectively as middle gray. On paper, zone 5 corresponds to a reflectance of 18%. Since computer monitors and printers vary greatly — Macintoshes have different brightness curves (gammas) from PC's — what you see here may not be an accurate representation of the zone chart. You will need to have a good feeling for Zone 5-- middle gray, so if what you see doesn't seem right, you should go to a camera store and buy a Kodak 18% reflectance gray card. These cards are used by professionals for exposure metering in the studio: They place the card next to the subject and meter from it. This is equivalent to incident light metering: measuring the light that reaches the subject. With incident metering, the exposure is independent of the subject's reflectance: dark subjects come out dark and light subjects come out light. Incident metering works very well in studio environments and for close-ups, but it isn't practical for landscapes. Meters built into cameras measure reflected light.

1	2	3	4	5	6	7	8	9

Ansel Adams' description of zones (geared towards black and white printing)

Low values Zone 0	Complete lack of density in the negative image, other than film base density plus fog. Total black in the print. We will omit zone 0 from the remainder of this tutorial; zone 1 will be considered pure black.
Zone 1	Effective threshold. First step above complete black in the print. Slight tonality, but no texture.
Zone 2	First suggestion of texture. Deep tonalities, representing the darkest part of the image in which some detail is required.
Zone 3	Average dark materials. Low values showing adequate texture.
Middle values Zone 4	Average dark foliage. Dark stone. Landscape shadow. Recommended shadow value for portraits in sunlight.
Zone 5	Clear north sky (panchromatic rendering). Dark skin. Gray stone. Average weathered wood. Middle gray (18% reflectance).
Zone 6	Average Caucasian skin value. Light stone. Shadows in snow in sunlit snowscapes.
High values Zone 7	Very light skin. Light gray objects. Average snow with acute side lighting.
Zone 8	Whites with textures and delicate values (not blank whites). Snow in full shade. Highlights on Caucasian skin.
Zone 9	Glaring white surfaces. Snow in flat sunlight. White without texture. (The only subjects higher than Zone 9 would be light sources; they would be rendered as the maximum white value of the paper surface.

In a scene — in the field — each zone represents a doubling or halving of the luminance — the light reflected from the subject — or equivalently, a difference of one f-stop. The eight steps between the nine zones in the chart (1-9) represent a luminance range of 256 (2⁸). On paper surfaces, this difference is considerably compressed. On good photographic paper, pure white is about 90% reflectance and pure black is about 2% reflectance. The maximum tonal range is around 45, equivalent to about 5.5 zones (log₂45). Reflectance differences between zones are less than a factor of two. The difference between zones at the ends of the scale (1 and 2 or 8 and 9) is much less than between zones in the middle (4, 5, and 6).

zone system research taken from: http://www.luminous-landscape.com/tutorials/zone_system.shtml