The Behavior of Light

Today's post comes from the book Existing Light: Techniques for Wedding and Portrait Photographers by Bill Hurter. It is available from and other fine retailers.

Unless it is traveling though a vacuum, the medium alters how light behaves. Four different things can happen to light waves when they hit a non-vacuum medium: they can be reflected or scattered; they can be absorbed (which usually results in the creation of heat but not light); they can be refracted (bent and passed through the material); or they can be transmitted with no effect. More than one of these results can happen simultaneously with the same medium, but what will happen is predictable. This is the key to understanding how lighting works in a photographic environment.

The natural shade of late afternoon is very soft, but also lacks sparkle. To give the light a little extra snap, photographer J.B. Sallee fired an on-camera flash that was set to output at two stops less than the daylight.

Reflection. One of the characteristics of light that is important to photography has to do with reflected light waves.When light hits a reflective surface at an angle (imagine sunlight hitting a mirror), the reflected wave will always come off the flat, reflective surface at the opposite angle at which the incoming wave of light struck it. In simple terms, the law can be restated as this: the angle of incidence is equal to the angle of reflection. Whether you are trying to eliminate the white glare of wet streets as seen through the viewfinder or to minimize a hot spot on the forehead of your bride, this simple rule will keep you pointed toward the source of the problem.

This rule also has applications in product and commercial photography. For example, when lighting a highly reflective object like silverware, knowing that the angle of incidence equals the angle of reflection tells you that direct illumination will not be the best solution. Instead, you should try to light the surface that will be reflected back onto the shiny object’s surface.

Scattering. Scattering is reflection, but off a rough surface. Basically, because the surface is uneven, incoming light waves get reflected at many different angles. When a photographer uses a reflector, it is essentially to distort the light in this way, reflecting it unevenly (or, put another way, so that it diffuses the light).

Translucent surfaces, such as the rip-stop nylon used in photographic umbrellas and softboxes, transmit some of the light and scatter some of it. This is why these diffusion-lighting devices are always less intense than raw, undiffused light. Some of the energy of the light waves is being discarded by scattering, and the waves that are transmitted strike the subject at many different angles, which is the reason the light is seen as diffused.

Refraction. When light waves move from one medium to another, they may change both speed and direction. Moving from air to glass (to a denser medium), for example, causes light to slow down. Light waves that strike the glass at an angle will also change direction, otherwise known as refraction. Knowing the degree to which certain glass elements will bend light (known as the refractive index) allows optical engineers to design high-quality lenses, capable of focusing a high-resolution image onto a flat plane (the film or image sensor). In such complicated formulas, now almost exclusively designed by computers, the air surfaces between glass elements are just as important to the optical formula of the lens as the glass surfaces and their shapes.

In lighting devices, refraction is used with spotlights and spots with Fresnel lenses. These lenses, placed close to the light source, gather and focus the light into a condensed beam that is more intense and useful over a greater distance than an unfocused light of the same intensity. Spotlights are theatrical in nature, allowing the players on stage to be lit from above or the side by intense but distant lights, but they also have many applications in contemporary

Absorption. When light is neither reflected nor transmitted through a medium, it is absorbed. Absorption usually results in the production of heat but not light. Black flock or velvet backgrounds are often used to create dense black backgrounds because they absorb all of the light striking their surfaces.

A dog’s coat, especially a dog like this with a lot of wrinkles, absorbs most of the light that strikes it. The only area of the dog’s face that efficiently reflects light is his eyes. The general rule of thumb with light-absorbing subjects is to give more exposure to the image if you want to record adequate detail in those areas. Photograph by Kersti Malvre.

Learning to see light takes patience. Read the main shadows to determine the direction and quality of the light. Read the catchlights (specular highlights) in the eyes and you can see the position of the light(s) relative to the person. Here Craig Kienast photographed his subject very close to a window to exploit the soft, directional light. The light falls off rapidly once it enters the large room. Note that light entering a portal, like a window or a doorway, follows the Inverse Square Law as if it were an artificial light source.

The modern-day photographer can record a magnificent portrait in next to no light—and, in this case, in light with wildly varying color temperatures. Marcus Bell recorded the vicar “waiting for the bride” in the darkened vestibule of the church. The bride is invisible except for the little bit of her veil at which the vicar stares.

The Intensity of Light
Another characteristic of light has to do with intensity. Illumination from a light source declines considerably over distance, which is to say that the light grows weaker as the distance increases between the light source and the subject. Light from sources other than the sun (see sidebar) falls off predictably in its intensity.

Marcus Bell likes fast lenses that he can shoot at maximum aperture and high ISOs to produce a miniscule band of sharpness. Here, only the eyes of 2006 Australian of the Year, Ian Frazier, are sharp. And not only are they sharp, but they contain a dash of cobalt blue in the midst of this monochrome portrait; another of the great advantages of digital is the ability to mix palettes within a single image.

Put precisely, the Inverse Square Law states that the reduction or increase in illumination on a subject is inversely proportionate to the square of the change in distance from the point source of light to the subject. For example, if you double the distance from the light source to the subject, then the illumination is reduced to one quarter of its original intensity. Conversely, if you halve the distance, the light intensity doubles. This law holds true because, at a greater distance, the same amount of energy is spread over a larger area. Thus any one area will receive less light.

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