Types of light

Main light

Main lights exist, obviously, to light the subject but they also exist to create contrast and character in the portrait. You can create a wide array of effects simply by altering the vertical position of the main light. The higher position of the main light, the more emphasis is placed on the texture and contours of the face. In some portraits, particularly those of the elderly, children, and babies, this is very desirable because it creates a stronger sense of personality and character in the model. Conversely, lowering the main light softens the facial textures and contours and is usually desired in glamour shots and traditional head shots. 

Fill light

The ratio between the overall light and shadow in a portrait depends upon the ratio of fill light to main or key light. The thing to remember about fill lighting, is that it is almost always at the same relative distance and position from the main light, no matter what you are using. The fill light is typically placed on the opposite side of the camera from the main light and set slightly lower than the main light. Fill light should be lower powered than the main light. Fill light is there to add enough light to soften any harsh or unflattering shadows created by the main light.

Backlight

Background lights are used commonly to add depth to or separation from foreground to background. Many fun and interesting techniques can be achieved with backlight, and using coloured gels or devices that add texture can be just the spice a portrait needs to shine. Backlighting makes the portrait dramatic and intense. Background light is usually placed low to the ground about equal distance between the model and background. Most lights used to provide backlight are low power and can be directed somewhat finely.

Mixed light

Mixing the colour temperature of the sources can give a more 'natural' look. Despite the fact the human eye corrects all light sources to what appears to be white light, it is visually accepted, and  in most cases to great effect, when we view images created using mixed light sources that there should be a difference between the colour temperature of the various sources of light within the frame.