Curves

Curves is a graph-based tone control in editing software. The horizontal axis shows input brightness and the vertical axis shows output brightness. A straight diagonal line means no change. Bending the line brightens or darkens specific tonal ranges. Photographers use curves to set black and white points, lift shadows, and shape contrast with more control than a single slider.

A contrast slider pushes highlights and shadows apart across the whole image at once. Curves target specific brightness ranges. A photographer can lift only the deepest shadows or compress highlights without shifting midtones as much. That precision matters in high-contrast scenes where a global slider can clip detail at one end of the tonal range.

An S-curve bends the diagonal line into a gentle S shape. The lower section dips below the default line, which deepens shadows. The upper section rises above it, which brightens highlights. The result adds punch and separation between dark and light areas. Mild S-curves suit portraits and landscapes; steep curves can look harsh or lose detail.

Curves cannot restore detail from severely blown highlights or crushed shadows. Those pixels contain little usable data. In a RAW file, mild clipping may respond to pulling the highlight end of the curve down, which recovers some tone. Severe clipping from overexposure stays lost. Curves work best when exposure left enough data to reshape.

RGB curves show one master curve plus separate curves for red, green, and blue channels. Adjusting a single channel shifts color as well as brightness. Lifting red in highlights can warm a sunset; adding blue in shadows can cool them. The master curve changes all channels together, which is the usual starting point for tonal work.

Curves usually follow basic exposure, white balance, and crop steps. Global tone needs a stable base before fine curve work. Many photographers apply curves near the end of editing, after local adjustments, so the curve reflects the finished image. Saved presets can store a favorite curve shape for batch work on similar scenes.

Yes, curves work on JPEG and other compressed formats. JPEG files carry less tonal headroom than RAW, so strong curve moves can band skies or add noise in lifted shadows sooner. RAW files tolerate larger adjustments because they store more brightness data. Curves on any format should stay moderate when the file was heavily processed in camera.

The histogram shows how many pixels fall at each brightness level. The curves graph maps those levels to new output values. Watching both together helps avoid clipping. If the histogram spikes at the right edge after a curve edit, highlights are blowing out. A spike on the left means shadows are crushed to pure black.

Tone curve is another name for the curves tool. It describes the same brightness mapping function whether the panel is labeled Curves, Tone Curve, or Parametric Curve. Some apps offer preset curve shapes like linear, medium contrast, or strong contrast. Custom points on the curve override those presets for scene-specific results.

Per-channel RGB curves can reduce color casts by shifting one channel relative to the others. A green cast in shadows might improve by lowering the green curve in the dark range. White balance tools usually handle casts more directly. Curves suit fine color shifts after white balance is close, especially when only shadows or highlights need a tint change.