I have used many different ways to adjust the lighting in photoshop, starting with photoshop 5 and now using cs 2 (9). The easiest way to learn about the techniques is to just play with them.
I am going to start with the most basic one, brightness and contrast. Every version of photoshop has this. But, before I get into that, the concept of a grayscale needs to be understood.
I'm only going to work with a black and white image this time, or only focus on what can be changed in it. Each of these steps will work with a color image, the next post will be about color correction.
So, a grayscale, as far as a computer sees it, is a chart made up of 256 different shades of gray (starting with pure black and ending with pure white). When an image is brightened, what happens is that the amount of gray is lightened over all. On a chart, it would look like the mid point (50% gray) is shifting to the left, with the left side (50%-pure black) being compacted and the other side (50% - pure white) being expanded.
Contrast does the same thing, but it actually decreases the number of shades of gray in an image. This creates additional contrast in the image, at the sake of shades of gray.
This does solve problems of underexposed images, but only slightly underexposed. The problem is that the brighter an image gets, the more grain it produces. So brightness and contrast should be used sparingly.
Next up is a bit of a more technical way to brighten an image, called levels. Levels is a visual representation of the grayscale chart. The only difference is that you can control the black, white and gray points. This means that you have control over how much each shade of an image is brightened. I'm going to be honest on this one, I just move sliders until I get the look I like.
The next 2 are much more complicated in what they actually do, but very easy to use.
The first is called shadow highlight. This will bring out shadow detail and allow highlight detail to be added. Once again, playing with sliders is the easiest way to learn this.
Exposure does exactly what it says, this allows the image to be changed as though the exposure in the camera is being changed. Once again, slide until you like what you see.
There are 2 things I want you to get out of this. 1. Playing with the tools is the easiest way to learn what they do and what the results look like.
The second thing is that the better the image you have starting off, the better the results will be.
next time, color correction.