Start in Darkness
To be in control of your scene's lighting, start in total darkness so that you can deliberately add all the light that will appear in the scene. When you shoot a film on location, starting in total darkness isn't always possible because there is usually some natural light in an environment. Fortunately, however, computer graphics are like a well-built sound stage, in that it is easy to start in total darkness for complete control. All you need to do is turn off any other light in your scene, such as default or ambient light, which could rob you of control over the lighting of each individual area of the image.
The Ambient Light Problem
The parameter called ambience (also called global ambience or ambient light) in most programs is an unrealistic effect that is not similar to its real-life namesake, and can impair your control over lighting the scene.
In real life, ambient light is the widely distributed, "indirect" light that has bounced off (or been transmitted through) objects in your scene. Ambient light illuminates even the areas not directly lit by another light source. Shadowed areas of a real room are sometimes made visible only by the ambient light. Real-life ambient light is tinted as it bounces around the environment and adds different colors to different sides of objects, based on colors it has picked up from the environment. Real ambient light varies in intensity in different parts of the environment and adds different tones to objects from different angles.
In 3D graphics, the "ambience" in most programs just means an amount of flat, uniform brightness added to objects in the scene, making objects visible even where no light source illuminates them. This is usually done without any calculation of an appropriate tone or direction of indirect illumination, and produces unrealistic shading that is uniformly applied. Global ambience adds the same color and intensity to all sides of an object, without regard for its position. You can see the unshaded ambience in the shadow area in the left frame of Figure 1. Focus on the lower-right area of the ball. The dents are not even visible in the area lit only with ambient light because the ambient light does not provide any shading based on the angle of the surface.
Figure 1 Global ambience (left) is an unrealistic effect that robs the scene of shading and depth.
In general, global ambient light will rob your scene of richness and variety, especially in areas not illuminated by other light sources. To get the most local control over your lighting, and the best quality of shading, turn off any global ambience in your scene. If you can choose a color for your global ambience, pure black is a very good choice. This way, there will be no light added to the scene other than the lights that you deliberately position and control.
After you have turned off global ambience, you can light a 3D scene to take advantage of the full range of tones that would be available when shooting a scene on film. Images shot on film can use a range of tones from pure black to pure white, limited only by the latitude of tones available on the film stock. Making this level of contrast possible, some portions of a filmed image can fall off into blackness where there is too little light to be visible relative to a scene's exposure. Global ambience in your 3D scene would take away the option to leave an area completely unlit, because the global ambience would be added to all parts of the scene.
Alternatives to Global Ambience
Some people seem to be addicted to the bad habit of using a uniform global ambience because they are worried that areas of their scene will fall off into a stark blackness without it. They think that using a small amount of global ambience is a harmless cheat. In reality, other techniques are better for adding secondary illumination to quality-oriented renderings.
Many artists add their own fill lights to a scene to provide secondary lighting that is more controllable than global ambience. A fill light can be any kind of light source (such as a spotlight or directional light) that is dimmer than your main light source and is used to brighten an otherwise unlit area. The right side of Figure 1 is lit with a set of fill lights that add shading and color variation to the ball, producing richer, more realistic shading than global ambience provides.
Definitions of ambient light vary among programs. Software that includes radiosity or other global illumination models may use a more accurate calculation for the addition of ambient light, instead of adding a uniform global value. If this alternative is available, and does not require an unreasonable amount of time to compute, it can add an enormous improvement to the secondary lighting in your scenes.