Out here in the real world, everybody makes mistakes and sometimes when shooting—just like you—I just loose track of my camera settings or don’t keep up with changing lighting conditions and that results, more often than not, in an underexposed image. Here’s how I fix that problem using Photoshop.
One technique many people use in this situation the Levels (Image >Adjustments> Levels) command: They slide the right hand triangle to the left to increase exposure but that approach also increases contrast at the same time. Another commonly used method is to use Curves (Image >Adjustments> Curves) to increase overall exposure but I prefer to use Photoshop’s Layers command.
Create a duplicate layer using the Layer > Duplicate Layer command. You can name it or not, then select “Screen” from the Layer’s palette’s Blending Mode pop-up menu. If you’re lucky the underexposure will be instantly corrected but you may have to modulate the effect by using the Layer’s palette’s Opacity slider to change the overall exposure of that duplicate layer. I used 90% for the corrected image at left. When it looks the way you want use Flatten (Layers > Flatten) create a single layer file.
The final image has its underexposure corrected and is lightly retouched. As with any underexposed image you can expect to find some level of digital noise—depending on how badly underexposed the image file is—and that’s when I use Dfine to minimize the noise.