Tone Curve
A graph mapping input luminance values to output values. It is the most powerful tonal adjustment tool for precisely controlling brightness, contrast, and tonal distribution in images.
A tone curve is a graphical representation of a function that maps each pixel's input value (horizontal axis) to an output value (vertical axis). A straight 45-degree diagonal represents identity. By reshaping this curve, photographers gain precise control over brightness, contrast, and tonal distribution.
- S-curve: The most fundamental contrast-enhancing pattern. It darkens shadows and brightens highlights, steepening the midtone slope for increased punch and separation
- Inverse S-curve: Reduces contrast for a flat, film-like aesthetic. It is foundational in cinematic color grading, creating a muted look for subsequent color treatments
- Per-channel manipulation: Adjusting RGB channels independently enables color cast correction and creative grading. Lifting the red channel's shadows introduces warmth into dark areas
- Clipping: Moving endpoints inward crushes tonal range to pure white or black. Intentional shadow clipping with a raised black point creates a popular faded film effect
In Photoshop and Lightroom, tone curves operate on a 0-255 scale but internal calculations use 16-bit or higher precision. Control points connect via spline interpolation to prevent banding. In video post-production, curves are baked into LUTs (Look-Up Tables) for real-time playback, applying complex transformations as simple array lookups per pixel.