What tone curves actually do

The two axes

The horizontal axis is the brightness a pixel has now. The vertical axis is the brightness it will have after. Left is black, right is white; bottom is black, top is white. That is the whole idea.

The diagonal line from bottom-left to top-right is the identity: every input maps to itself, nothing changes. Every edit you make with the curve is a deviation from that diagonal. Above the line means brighter, below means darker, and the steepness at any point is the local contrast at that brightness.

That last part is the insight worth keeping. Contrast is not a slider, it is a slope. Where the curve is steeper than 45°, tones in that range are pushed apart and you see more separation. Where it is shallower, tones are squeezed together and detail flattens. You cannot steepen one region without flattening another — the curve starts and ends at fixed corners, so every unit of contrast you add somewhere is taken from somewhere else. This is the single most important fact about tonal editing and it is invisible in the Basic panel.

The classic shapes

• The S-curve. Pull the quarter-tone down and the three-quarter-tone up. Midtone slope increases, so the midtones gain punch; the shadows and highlights flatten to pay for it. This is what the Contrast slider approximates.
• The inverted S. The opposite: midtones flatten, shadows and highlights gain separation. Useful for recovering a scene that was contrasty to begin with.
• The lifted toe (matte, or "faded film"). Drag the bottom-left point straight up. Pure black is no longer available — the darkest the image gets is a dark grey. This is the entire basis of the faded-film look, and it costs you shadow contrast, which is why matte looks feel soft.
• The rolled shoulder. Pull the top-right point down. Highlights compress instead of clipping, mimicking how film reacts to overexposure. Gentle, and much more forgiving than the Highlights slider.
• The flat line. A horizontal segment maps a whole range of inputs to one output. That is posterisation. Avoid unless you want it.

Parametric versus point

Lightroom gives you two curves in one panel. The parametric curve has four sliders — Highlights, Lights, Darks and Shadows — plus three split points along the bottom that set where those regions begin and end. It cannot posterise, because it is constrained to smooth adjustments of predefined regions. It is the safe one.

The point curve is the real thing: you place up to sixteen anchors anywhere and drag them. It can do anything, including ruin an image. Since the panel redesign in Lightroom Classic 9.3 you switch between them with the small icons at the top right of the Tone Curve panel: a wavy-line icon for parametric, then white, red, green and blue circles for the master and per-channel point curves. Older guides referring to a "Channel" dropdown are describing an interface that no longer exists.

The two curves stack. A parametric adjustment and a point adjustment both apply, in that order. This trips people up constantly — if your point curve is not doing what you expect, check whether the parametric curve is also non-neutral.

The channel curves: where colour grading really happens

The red, green and blue curves work the same way, but on one channel at a time. Lifting the bottom-left of the blue curve adds blue to the shadows. Pulling the top-right of the blue curve down removes blue from the highlights — and removing blue is the same thing as adding yellow. That pair is the classic teal-shadow, warm-highlight cinematic split, expressed as two drag operations.

The complementary pairs are worth memorising, because every channel curve move is really two moves:

Because each channel curve can be shaped independently along the tonal range, channel curves can do something no white balance slider can: shift colour differently at different brightnesses. Temp and Tint move the whole image. A channel curve can cool the shadows while warming the highlights in one operation. That is the entire reason film stocks look the way they do, and it is why measuring a look produces per-channel curves rather than a pair of white balance numbers.

Reading a curve you did not draw

When you look at a plotted preset — the tone-curve panel on the PresetForge hero plots exactly this — read it in three passes. First, where is the master curve relative to the diagonal? Above means the look brightens, below means it darkens. Second, where is it steepest? That is where the look puts its contrast. Third, do the three channel curves separate from each other, and where? Where they fan apart, the look is introducing a colour cast at that brightness; where they sit on top of each other, it is tonally neutral there.

Three or four well-placed anchors will describe almost any real edit. More than that and you are usually fitting noise rather than intent — a curve with sixteen anchors chasing every wobble in one photograph will posterise on the next one.