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The Black Offset level you set also becomes the minimum picture level when you lower exposure, either using the Global Exposure changes, or when you use any of the “darkening” zone controls such as Shadow, Dark, and Black. The following image shows the result of lowering Global Exposure; the shadows of the image are compressed more heavily than the highlights as exposure is lowered, with the result that the minimum value of the image remains at the Black Offset level you set. With
shadow detail impressively preserved, this compression rolls off smoothly and nonlinearly through the midtones so that the image continues to look as natural as possible.
(Left) Before, (Right) After lowering Global Exposure to compress image contrast down to the Black Offset level; notice the signal never goes lower than the Black Offset level
This is also true when using other darkening zone exposure controls, such as the Shadow control, which selectively pushes the exposure of the darker part of the image down, while leaving the highlights alone. While reducing Shadow exposure, the darkest pixels still compress so that nothing goes beneath the Black Offset level you set.
Bottom line, you can adjust Black Offset at any time, either before or after any other Global or Zone adjustments, to fine tune the image as necessary.
Making Zone-Based Adjustments
After you’ve made whatever initial global adjustments you want, you can optionally make more tonally- specific adjustments to the image via the Zone-based color and contrast controls to the left. These divide the image into multiple overlapping “zones,” which are somewhat similar in principal to Ansel Adams’ zone system, which divides images into tonally-specific regions based on image luminance, from pure black, through progressively lighter shadows, to 18% gray as the middle value centering the midtones of most images, and then up through progressively lighter highlights on the way to the last zone of pure white.
Simplified illustration of Ansel Adams’ zone system