How do you increase or decrease the contrast of an image?

Here's the short answer: Create a Curves Adjustment Layer and place two anchor points on the line. One anchor point should be 1/4 of the way up the line, and the other should be 3/4 of the way up the line.

To increase contrast, click and drag the 1/4 anchor point (shadows) down and a little to the right. Click and drag the 3/4 anchor point (highlights) up and a little to the left. This creates a curve in the line that resembles a shallow "S".

To decrease contrast, click and drag the 1/4 anchor point (shadows) up and a little to the left. Click and drag the 3/4 anchor point (highlights) down and a little to the right. This creates a curve in the line that resembles a shallow, inverted "S".

Here's the long answer: When working with contrast, Curves offer a great opportunity to increase or decrease contrast without affecting the white point or black point. Contrast itself is the range of brightness between the highlights and shadows of an image. The larger the range, the greater the contrast. An image with extreme contrast has mostly highlights and shadows with very few (if any) midtones.

Our goal in adding contrast to an image is to lighten the highlights and darken the shadows of the image. If we want to decrease contrast instead, we'll darken the highlights and lighten the shadows of the image. It's important in this procedure to leave the white point and black point alone.

Because we can affect highlights and shadows separately in Curves, we use Curves to adjust the contrast of the image. Here are some things to remember when working with Curves Adjustment Layers:

• A little goes a long way. You don't have to drag an anchor point very far to see a substantial change.
• If you move an anchor point so that the line becomes flush with the top or bottom of the box, you've essentially moved the white or black points, respectively. In doing so, you run the risk of cutting off valuable highlight or shadow information in your image, causing highlights to turn pure white and shadows to turn pure black. Not good!