Ambient Light refers to any light in a given scene that isn’t artificial light, light supplied by the photographer. Sunlight, candlelight or light emanating from surrounding lamps can produce ambient light in a photograph.
In general, photographers use ambient light on or around an object to produce a certain mood or feeling for the resulting image. This subtle photographic technique is known in cinematic circles as low-key lighting.
When taking a photograph of lights, for instance at Christmas time, experts suggest making the most of ambient light. Flash photography indoors can be harsh, overpowering the serenity and softness of a image. Consequently, rather than use flash, adding candles or incandescent lights can enhance the ambient lighting of a scene without overpowering it.
Another common manner in which ambient light creates a mood lies in the deliberate ways certain scenes are lit. For example, if a photographer wants to engender a creepy or haunting effect, he tends to use ambient light that comes from beneath the image, making it seem larger and, therefore more menacing.
However, if there is not a clear distinction between an image and its background, ambient light shouldn’t be the sole source of light for the photograph. In these cases, ambient light should be enhanced either through flash photography or other sources of light the photographer supplies.