Among the many tools in the Photoshop arsenal, a levels adjuster is perhaps the most effective for the widest range of not-so-hot photos. The Digital Photography School blog offers a gentle introduction to levels, graphs, sliders, and what they all do.
Your morning editor once worked as an assistant features editor at a regional newspaper, which meant scanning and processing a good batch of band and event photos every day. A surprising number of these photos submitted for publication look like they'd been shot in a friend's basement with a 35mm film camera and the flash set to Magnesium Flare or with some kind of grey film covering the lens. After a photographer spent some spare time teaching the ways of Levels, I could rescue more of the amateur work that came my way and spend less time cajoling entertainers to find just one more shot to send.
DPS' entry from a contributing writer walks through the fixing of just such a worthy but muddy-looking photo and explains what you're doing when you move the white, black and mid-point sliders around. We've previously linked to a more multi-tool guide to processing a digital photo, but this post offers more screenshots and a deeper dive on on particular tool. Levels is a tool available in the free GIMP open-source image editing tool, along with Photoshop and other commercial products.
Rescuing Poorly Exposed Photos with Photoshop Levels [Digital Photography School]