Photoshop has included a curves adjustment since its first version. Lightroom adopted it when it was released in 2007 for global processing, then in 2023 for local processing.
It is an essential adjustment.
Curves are ubiquitous in image processing software. They appear under the names tone curve, tonal response curve, gamma, and LUT (look-up table). They are a graphical representation of a fundamental computer function: the conversion of input data into output data. In software such as Photoshop, they are widely used to correct density, contrast, or chromaticity (see, for example, our tutorial Photoshop: Make color corrections before printing).
Essential adjustments in Photoshop and Lightroom
Curve adjustments have been a feature of Photoshop since its first version, released in 1990. Six years later, it appeared as a non-destructive adjustment layer in Photoshop 4. Lightroom 1.0 dates back to 2007. Its development module offers a tone curve panel, but it wasn’t until April 2023 that the curve became available for mask adjustments.
Combined with Lightroom’s “smart” masking, it offers powerful targeted adjustment capabilities. Curves greatly increase the possibilities for adjustment. In fact, regardless of the software, this type of adjustment is the solution for most of our needs in terms of controlling density, contrast, and chromaticity.
The tone curve in Lightroom’s Develop module functions similarly to the one in Photoshop, though it is the only available curve for global adjustments. A key feature is the Refine Saturation slider, which allows the user to mitigate any excessive saturation that may result from aggressive curve manipulation.
Curve shapes
By default, a curve is displayed as a grid with a diagonal line. The straight line at 45° indicates that no adjustment has been applied: the input values are identical to the output values.
The horizontal axis represents the original tonal values (input data). Black is on the left; the values lighten towards white on the right.
The vertical axis represents the modified tonal values (output data). Black is at the bottom; the values lighten towards white at the top.
If the targeted adjustment tool (small circle in the upper left corner of the Lightroom Tone Curve panel) is enabled, a circle appears on the curve. It represents the tonal values over which the tool is hovering. When a point pulls the curve down, the image darkens; if it pulls it up, the image lightens. When a point is set at the bottom of the line to lower it and another point is applied at the top to raise it, the line takes on an S shape; the contrast of the image increases. If the slope of the curve is steeper than the diagonal, the contrast increases; if it is shallower, the contrast decreases.
An S-shaped curve contrasts the image. A point below the diagonal darkens the image, while a point above it lightens it. Another principle: if the slope of the curve is steeper than the diagonal, the contrast increases; if it is shallower, the contrast decreases.
Parametric curve, point curve
In Lightroom, the tone curve comes in two forms: the parametric curve and the point curve. The first modifies the image using sliders, by region: highlights, midtones, shadows, and blacks. It is equivalent to adjusting levels in Photoshop, targeting specific tone areas. To unlock its full potential, the point curve is essential.
All three channels modify the brightness, contrast, and even the saturation of the image; individual adjustments to the red, green, or blue layer change its hue.
The parametric curve modifies the image using sliders, by region: highlights, Lights, Darks, and Shadows.
The curve offers four adjustment options: across all layers and on each RGB layer. Here, the blue layer is slightly lowered to reduce the blue cast in the image.
Curve and mask
The tone curve is available for masks. It offers more flexibility than the exposure slider, as you can adjust the desired tone range rather than the overall tone. Unlike Photoshop, you can only use one tone curve for the entire image in the basic settings.
To multiply the curves across the entire image, you need to use masks and paint with a brush on the entire image.
The upper right part of the sail needed to be darkened. A local gradient mask, combined with a tone curve, reduced its brightness.
In Masking mode, if the entire image is painted with the brush, the adjustment applies to the entire image. This tip is useful, for example, when editing a file for printing, where a specific adjustment to the curves can change the image’s brightness, contrast, and hue.
Recording custom curves
As you process images, you will notice that you tend to use the same types of curves. You can save these custom curves for both basic and local adjustments. They are saved in XMP format in the Camera Raw preferences. On a PC, they are located in the Curves folder, found at “C:\Users\User\AppData\Roaming\Adobe\CameraRaw\Curves.”
On a Mac, they are located at /Users/User/Library/Application Support/Adobe/CameraRaw/Curves. This is a very useful feature when you regularly apply the same types of adjustments to your images.
Custom curves can be saved and stored in Camera Raw preferences (Curves folder, Mac or PC). They can be recalled when you regularly make the same types of adjustments to your images, for example, when preparing a print for a lab like PictoOnline, using a Fujifilm Frontier printer.
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Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos