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Lighting for Portrait Photography (Part 1): Behavior of Light


By Steve Barnes

Light is the raw material of a photographer. Much as the painter works with paint and the sculptor works with stone, the photographer works with light. This analogy is not precise however, because as the painter and sculptor work with actual material substances, the photographer works with a form of energy. Understanding the behavior of this form of energy that we call light, is foundational to your success as a portrait photographer. A painter may not need to know the chemical and physical properties of each component of her paints, but she must completely understand how to blend the different colors, and how the paints behave as she applies them to the canvas. Just as a painter or sculptor must gain masterful insight into the behavior of the raw materials of their arts, so must the portrait photographer gain a keen understanding of the behavior of light.

The first prerequisite for photography is light being emitted from a source. Just think about it, without light, photography is impossible. Light may be emitted from a natural source, such as the sun, or from an artificial source, such as strobes or constant light sources. In 1931, the strobe was developed for use in still photography by Harold Edgerton, an electrical engineer from MIT. Today, the strobe is by far the most used light source in the portrait studio. Advantages of strobe lighting for portrait studio photography include: reasonably precise control of light intensity and light color temperature, low heat generation compared to a constant light source, and low power consumption for the amount of light output.

The most important property of light to the portrait photographer is the light's intensity or brightness. There are several ways of controlling the intensity of light striking the subject. In the studio, the power supply of modern strobes may be adjusted. The strobes may be positioned farther away from the subject. Outdoors, you may take advantage of cloud cover or the overhang of a tree or building, or even the time of day, to control the intensity of the incident light on the subject. These methods are effective for controlling the average (overall) light intensity of the composition. Many devices have been developed to control the relative intensities of light (specular highlights) of specific areas within a composition. Devices such as scrims, gobos, snoots, grid spots, and barn doors, are commonly used to partially block, direct, or otherwise control the relative light intensities within a composition.

Another property of light of great importance to the portrait photographer is the light's color temperature. Pure white light is the result of an equally balanced mixture of the three primary colors: red, green, and blue. In different lighting conditions (e.g. cloudy versus full sun), the proportions of the color mixture may vary. Normally, the human brain automatically compensates for this, and you do not notice the difference as you leave one lighting condition and enter another. Film can not make this same automatic compensation. Therefore, differences in color temperature must be manually adjusted for by the photographer. Color temperature of various light conditions is commonly stated in degrees Kelvin. There are three standard color temperature rated films commonly used by photographers. "Daylight" film is designed to be exposed by 5500K light, and "indoor" film is designed to be exposed by 3400K light, or 3200K light for professional "indoor" film. For a greater degree of control over the white balance when using film, color correction filters are used. Most if not all digital SLR cameras have a white balance adjustment to electronically compensate for changing color temperatures encountered in various light conditions. In digital photography, when shooting in RAW format, the color temperature can easily be corrected in Photoshop.

A third property of light that is very important to the portrait photographer is contrast. A light source has high contrast if its rays all strike the subject at approximately the same angle. A light source that is diffuse has low contrast, because its rays strike the subject from many different angles. High contrast light sources produce shadows with a hard edge, while low contrast light sources produce shadows with a soft edge. This is because with a high contrast light source, where the rays all approach the subject from approximately the same angle, no light enters the edge of the shadow and the shadow's edge remains distinct. A light source's relative contrast is generally determined by the size of the light source and its distance from the subject. The sun on a clear day is relatively small in our sky, and therefore it is a high contrast light source producing hard edged shadows. On a cloudy day, the light from the same sun is spread out and diffuse. Effectively the entire sky becomes a low contrast light source, producing very soft edged shadows. In the studio, we have many light modifiers available to us, to control the effective size of the light source and thereby control the level of contrast. For any given size of a light source, as it is positioned farther and farther away from the subject we see that it effectively becomes smaller and smaller, yielding higher and higher levels of contrast, albeit lower and lower intensity.

Light acts on any subject it may strike. This much may be obvious. But every subject also acts on any light that strikes it. A subject may act on light in three distinct ways: refraction, absorption, and reflection. Refraction is the bending of light waves as they pass through a transparent material such as glass. In fact, the refractive property of glass is what is manipulated within the photographic lens, to focus an image onto the film (or digital image sensor). Absorption is the process whereby certain materials convert light energy into some other form of energy (usually heat). The absorptive property of a black painted foam core board may be used by the photographer to selectively "subtract" light, so that it does not bounce around the studio in an undesirable way.

Of the three ways a subject may act on the light striking it, reflection is the most important to the photographer. Reflection is an abrupt change in the direction of propagation of light waves that strikes the surface of the subject. In direct reflection, the light rays bounce from a smooth surface at the same angle at which they hit it. The intensity of the direct reflection mirrors the intensity of the light source. Glare, such as observed on the surface of a body of water, is a polarized direct reflection. Unlike direct reflection however, glare reflection always has a lower intensity than the light source producing it. Glare reflection may be controlled or eliminated by using a polarizing filter. Diffuse reflections occur when light from a source is reflected equally in all directions by the surface it strikes. In theory, diffuse reflections are the same intensity no matter what angle they are viewed from. The intensity of a diffuse reflection increases as the light source is moved closer to the subject. The Inverse Square Law says that the intensity of the diffuse reflected light is inversely proportional to the square of the distance between the light source and the subject. This implies, a light source at any given distance from the subject will light the subject with an intensity that is four times greater than the same light source moved to twice the distance from the subject.

An understanding of the behavior of light is a prerequisite to understanding how to control the light. We see that light can act on any subject it strikes. Intensity or brightness, color temperature, and contrast are the three properties that are of most concern to the portrait photographer. Any subject also acts on light that strikes it, either through refraction, reflection, absorption, or some combination of the three. In portrait photography, light is controlled to achieve optimum overall exposure of a composition, to develop of specular highlights, to reveal and enhance textures, forms and color saturation, and to build a three dimensional perspective. In Part 2 of this article, the fundamentals of controlling the overall exposure of a composition using the camera are discussed. Until then, good day and happy clicking.

Steve Barnes is a professional portrait photographer, free lance writer, and co-owner of Hayley Barnes Photography, in League City, Texas. Please visit his website at: Hayley Barnes Photography. Elegant portrait photographer. Children, Families, High School Seniors, and Quinceaeras. "Custom Designed, Uniquely You"

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