This command is at Image ---> Adjustments ---> Curves
This commands brings up a dialog box with no curves at all - just a straight line! It helped me to use this straight line to understand how curves work.
The straight line is a representation of the relationship between input and output. It is a simple x=y graph where input=output. In other words, the dark pixels make just as much of an impression as the light pixels. Not more not less. One way to think of this is to hearing a song sung by two people - one with a loud voice and the other with a quiet voice. The loud voice will likely drown the quiet voice - unless you can tone down the loud voice and (maybe) turn up the quiet voice so that you have a balance where you can hear both voices about equally.
This is what Curves lets you do - convert the straight line relationship into a curve where the shape of the curve dictates how much you want the shadows to brighten up and how much you want the highlights to darken so that your image has an overall balanced look.
What happens is that you have changed the y=x to a gradual scale of
With digital cameras (circa 2004), the problem most often noted is that they blow out highlights. In other words the brighter parts of a photo go straight to pure white, i.e. no information at all. However, these cameras manage to hold detail in darker shadowy areas rather well. But since most cameras do the x=y, the darker areas all look black. One way to get around this limited dynamic range of digital camera sensors is to expose for the highlights confident in the knowledge that you will keep the highlight information without blowing it out and can later use Curves to dig out the darker portions of the image.
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