Using the Curves tool in your image editor to adjust image tones and contrast.
Whereas the Levels adjustment tool in image editors is a global adjustment that acts across the entire image, when you need to tweak tonality in one sector of an image, the Curves tool is the one to use. It lets you adjust points within an image’s tonal range. To apply a Curves adjustment, do one of the following:
Click on Curves in the Image > Adjustments panel.
Choose Layer > New Adjustment Layer > Curves. Click OK in the New Layer dialog box.
The adjustment process is based upon a graph in which the horizontal axis represents the input levels (original image values) while the vertical axis represents the output levels (new adjusted values). The ‘shape’ of the tones within the image is represented as a shadowed area on this graph.
The image we have chosen has a restricted tonal gamut, which can be extended by drawing out the ends of the tonal graph on the Input axis. This sets the black and white points (pure black and pure white values) in the image. The Input value changes as you drag.
Selecting the Curves control in Photoshop.
Tonal Adjustments with Curves
Clicking on the diagonal line allows you to select the tonal area you wish to adjust and also anchors the tones at the point selected. It’s a good idea to start by anchoring at least three points on the diagonal to cover shadows, mid-tones and highlights. You can add up to 14 control points to the curve.
Points on the curve remain anchored until you move them, allowing you to adjust one tonal area without affecting other areas. To remove a control point, simply drag it off the graph.
Moving a control point in the right side of the curve will adjust the highlights. Moving a point in the centre of the curve adjusts the mid-tones and moving a point in the left section of the curve adjusts the shadows. Dragging a control point up or down lightens or darkens the tonal area you’re adjusting.
Adjustments can also be made by clicking on a point on the curve and entering numerical values in the Input and Output text boxes. Alternatively, you can select the pencil icon and draw a new curve over the existing one. To even out irregularities, click the Smooth the Curve Values icon to smooth the curve you drew.
Dragging a control point left or right increases or decreases the contrast in the selected tonal area. The image we’ve chosen is rather flat so we will boost the contrast in the middle tones by dragging the mid-tone point (circled) to the left.
Adding points as you go allows you to make subtle adjustments to brightness and contrast and turn a flat, uninteresting image into one with the tonal range you wish to present to viewers.
The Curves dialog box.
A. Curves presets dropdown menu
B. Channel selector. (The Curves adjustment can also be applied to CMYK, LAB, or Greyscale images. For CMYK images, the graph displays percentages of ink/pigment. For LAB and Greyscale images, the graph displays light values.)
C. Edit points to modify the curve.
D. The output levels axis.
E. The black point.
F. The input levels axis.
G. Sample in image to set black point.
H. Sample in image to set grey point.
I. Sample in image to set white point.
J. Set grey point.
K. The white point.
An unedited image with the Curves dialog box superimposed.
Moving the arrows on the input levels axis (circled in red) in to meet the ends of the image’s tonal graph increases the overall contrast in the image.
To lighten highlights, move a point near the top of the curve upwards. (Moving it down will darken them.)
Dragging the mid-tone point (circled) to the left increases contrast in the middle tones without affecting the highlights or shadows.
The inset Curves graph shows how control points have been added to enable the contrast in the shadow areas to be increased slightly, while the contrast in the lighter mid tones is reduced to emphasise the misty atmosphere in the scene.
This is an excerpt from Photo Review Issue 56.