Scene ambience simulates a certain amount of indirect light, coming from no particular source, that is applied to all objects in the scene. It is a simple approximation of the indirect lighting that normally bounces around an environment and contributes to the illumination of objects.
The scene ambience is not controlled by a light, it is a property applied to the entire scene. The effect of ambience on the scene is to brighten or darken all objects to a certain degree. The scene ambience is multiplied with each object's ambient area of illumination to produce the object's final ambient component. For more information about an object's ambient area of illumination, see Surface Illumination [Materials and Shaders].
You can edit the ambient lighting for the whole scene, or you can apply a local ambient lighting property to selected objects and pass partitions in the scene.
To set global ambience: Choose Modify 
Ambience from the Render toolbar.
To set local ambience: Select an object, multiple objects, a group of objects, or a render pass partition, and choose Get Property Ambient Lighting from any toolbar.
| Ambience |
Sets the color of the ambient lighting. If the scene ambience is set to black, nothing can alter the ambient color of an object except, of course, a light. Set to dark gray by default. |
Tips for Setting Scene Ambience
Keep the ambient lighting color value dark or low so as not to affect your scene's lighting in an unpredictable way.
If the scene ambience is set to black, nothing can alter the ambient color of an object except, of course, a light.
For best results, ambience should be set to 0 (or close to it) in scenes that use flat lights or in scenes with photon lighting effects like global illumination and caustics. These effects create indirect lighting, but much more realistically, so ambience is unnecessary. For more information, see Creating Flat Lights and Global Illumination and Caustics.
Setting a Realistic Ambient Color
Perhaps the most realistic way to set a scene's ambient color is to match a color from a rotoscoped image from the scene you use for compositing.
Open the Ambient Lighting property editor as described above.
Open a Rotoscope view in any viewport and load an image or scene to be used for compositing.
From the Ambient Lighting property editor, click the color box to open the color editor.
From the color editor, select the color picker ; the cursor changes to an eye-dropper.
Use the color picker to select the ambient color from the image or scene to be used for compositing. For best results, choose an ambient color you find in the shadows of an image.
Close the color editor to accept the chosen color as the new ambient lighting color.
There are some color range limitations to safely output for TV or film. A true black or a full red isn't supported by NTSC and some other formats.
Also, adjusting the gamma value doesn't alter a black color generated by a non-lit scene/object; that is, you can adjust the shadowed area of a scene without adjusting or adding lights.
