Contrast masking is a useful image editing technique for adjusting the brightness and contrast in an image. It brings out more detail in the underexposed areas of a photo.
When to use contrast masking
Use contrast masking when the main subject in a photo is dark. Dark areas that show little or no detail are often a consequence of backlighting. Contrast masking inverts the dark and light in a photo to reveal more detail in underexposed areas.
How to apply a contrast mask in Photoshop
1. Open photo in an editing program that supports layers.
2. Duplicate the background layer.
3. Desaturate the new layer.
Image menu > Adjustments > Desaturate
4. Invert the layer.
Image menu > Adjustments > Invert
5. Apply a Gaussian Blur.
Filter menu > Blur > Gaussian Blur (set at a radius between 20 and 30 pixels).
Adjustments may need to be made to the amount of Gaussian blur used to avoid a halo effect where bright and dark areas meet.
6. Select Overlay from the layers palette.
Once the contrast mask is applied, its intensity can be adjusted by changing the opacity of the layer. You can also fine tune by erasing portions of the mask. Do any additional editing, such as sharpening, before saving the image.
Note: Newer versions of Photoshop CS and Photoshop Elements have a Shadow/Highlight command, which produces results like Contrast Masking. The Shadow/Highlights command is easier to use and allows for greater adjustments.
Related reading: High Dynamic Range photography and software