How and When to Adjust The Black Level
The first thing you should adjust in any correction is the black level, which is sometimes also referred to as pedestal, lift, or setup. The reason for this is that in most color engines, all of the other computations are based on this level. It gives you a strong “base” to build the rest of your correction on. Normally if your picture looks a little washed out or there is not much detail in the blacks, you need to look at this level and get it to around 0 IRE. The exception to this rule is if there is nothing in your image that should be black.
If your image is:
- Shot through a fog or smoke
- Is mostly snow
- Is a long focal length shot of some distant skyscrapers
- Has a subject that is entirely midtones and highlights
Then there probably isn’t anything that needs to be pure black. In cases like these, you should rely on your subjective judgment.
One of the primary tools for setting luminance or black levels is the waveform monitor. The specific position of the deepest black or the brightest white is not as important as watching how compressed the waveform is at the top and bottom. If you lower the setup to get it down to 0 IRE but notice that the lower levels of the waveform are starting to compress into a single, thick line at the bottom, you know you’re losing detail down in the shadows. Sometimes this means compromise. Usually you want that black point to be all the way to 0 IRE, but if you start compressing the shadow details before you reach 0 IRE, then you may need to use your eye to tell you when the blacks are rich enough. You can also bring the blacks all the way down to 0 IRE and then use your gamma control to “uncrush” the details in the shadow by lifting the gamma.
So the clue that you are clipping or crushing is not really the position of the waveform but the overall shape of the waveform. Examine it carefully as you move the shadows toward 0 IRE and the highlights toward 100 IRE. When the shape—or excursion—starts to compress into a single, tight line, then you know you are killing detail.
Another analogy for controlling your adjustments is that it is similar to focusing a camera. To make sure you are in perfect focus, you have to go a little bit beyond your focus point then come back to the sharpest focus. Color correction is similar. The only way to know if you have taken the image far enough is to take it too far, then come back to the place where it belongs.
Excerpt from Color Correction for Video, Second Edition by Steve Hullfish and Jaime Fowler, © 2009 Elsevier Inc. All rights reserved.