The Zone System
Most of the charts on the internet depicting the Ansel Adams Zone System are completely wrong. They were produced in the wrong color space.
The Zone System is a perceptual method of reproducing real-world tones in a target image. It was conceived for use in black and white photography, but the concepts still hold true for modern digital images. In fact, Reinhard’s influential paper on tonemapping, “Photographic Tone Reproduction For Digital Images”, was an attempt to apply the zone system in the context of digital HDR imaging. Unfortunately, even the zone system chart in Reinhard’s paper is wrong¹. Ouch.
A zone chart has 11 grayscale values, centered around middle gray (what photographers call 18% reflectance, or Zone V) and should contain entries of increasing brightness, doubling with each entry. For whatever reason, Adams used roman numerals, so Zone X refers to the lightest value and Zone 0 is the darkest. To use the zone system, a photographer decides which zone they want their subject (or some other reference point) to fall under. They then spot meter the subject and adjust their exposure based on it’s offset from Zone V. Light meters are calibrated to expose for Zone V. This system allows the photographer to keep bright subjects bright, and dark subjects dark, instead of everything coming out middle gray.
In lieu of a full lecture on sRGB, realize that the “0-255” RGB numbers you see in your image editor are usually encoded in sRGB. Keying in “128” won’t get you a value halfway between black and white, but actually something much darker. The right way to build the chart is in linear space, then convert it to sRGB².
Here is how most people build the zone system chart –
And here is the correct chart³ –
Notice how each zone feels twice as bright as the previous? These correspond directly to photographic “stops”. On my monitor, I only get about 8 stops of usable dynamic range (Zones 1-8).
Also note the choice of Zone V is arbitrary. I chose that particular value by visually matching my monitor to an 18% gray card. Depending on the image, you could probably get away with moving Zone V down a stop. Unfortunately it’s standard practice to use “255” white for UI backgrounds, so most monitors are set too dark to move Zone V down much more than that.
The dynamic range is not nearly as expressive as most charts may lead you to believe. It’s no wonder why digital images tend to have completely blown out highlights. Tone mapping can mitigate this a bit, but if you ask me the solution is HDR displays.
¹ http://www.cs.utah.edu/~reinhard/cdrom/ – To be fair, I didn’t actually try printing the paper out before judging, but certainly viewed on a monitor their chart is wrong.
² For a better understanding of sRGB and linear color space conversion, I refer you to John Hable’s presentation.
³ The values were estimated using gamma 2.2, so not sRGB exactly.