The tone curve and Levels
The Tone curve section is the last in Tone (⌃1). It is the tool for contrast the single Contrast slider cannot express: an S-curve, a flattened film-like curve, deliberately crushed blacks. It runs after the film simulation, the look and the sliders above it, so the curve is the last word on the frame's contrast.
Two buttons at the top of the section choose its mode: Tone curve or Levels. Both drive the same step of the render; they are two ways of saying one thing.
Drawing a curve
The plot runs input left to right and output bottom to top, both from 0 to 255, so a point at 128 is the midtone. At rest it is a straight diagonal. The frame's own histogram sits behind the curve and is measured from the frame as it renders now, including the curve you are drawing.
- Click anywhere on the plot to add a control point.
- Drag a point to move it. The photograph follows live; the change is stored when you let go.
- Double-click a point to remove it, or drag it off the plot and let go. It is drawn hollow while it is outside so you can see the removal coming.
The two endpoints cannot be removed, only moved: drag the black point inward to crush everything below it, or the white point inward to clip the highlights. Points cannot cross one another, so the curve is always a curve. Where your points rise, the curve rises; it never dips below a point it has just passed through.
The four channels
A picker above the plot chooses RGB, R, G or B. RGB is the master curve, applied to all three channels together, and the one most frames ever need. The three colour channels are trims on top of it, for a cast or a split-toned look. A channel that holds a curve shows a small dot on its chip, so a red curve set earlier is not invisible while you are looking at RGB.
The reset arrow on the section's header puts all four channels back to the diagonal.
Levels
Press Levels and the section renames itself Levels and shows three sliders instead of the plot.
| Slider | Range | Starts at |
|---|---|---|
| Black point | 0 to 60 | 0 |
| White point | 40 to 100 | 100 |
| Gamma | 40 to 220 | 100, which is straight |
The note under them says it exactly: Levels are the curve said in three numbers — where black lands, where white lands, and where the middle sits between them. The render is the same step either way.
With Levels on, the frame renders through these three numbers in place of the RGB curve. The R, G and B curves still apply on top of them.
Levels only holds on a frame whose R, G or B channel already carries a curve. On a frame where all three are straight, pressing Levels puts the section straight back on Tone curve and keeps nothing. If the frame carries a curve on RGB alone, pressing Levels also deletes that curve; press ⌘Z to bring it back. The same goes in the other direction: pressing Tone curve to leave Levels, on a frame where all four channels are straight, drops the three Levels numbers.
To use Levels, give one colour channel a small trim first. Choose R, G or B above the plot, click the plot a few levels off the diagonal and let go, then press Levels. A trim that small barely changes the picture, but it is an adjustment: it is listed in the Changes tab, and if you straighten it again in the curve mode while RGB and the other two colour channels are straight too, the Levels numbers go with it.
With a trim in place nothing is thrown away when you switch modes. A shape you drew on RGB is still there when you go back to the curve, and a black point you set is still there when you leave Levels. The section's reset arrow clears the whole section, all four channels and the three numbers, and puts it back to the plot.
When to reach for it
Use the sliders above for the everyday work and the curve for the shape of the contrast: lift the lower-middle for open shadows that stay black at the very bottom, or pull the top end down for a soft, filmic highlight. Levels is for when all you want is to set where black and white land, by number rather than by point.
This section has an eye, so you can see the frame with and without the curve in one click, and a line in the Changes tab for each channel that holds a curve. Levels has no line there; see Changes, Versions and History.