As you can see, the flash gave me a proper exposure on the left side of the image, but the right side is too dark. Since this uneven lighting is linear and gradual, this is the perfect time to use a Levels Adjustment Layer with Gradient Mask. We could darken the left side of the image, but for this illustration, let's lighten the right side!

So, start by downloading my image "problem" and work along with me. Click the next image when you are ready.