In the studio, you usually use more than one light to illuminate a scene. The goal is often to create light that looks like that found outdoors. The lights can be hot lights, strobes, or slave flash units -or even fill cards. Two or three lights are often used.

The use of main, fill, background and rim lights is a classic studio lighting setup for portraits, but its principles can be adapted to desktop photography. Once you know how to do it well, you can explore variations off this basic theme.

• The main light is positioned somewhat to one side of the subject and somewhat above it.
• A fill light is placed opposite the main light, but more nearly at the subject's level.
• A background light is used to control the lighting on the background behind the main subject.
• A rim light is placed quite high and behind the subject to highlight edges and separate the subject from the background.

  Although this lighting setup gives you almost total control, for most purposes you can get by with just the main light and a fill light. In fact, you can often get along with just the main light by replacing the fill light with fill cards to bounce light into the shadows. The way you position a light relative to the subject is very important.

• As you move a light farther away from the subject you reduce the light falling on it. Because there is less light you may have to use a larger aperture which gives less depth of field. Light falls off with distance because the beam of light expands as it moves father from the camera. The same amount of light covers a much larger area. As a result, subjects nearer the light source will be illuminated with a more intense light than subjects farther away. The rate at which the light falls off is described by the inverse square law. If the distance between the light source and subject is doubled, the light spreads out to cover an area four times larger. As a result only one quarter of the previous amount of light will illuminate the subject. Conversely, when the distance is halved, four times as much light falls on a given area.
• Moving a light back hardens its light, while moving it closer softens it.
• Positioning the light at an angle to the subject will make the light uneven over the subject with the part closest to the light getting more light. The exposure will only be correct for those at one distance— normally those in the middle of the area metered by the autoexposure system. Parts of the setup located farther from the light source will be increasingly darker the farther away they are.

Given that light on a subject can be increased or decreased just by moving the light source closer or farther away gives you a way to control the lighting on your setup. This is especially true when you use two lights of the same intensity. By moving a light farther away, you can reduce the light it illuminates the subject with. On strobes, you do it by adjusting the light's intensity. On continuous lights you can do the same with a dimmer switch. As you'll see in the section on positioning the fill light, you can have one light illuminate the subject with more intensity than another light. The difference between the two lights is called the lighting ratio.