Development / Zone
Actions
- zone/setup : Create layer to highlight a zone.
- zone/* [l] : Change which zone gets highlighted.
The zone actions provide a way to check the tonal range of an image, by highlighting (in red) a tonal zone in the image. The zone action produces a non-destructive Adjustment Layer, so it can be used during development while actively adjusting brightness and contrast. The zones are based on Ansel Adam's Zone System.
Zone 0 : Pure black, no detail.
Zone 1 : Threshold of tonality, no texture.
Zone 2 : Slight texture.
Zone 3 : Full detail in the shadows.
Zone 4 : Light areas in shadow, dark foliage, shadow for portraits in sunlight.
Zone 5 : Middle gray, lightest highlight in a shadow, clear blue sky, sunlit grass, dark skin.
Zone 6 : Lightest medium tone, white skin, shadows on sunlit snow.
Zone 7 : Full detail in the highlights, light skin, side lit snow.
Zone 8 : Slight texture, highlights on white skin.
Zone 9 : Threshold of tonality, no texture.
Zone 10 : Maximum white value, no detail, light sources.
To use, select the top Layer in the document and run the zone/setup action. This will create an Adjustment Layer named zone/5 which highlights zone 5.
To change the highlighted zone make sure the zone layer is selected and run one of the zone/* [l] actions.
The zone layer can also be left on above the development layers. In this way when making changes, such as an adjustment to the contrast overlay below, it becomes easy to see how the adjustments effect the tonal spread of the image.
Technical Details
The zones are roughly based on Ansel Adam's Zone System. This action uses 9 actual zones, zone 0 is just pure black and zone 10 is just pure white. The numerical values of the 9 zones were computed by splitting the CIE lightness range into 9 equal spaced zones and then converting lightness into an 8-bit level. CIE lightness is an approximation of how human vision perceives brightness. The location of each zone can be seen by the changing gradients in the histograms found in the image below. The red line marks the level which is perceptually half way between black and white.
Linear/Gamma 1.0
| Gamma 1.8
| Gamma 2.2
|
The equations used to compute the numerical location of each zone are from CIE standards.
VARIABLES
G = gamma (0=linear, 1.8, or 2.2)
L = input lightness (range 0-100)
Y = intermediate luminance (range 0-1)
P = output level (range 0-255)
LIGHTNESS TO LUMINANCE
if (L <= 216/27)
then Y = L*(27/24389)
else Y = ((L+16) / 116)^3
LUMINANCE TO OUTPUT LEVEL
P = 255 * Y^(1/G)
External References
- A simplified Zone system for making good exposures by Norman Koren
- ColorFAQ - An excellent FAQ by Charles Poynton.