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Digital Photography Tutorials
Someone mentioned opening the blinds and turning lights on.
Each kind of light source generates light that has a certain color temperature.
Our brain alters our perception of the light we see with our eyes so most light look close to white to us.
The camera cannot do that on it's own. So it has an adjustment called White Balance, though ironiocally White Balance is rarely about white.
Understanding White Balance
Incandescent light, sunlight, and flourescent light each have a different color temperature. If your camera's white balance is set to sunlight, and the scene is lit with incandescent lighting, the photo will have a strong orange/yellow color cast to it. If the light source is flourescent and your camera's white balance is set to sunlight, the photo will have a strong greenish color cast to it.
Your camera has White Balance settings for vaious light types. Unfortunately, your camera can only be adjusted for one light type at a time. If you try to shoot a scene lit by more than one light type, called mixed lighting, you can accommodate only one of the light types with the camera's white balance adjustment.
This is just one of the problems encountered when trying to shoot using available light, compared to using flash. By the way, all light is natural light.
To shoot indoors without flash you will still need to learn how to do photographic lighting. As you learn about photographic lighting you will discover that using only available light is pretty limiting and not all that easy since to have a measure of control over the available light sources you will need some accessories like reflectors of various sizes, types, and colors, as well as diffusers.
Introduction to Portrait Lighting