Source: http://lifehacker.com/5889078/fix-your-photos-exposure-problems-in-seconds-with-this-simple-trick
Despite our best efforts to take great photos, we can't always control the light in our images. When you end up taking a picture with an uneven exposure, this little trick can fix the problem in just a few seconds.
You can perform this trick with just about any image editor you like (so long as it supports layers, is able to invert the image, and offers the Overlay blending mode). To see how this works, watch the video above. Alternatively, just follow these steps:
- Open up the photo and duplicate the layer containing it.
- Invert the duplicated layer. (The shortcut is Control/Command+I in Photoshop and is likely the same in other image editors.)
- Set the duplicated layer's blending mode to Overlay.
- Optional: Reduce the opacity of the duplicated layer if the effects are too much.
- Optional: Merge the two layers and adjust the levels or curves to suit your taste. (If you're in Photoshop, you can just make a new adjustment layer to do this.)
Problem solved! Pretty easy, right?