Natural light is probably the best and most appealing for many types of portraiture. The easiest starting point when indoors is to work reasonably close to a window where the light is indirect, diffused and directional. Light levels decrease significantly as distance from the window increases, so move the subject around the room and observe how highlights, shadows and tonal gradation change. Contrast is likely to be a problem very close to a window, particularly if working in colour without any secondary light source. A second window on the far side of the room might provide enough light to fill shadows, otherwise use a reflector placed close to the model.
Contrast can be reduced by excluding most of the window areas from the frame or by using flash, perhaps bounced off a reflector or white ceiling. Tungsten photofloods can also be used but their lower colour temperature produces a warm yellow cast on daylight film. This can be rectified by using an appropriate blue colour correction filter such as an 80B, or by changing to Type A film that is balanced for photofloods. With a digital camera, adjustment of the white balance should remove the problem. However, when natural and artificial light sources are mixed the situation gets more complicated. The simplest solution is then to ensure that one type of light dominates, and use the appropriate film, filters or white balance. Imbalance from other sources usually remains tolerable.
The alternative approach to photographing people indoors in low light is of course to rely entirely on artificial lighting. This may be flash, photoflood or standard room lighting - usually tungsten. Direct flash produces harsh lighting and is likely to lead to bland images with hard shadows on the face and behind the subject. Bounced flash produces a much softer result so look for a near-white wall or ceiling to use as a reflector. Photofloods are practical only when the photographer has total control of the situation, and are certainly of no use at social gatherings. In many cases lighting is not controllable and the photographer must make the best of what is available. Fast films, or high ISO settings, are always an asset under such difficult conditions. Use speeds in the range ISO 400 to ISO 1,600 and accept the consequent grain or noise. It should then be possible to use a telephoto lens to capture close-ups of faces. The alternative is a camera-mounted flash which is convenient and portable, but may produce red-eye if used incorrectly.