Within Adobe Photoshop, Curves provide a superb method for controlling colour and contrast in a similar, but complimentary to Levels. The difference being, with Curves we can tweak and alter any point along the tonal range. This gives us the opportunity to precisely target colours or tones for exact adjustments.
Curves can be applied to to selections, layers and also as an Adjustment Layer.
Curves can be opened via (menu)->Image->Adjustments->Curves (CTRL+M). Curves are not a replacement for Levels and are better used as an after-tweak.
On later versions of Adobe Photoshop, the Curves dialogue box displays a visual representation of the tonal range in the form of a Histogram. Whether your version has this feature or not, the same principle is true. The horizontal tonal range bar equates to the tonal information from black (left) to white (right).
As with Levels, you can work on curves as a composite (All colours: RGB, CMYK, etc) or individual Channels.
Within your Curves graph, try the following:
- Click and drag down the top right marker: this will darken the lightest pixels in the image.
- Click and drag up the bottom left marker: this will lighten the darkest pixels in the image.
- Click and drag the bottom left marker to the right: this will increase the contrast in the darker areas of the image.
- Click and drag the bottom right marker to the left: this will increase the contrast in the lighter areas of the image.
The above exercises may prove useful, but must be used with caution. Such methods may result in loss of pixel information in the extremities of the tonal range.
- Establish anchor points along the length of the diagonal line that runs from top-left to top-right.
- Any number of anchor points can be created along this line by clicking and dragging to position.
- Click and drag anchor points away from the graph to remove.
Remember, these techniques can be applied to individual channels.
A typical scenario for correct usage is the alteration of shadow and highlight temperature. By anchoring on the red Channel, you can increase the curse at the lower end of the tonal range, whilst decrease the reds at the lighter end.
To establish just where in the tonal range any colour/tone resides, simple click over that region with the eyedropper tool. You will see a circle momentary appear along your curve. You can fix anchor points by doing this if you hold down the SHIFT key whilst clicking on a region.
Some later versions of Photoshop offer Custom curves – predefined settings. You can also, in many instances use the Auto function which will automatically adjust your image to the best settings. Auto doesn’t always work as it purely depends on the nature of your image. It is good practice to know how to manually fix your image’s curves for those times where Auto gets it wrong.