Debi wrote Photoshop 911 with this problem of under exposure in her photographs:
> Hi, I have a picture of my wedding reception.> Becuase of the lighting around the room, the> Can you tell me how to lighten it, without changing us?> background of all the pictures look dark.> I am a little confused on the layers.> Then a duplicate layer where I did some adjustments> What I have so far is the main layer that I did not> do any changes to. > to our skin (named skin).> Thanks so much> Please tell me where I go from here to follow your steps.
Use an adjustment layer filled with black to mask the "good" parts of the image, while painting in white to reveal the dark, shadow areas of the image for adjustment.
Adjustment Layers for Shadow Balancing
Although there are a dozen ways to lighten or darken small portions of an image, the use of adjustment layers and masks has made the technique easy and non-destructive.
Our group was about to embark on a raft trip down the Nile in Uganda, Africa, when we encountered this young African "Jack Fruit" girl hoping to sell to our group. She was moving in and around the group quickly, so I had to get the shot as best as possible.
Move the right slider toward center, and the center slider toward the left. The beauty of the Adjustment Layer is it can be infinitely tuned until just the right look is achieved.
Notice however that during the process, we've made the scene too light -- -- taking away the richness of foliage along the banks of the Nile. That will be easy to fix by modifying the adjustment layer's mask.
Paint White to Reveal: Now, use an appropriate brush size and opacity and begin painting white into the mask to reveal just those areas that need lightening.
We have pre-tuned the Levels in this adjustment layer to provide the desired lightness of the girl. The BLACK completely hides those adjustments. As we paint white into the mask, those adjustments once again come into play.
Notice, I'm painting along the platter, down the arm, and within the face. (See the white shapes in the black layer mask at the far right above.) I'll also just lightly touch areas of the little boy's features to lighten them as well.
Before finishing, I want to add a slight bit of "richness" to the overall scene with a Contrast/Brightness Adjustment Layer. Now the image is ready for flatening and further use.
You might also find this Photoshop Tutorial on Exposure Balancing of some help in future layers work. The pros might take a somewhat different approach, using curves or further modifications. You should experiment with the above techniques to see if that's the cure -- or if you want to investigate other options.