Indoor lighting differs from outdoor lighting in several ways. To use it well, you need to know how to work with these differences and use them to your advantage.
The most obvious differences between outdoor and indoor lighting are intensity and color. Without a flash, indoor light is usually five to nine stops darker than sunlight. This means that if you were using ISO 100 film outdoors, you'd have to use ISO 6400 film indoors with the same shutter speed and f-stops. More likely, you'll use ISO 400 or 800 film for shooting indoors without a flash, which means larger apertures (f/2.0) or slow shutter speeds (1/15).
Ambient light is the existing light in a room. It can be directional or not, depending on the source, and is created by lamps, ceiling lights, window light, light shining in from windows, and so on. Rooms are often illuminated by several lights. Each casts its own pool of light, which means that each can serve as a main light if the subject is in that pool of light. The other lights then become fill lights or back lights for your subject.
You can choose how the various lights in the room will affect your picture through how you meter the scene. If you want to use one primary lighting source and your camera is likely to include a source in the background, then make a closeup reading of the subject with your camera and lock in the exposure. You can also take an incident light reading with your meter at the subject's position and set your camera manually. Make sure the shutter speed is fast enough for handheld work, or use a tripod or other object to steady the camera during the longer exposure.
Classrooms and offices usually have overhead lighting, which is very even and often dull. Because of the lower contrast of this type of lighting, you're less likely to be aware of the shadows that it casts under eyebrows, and you may not see the generally muddy look it creates until you get your prints back from the lab. Shooting in these circumstances with a digital camera allows you to be aware of bad lighting and compensate for it.
However, if you're photographing a large scene, such as an entire classroom or a large office space, you'll appreciate the evenness of overhead lighting. There will be no contrasting pools of light such as those you see in your living room.
Shooting with existing light both indoors and outdoors is called existing-light photography. You can also add artificial light, either with flash units or from other sources. For more on using flash for indoor photography, turn to Chapter 11. Lighting for formal indoor portrait situations is discussed in Chapter 17.
It's not totally accurate to characterize window light as indoor lighting; it's really more like a specialized version of open shade. Window light is never direct sunlight streaming in the window. Your light may come from the outdoor scenery or sky. If there is sunlight coming in the window, you will cover the window with a translucent shade or curtain.
You can use color film and get accurate color with window light as long as no incandescent lights are overpowering the window light. Generally, digital shots will accurately portray the color, particularly when shooting with a digital SLR.
You can control the quality of light by the distance of the subject from the window. The closer to the window, the larger the light source; thus, the line between shadow and highlight is softer. As the subject moves away from the window, the contrast usually drops because the bright window light no longer overpowers the existing light bouncing around the room. But the main light becomes more directional because it's a smaller source of light.