Histograms are a much misunderstood species, but can be an incredibly useful tool for any photographer and Photoshop artist. In my previous tutorial about clearing a colour cast I have already shown one example of how you can use these graphs; there are countless other ways they make themselves invaluable. And it’s not just in Photoshop that histograms are a useful guide. They can also be found in the display options of all digital SLR cameras, and most point-and-shoot cameras, and can be a useful aid in achieving correctly exposed photos.
I use them constantly while I’m working in Photoshop, so before I write any more tutorials that use these I’d like to give a quick explanation as to what it is you’re looking at, and why they’re so helpful.