Photoshop Tutorial: how to use Curves more creatively

The exposure and dynamic range optimisation systems in modern digital cameras are sophisticated, but they aren’t foolproof or psychic, and most images benefit from some post-capture adjustment to their brightness and contrast.

Photoshop provides a number of ways to make these adjustments, but the Levels and Curves controls are two of the most powerful. Both of these adjustments can be applied as Adjustment Layers, so it’s possible to protect your original image, revisit your editing and try alternative treatments.

Whereas Levels has just three sliding controls, which enable you to set the black, white and mid-grey points, Photoshop’s Curves allows more varied and targeted editing of image brightness. It’s a bit like a graphic equaliser for image brightness. And although it’s incredibly powerful, it’s still extremely easy and intuitive to use.

Just like Levels, the Curves Panel image brightness is displayed as a histogram, and the tones are adjusted by manipulating the shape of the curve above. Pushing the curve up at any point on the line creates a new control point, and brightens the pixels at that level of brightness, while moving it down darkens them.

As well as manipulating brightness, Curves provides an extremely powerful way of manipulating colour, because each of the colour channels (red, green and blue) can be adjusted separately. This enables a huge range of creative effects to be applied to images. Read on to find out how.

Left third: start image. Middle third: contrast boost. Right third: channel tweak.

How to use Curves

You can apply a Curves adjustment directly to an image via Image>Adjustments> Curves (or Cmd/Ctrl+M), but as with any image adjustment, it’s better to edit on an Adjustment Layer via Layer>New Adjustment Layer>Curves. This applies the adjustment in a separate layer that can be re-edited at any time, provided that you preserve the layers when you save the image.

Curves is most commonly used to boost contrast. This is done by manipulating the straight diagonal line into a slight S-shape. To do this, click the straight line about halfway between its mid-point and the top right corner and drag upwards. Then click the line between its mid-point and the bottom left corner and drag down. As soon as the first point is moved, the straight line becomes a curve.

This adjustment brightens the highlights and darkens the shadows, thus increasing global contrast. Moving the curve in the opposite direction to create an inverted S reduces the contrast.

Understand the Curves Histogram

The histogram in the Curves Panel indicates the image’s brightness distribution, and shows the proportion of image pixels of each brightness from 0 to 255, with 0 being black and 255 being white. The larger the peak at any particular point along the histogram, the greater the number of pixels with that corresponding brightness. Histograms are usually shaped like a hump-backed bridge.

Use Curves more creatively

For many images a gentle S-shaped curve will suffice, but sometimes you want to do something a bit more dramatic. The Curves Dialog Box has all the tools you need to recreate traditional effects such as cross-processing or split-toning.

Coloured effects, such as cross-processing, are created by adjusting the three colour channels separately. A common approach is to give the red and green curves an S-shape while pulling the blue curve in the opposite direction to make an inverted S. More dramatic changes in the curve shape increases the colour intensity.

Using a similar technique with an image that has been converted to monochrome using a Black and White Adjustment Layer gives a split-toned effect. Choosing a colour in the Channel menu and moving the curve up increases the amount of that colour, whereas moving it down introduces the opposite colour. Hence, giving the blue channel an inverted S-shape adds yellow to the highlights and blue to the shadows.

Make targeted adjustments with Curves

When you use two points to adjust the curve, the mid-point between them remains in the same position. This means you can use two or three points to anchor part of the curve.

To do this, click the relevant part of the curve and leave each anchor point alone. You can then put down another point (or points) to adjust the part of the curve that needs work.

This is a useful technique for bringing out more detail from the shadows in a high-contrast image. By working on the separate channels, the technique can also be used to make targeted colour adjustments – for example, removing the blue colour cast from shadows.

Curves presets
Photoshop has numerous preset curves adjustments that can be applied to the image using the Preset drop-down box in the Curves Panel. Because the curves can still be edited, these presets can act as a starting point for your own adjustments. You can also save Curves adjustments as a preset for use in the future or on other similar images. To do this, create your curve and select Save Curves preset from the Preset drop-down menu and give it an appropriate name.

This entry was posted on Saturday, February 18th, 2012 at 7:00 am and is filed under Tutorials. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a comment, or trackback from your own site.