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Levels and curves

Levels and curves define how an input channel value becomes an output value. They make tonal compression, expansion and selective contrast explicit.

Read tone remapping against the source histogram.

Source histogram behind the editable tone transfer0255Input → output
Source tonal study
Source
Remapped tonal study
Mapped
Transfer control
128 → 127255 × clamp((x − 35) / (222 − 35))^(1 / 1)Input values mapped to black / white: 36 / 34

The histogram counts source tones; the line remaps channel values. Move the probe into a flat segment to find where distinct inputs collapse. Curves use the same shape-preserving interpolation as the Lab. A histogram records frequency, not the spatial location of detail.

The black and white points squeeze the tonal range; then a curve lifts and lowers the middle tones. Use any control to take over.

What to try
  1. In Levels, move Input black or Input white and probe a value in a flat segment.
  2. Switch to Curve and move its output at input 128; compare the curve and the image.
Open in the Lab ↗

What to look for: The probe links one input to one output. Flat regions remove distinctions; steep regions expand them. The histogram shows how often source tones occur.

How it works

Levels sets input black and white points, a midtone gamma and output endpoints. Values below input black become output black; values above input white become output white. Moving the input endpoints inward stretches the remaining range and can increase contrast while clipping the tails.

Curves provides a more flexible mapping. The horizontal axis is input; the vertical axis is output. A point above the diagonal lightens that range, while a steeper section increases local contrast. The Lab composes a master curve with individual red, green and blue curves and shows a source histogram behind the graph.

Use it in the Lab

Use numeric point controls for precise changes and the graph for shaping. Preview on a selection when comparing a local treatment. Curves and their control points survive saved-chain replay in the Lab.

Levels ↗Curves ↗

Limits & distinctions

A histogram shows how often values occur, not where they occur. Two very different compositions can have the same histogram.

Sources & references

Guide updated