When it comes to realism, lighting in a still image or animated scene can make or break a persons viewing experience. When comparing a computer generated imagine to a photograph the first thing that comes to mind is whether or not it looks as real as the picture. The more real it looks, the more satisfaction is had by the viewer.
Lighting is probably the last thing thought about when it comes to computer graphics, whether you're the animator or the viewer, even though its the key factor in realism. Its a tricky and time consuming process in the 3D world but when the right results are achieved, it's very gratifying.
When setting up a well lit scene multiple factors need to be considered. The time of day and the mood the scene is going to convey are just a couple. If it's during daylight hours the color of the lights should be warmer such as yellow and orange. If its a night time scene then colors should have a more cool blue tone to them.
Normally two or three lights per scene also helps achieve realism. Since real world lighting for the most part always has more than one light source its always best to do the same in the virtual world. Additional things that should be considered are the rays of light, whether its from the sun, and indoor light fixture or spotlight. The type of light is very important.
To achieve a type of mood shadows usually play the most important role. The darker the scene and the more prominent the shadows are, the more negative the mood is. The brighter the scene and the less shadows there are, the happier or more positive the mood is. Another thing to keep in mind when considering shadows is its edge. The softer the edge the more realistic it is. Harder edges are usually used for more non-realistic imagery.
These are just a few of many of the important things to be thought about when it comes to lighting a virtual scene. Any virtual object and or scene can appear real if it's lit properly. Like anything the more time and thought put into it, the better the results. In the end, the greatest compliment a 3D artist can receive, is the viewer not realizing that imagery is computer generated.