DIAGRAM 1-7. Light travels in a straight line. When it strikes a surface, the angle of incidence equals the angle of reflectance. This concept is key to lighting your images successfully.

Direction. The placement of the main light source relative to the subject is another controlling yet subjective factor in the quality of the light in your final portrait. Light travels in a straight line, and when it strikes a surface, its angle of incidence is equal to its angle of reflectance. For example, with front lighting, the light is directly at or behind the camera, producing a zero-degree angle of incidence and thus a zerodegree angle of reflectance. The result is flat lighting—an image that has no distinct transitions from shadow to highlights. For example, if the light source is moved 45 degrees to the left or right, off the lens axis, it will reflect back at the same 45-degree angle. This concept is important when positioning your light and modifiers to show shape and create depth on your subject.

THE INVERSE SQUARE LAW

Understanding the Inverse Square Law is very important to becoming skilled in the art of photographic lighting. This law describes how a change in the distance of the light source to the subject will change the intensity of the light on the subject. The Inverse Square Law states that when the distance of the light to the subject doubles (2x the distance), the light on the subject will be reduced to 1/4 of its original intensity—because the inverse square of 2 is 1/4 (1/22). This means that when you change the distance of the light to the subject, the exposure (or the output of the light) must be adjusted to compensate.

DIAGRAM 1-8. The Inverse Square Law. If you move your subject from 3 feet to 6 feet from the main light, you’ll need four times the amount of light to arrive at the same exposure—two full stops more light. The beam of light widens as it travels farther from the source; because it illuminates a broader area, the light becomes less intense.