This technique will show you how to fix an underexposed photo. I have fixed underexposed pictures that was underexposed by as much as two stops using this technique. Many people are unaware of how easy it is to correct underexposed pictures with photoshop.

Step One :
Drag your layer onto the create a new layer icon in the layers palette. Change the blend mode from normal to screen.

Duplicate layer and select Screen

Step Two :
In slightly underexposed pictures you will find that step one might have ligten the picture up too much, or in severely underexposed images that the image is still not light enough.

If the image is too light, darken it by lowering the opacity of the new layer you created.

If you find that you need to bump up the exposure a little more just duplicate the original layer and change the blend mode to screen until correct exposure is achieved.

Step Three :
Use the opacity setting on each level to achieve optimal exposure. Flatten your layers and sharpen.

Underexposed photo

Same photo after correcting exposure in Photoshop