Here are two versions of the same image, one that has been photoshopped (the brighter one) one that hasnt. The reason i have done this is firsty because my cousin tried lightening up an image of mine from flickr and succeded, heres how i got on at doing another one.
Below you can see the Un-photoshopped image of the rock pools... this image has had very minor photoshop work done to it such as levels and brightness/contrast adjustments.
Colour balance. When lightening/darkening i used the Magic wand tool to adjust the brightness seperately.
This post was mainly here to show how easy it is to adjust any under or over exposed images using very minor photoshop adjustments.