How to Fix an Underexposed Digital Photo
If your take a photo that you love but it’s underexposed, you can rescue the image in your image-editing software. Many image editors let you correct underexposed pictures, making them underexposed no more. This is how you do it in Photoshop Elements:
Open your image editor and choose File→Open.
Navigate to your underexposed image and click OK. The image opens in your image editor.
Choose Layer→Duplicate Layer.
A duplicate layer appears, as shown in this figure.
In the Layers palette, change the duplicated layer’s blend mode to Screen.
This blend mode brightens the image considerably.
If the image is too bright, lower the opacity of the uppermost layer.
Lower the opacity until you get the image you want.
If your image has dark shadows but is otherwise properly exposed, choose Enhance→Adjust Lighting→Shadows and Highlights to open the Shadows/Highlights dialog box. From within the dialog box, you can lighten the shadows, and, if desired, increase contrast.
If your image still isn’t bright enough after applying a duplicate layer in Screen blend mode, duplicate this duplicate layer. When you duplicate a layer, the blend mode of the original layer is inherited by the duplicate (so you get double the Screen).