How to use Curves adjustments to control image colour, brightness and contrast.
The dialog box for Curves adjustments. To make adjustments, you add control points at different places on the diagonal line (one is indicated by the red arrow) and then and move them up or down to create S-shaped curves.
While Levels adjustment is often the first step photographers use for adjusting parameters like brightness, contrast and colour balance, when you want those adjustments to be subtle, the Curves control should be the tool of choice. Like Levels, the Curves adjustments are based upon a histogram, which can be globally adjusted through the default RGB channel or allow each colour channel to be adjusted individually.
But unlike Levels, which only provides adjustments for black, white and midpoints, Curves can be controlled with any number of points. This makes the Curves function a very powerful tool.
In the dialog box for Curves adjustments, shown on this page, the horizontal axis plots the input (original image values), while the vertical axis plots the output (adjusted values). The diagonal line (curve) maps inputs to outputs. When the curve is a straight diagonal line from bottom left to top right, all tones are mapped to their default values, with a linear relationship between adjacent tones.
The bottom left corner is pure black (0,0,0) while top right corner is pure white (255,255,255). Everything in between maps to its equivalent value; for example, the midpoint on the graph will be 128 on both input and output axes, as shown in the illustration above.
You can add as many control points (or as few) as you need. More points provide more scope for making adjustments, while fewer points allow greater subtly in the adjustments applied.
Moving a point in the top part of the curve adjusts the highlights. Moving a point in the centre of the curve adjusts the midtones, and moving a point in the bottom section of the curve adjusts the shadows. Moving a point up, makes those tones lighter, while moving it down makes them darker. Contrast increases when the curve becomes steeper and decreases as it is made flatter.
The S-shaped curve shown in the graph on the left indicates an increase in contrast, caused by pushing up the highlights and lowering control points in the shadows. The curve on the right shows reduced contrast, achieved by pushing the control points in the opposite direction. The midpoint remains unchanged. The effects of each change are shown below the graphs.
By its nature, the Curves adjustment won’t allow you to increase contrast in one tonal region without reducing it in another. Similarly, if a tone was initially brighter or darker than another, it will remain that way after a curves adjustment, although not necessarily by the same amount.
The exception comes when there are gaps in the tonal range as seen in the image histogram, either at the ends of the range or between tonal peaks. Closing the gap enables you to focus upon tones that are present in the image, as shown in the example below.
Moving the end of the input slider (circled in the left hand graph) to the left (circled in the right hand graph) provides greater facility for adjusting the other tones in the image.
This time, the Curves adjustment has been used to close a gap in the image tonal range that reduced all shadowed areas to near black. Note the adjustments in the circled area and the way the tones in the sky have remained unchanged after the adjustments.
All the adjustments shown so far have been made with the RGB channel setting. But you can select each colour channel ““ Red, Green or Blue ““ and work on them separately. This gives you enormous subtlety when you want to change colours in an image.
First, however, you need to know how the colour channels work in the Curves tool. In each channel, pushing the control point upwards will boost the colour of that channel. Pushing it down shifts to colour balance towards the complementary colour. For example, if you’re in the Red channel, pushing a control point in the shadows area up will make the shadows a little warmer (redder), while pushing it down changes the colour balance in the shadows towards cyan (the opposite to red).
The examples on this page demonstrate how you can change the colour balance in images by using the Curves tool. They aren’t necessarily the ideal adjustments for the selected picture but have been used to demonstrate how the Curves channels can be used for selectively adjusting the colours in highlights, midtones and shadows.
The image and associated Curves dialog box on the left show balance of hues and tones in the original image. To the right are three variations produced by adjusting Curves in each colour channel (red, green and blue) by pushing highlights up and shadows down.
The adjustments made to the original image (shown on the left) are the opposite to the previous adjustments. For each colour channel, the control points for the highlights have been moved down, while the shadows control points have been moved up.
In most cases the adjustments will be more subtle than those shown in these comparisons. The Curves control is particularly useful when you want to tweak a particular colour or correct colour imbalances. Examples are shown on this page.
You can add subtle warmth to portraits and enhance the colour of blue eyes with small adjustments in the Blue colour channel.
Raising the midtone control points in the Green colour channel made the greens in this shot richer and closer to their original values.
Small adjustments to the Curves in the Blue and Red colour channels corrected the purplish caste in the original image (top) and restored a natural colour balance (below).
Article by Margaret Brown – see Margaret’s photography pocket guides