Contrast Adjustment Explained

If you strip away the color in an image (you can do this by setting the Saturation control to 0), the grayscale image that remains represents the luma component of the image, which is the portion of the image that controls the lightness of the image. As explained in The Y′CBCR Color Model Explained, the luma of an image is derived from a weighted ratio of the red, green, and blue channels of the image which corresponds to the eye's sensitivity to each color.

Figure. Chroma and luma compared.

Although luma was originally a video concept, you can manipulate the luma component of images using the contrast controls in Color no matter what the originating format. These controls let you adjust the lightness of an image more or less independently of its color.

Note: Extreme adjustments to image contrast will affect image saturation.

What Is the Contrast Ratio of a Shot?

One of the most important adjustments you can make to an image is to change its contrast ratio. The contrast ratio of an image is the difference between the darkest pixel in the shadows (the black point) and the lightest pixel in the highlights (the white point). The contrast ratio of an image is easy to quantify by looking at the Waveform Monitor or Histogram set to Luma. High-contrast images have a wide distribution of values from the black point to the white point.

Figure. High contrast image and waveform.

Low-contrast images, on the other hand, have a narrower distribution of values from the black point to the white point.

Figure. Low contrast image and waveform.

The Shadows, Midtones, and Highlights contrast sliders let you make individual adjustments to each of the three defining characteristics of contrast.

Note: Contrast adjustments made with the primary contrast sliders can affect the saturation of the image. Raising luma by a significant amount can reduce saturation, while reducing luma can raise image saturation. This behavior is different from that of the Color Corrector 3-way filter in Final Cut Pro, in which changes to contrast have no effect on image saturation.