Last update: Part III – Selective Control of Exposure (added on 15 June’07)
Steve Barnes, a professional portrait photographer and co-owner of Hayley Barnes Photography in League City, has written great tips on lighting for portrait photography. He writes about Behavior of Light, Controlling Exposure within the Camera, and Selective Control Exposure;
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I. BEHAVIOR OF LIGHT
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.
II. CONTROLLING EXPOSURES WITHIN THE CAMERA
The term “photography” derives from the Greek “phos”, meaning light, and “graphis”, meaning to write or draw. If photography is defined as, the art and science of fixing images on a sensitive surface through the action of light, we see that at least some understanding of the nature of light and how to control it is fundamental to one’s success as a photographer. In Part 1 of this article, the behavior of light as it concerns the portrait photographer is addressed. Now we shall begin to explore methods used by the portrait photographer to control the behavior of light in rendering artistic interpretations of their subject. There are many aspects involved in manipulating light for the purpose of portrait photography. One fundamental aspect is exposure. The degree of sensitivity to light (photosensitivity) of the surface on which an image is to be fixed, dictates the required length and intensity of exposure.
The aperture of the modern camera lens is designed to control the intensity of light falling on the film (or digital image sensor). In simple terms, the aperture is a hole or opening, through which the light reflected by the subject is admitted to the camera. The intensity of the reflected light being admitted to expose the film (or image sensor) is controlled by the size of this opening. The size of the aperture opening is commonly stated in “f/stops”. F/stops may seem confusing at first. The f/stop value represents a fractional opening of the aperture, and therefore a decrease of one f/stop results in the intensity of light being admitted into the camera to approximately double, and an increase by one f/stop results in the intensity of light being cut by half. One may prematurely conclude, proper exposure is obtained simply by adjusting the size of the aperture until the intensity of light admitted is just right. However, the depth of field (range of distance in front of and behind the subject that is in focus) is also a function of the size of the aperture opening. In general, depth of field increases as the size of the aperture opening decreases.
Controlling exposure is also achieved by controlling the duration of the light striking the film (or image sensor). To control the duration of the exposure, modern cameras employ a shutter. The shutter may be thought of as a curtain with an opening or slit that passes in front of the film (image sensory) at a controlled duration or speed. Shutter speeds are expressed in seconds, and fractions of a second. A shutter speed of 1/100 allows twice the duration of exposure as a shutter speed of 1/200. The resolution of an image is partly determined by the duration of exposure in capturing the image. A typical approach in portrait photography is to set the size of the aperture to yield the desired depth of field, and set the shutter speed to achieve an acceptable exposure level.
A third fundamental parameter that is manipulated to control exposure when capturing an image is the film speed, commonly stated as an ISO/ASA number. Film speed is a quantitative description of the chemically derived photosensitivity of the material used in the film. The higher the ISO number, the more photosensitive the film is. Faster film speeds enable action shots and low light images to be easily captured. However, faster film speeds can also result in increased perceived graininess in an image and decreased sharpness and detail. Similarly, the ISO number on many modern digital cameras may be adjusted to control the sensitivity of the digital image sensor, with similar effect.
Light is controlled within the camera by manipulating the intensity and duration of exposure, and by selecting an appropriate speed of film for the light conditions, or adjusting the sensitivity of the digital image sensor. These methods work very well to control the average or overall exposure of the composition. A finer degree of control of the light to enhance specular highlights, falloff, and softness of shadows, is best achieved outside the camera.
III. SELECTIVE CONTROL EXPOSURES
In Part 2 of this article, we explored the fundamentals of controlling overall exposure of a composition within the camera. The methods discussed are very effective for achieving the desired exposure of the overall image, the depth of field, and the perceived sharpness and graininess of a photograph. However, the photographer will generally want to emphasize a particular element within the composition, enhance a particular perspective, or suggest a particular mood. This may be accomplished in part, through carefully choosing or designing the elements of the composition. We have many classic rules to guide us in composition (e.g. the rule of thirds, negative space, color theory, etc.) Also, we may give special attention to posing our subject, or choosing a creative perspective or angle of exposure to emphasize a particular element, perspective, or mood. In my experience however, effective control over photographic lighting within a composition can be the difference between a masterful work and a mere snapshot. Light is the paintbrush in the art of photography. Controlling it is essential.
Over the years, many tools for effectively controlling lighting of a photographic composition have been devised. From scrims of all sorts, to flags, snoots, barn doors, and grid spots, to reflectors, umbrellas, and soft boxes, the tools in a photographers tool box may be neatly divided into three main classes. There are the tools for subtraction of light, the tools for addition of light, and the tools for modification of light.
Tools such as scrims, flags and even overhangs in the outdoors, are used to subtract light from the subject selectively to control emphasis, perspective, or mood. Filters and gels are a special case of subtractive tools. Some filters are used to subtract specific regions of the light spectrum, while gels are used to modify the color of light striking the subject also by subtracting specific regions of the light spectrum. Tools such as reflectors are used to add light selectively to the subject to control emphasis, perspective and mood within the composition.
Tools such as snoots, barn doors, grid spots, umbrellas, and soft boxes are used to modify a property of the light source used for photographic lighting. The properties commonly manipulated by the photographer are relative size of the source, specific placement of the light on the subject, fall off, and diffusion. In Part 1 of this article we learned that, “A light source’s relative contrast is generally determined by the size of the light source and its distance from the subject”. The light modifier tools of an indoor (studio) photographer function to control the effective size of the light source and thus control the contrast and fall off of the light striking their subject. For example, a large soft box may be used to modify a light source to increase it’s effective size, and thereby decrease the contrast of light striking the subject. On the other hand, a six inch parabolic reflector may be used to decrease the effective size of the source and increase the contrast of the light striking the subject. Generally the light modifier tools can be ranked in order of increasing size (decreasing contrast of light striking the subject): grid spots, 6 inch parabolic reflector, 16 inch parabolic reflector, 30 to 36 inch umbrella, small soft box (24″x36″), medium soft box (36″x48″), and so forth. The snoot and barn doors are typically used more to control specific placement of the light on the subject than as a means to control the level of contrast.
A basic understanding of the behavior of light, and how to effectively modify it using simple tools to control emphasis, perspective, and mood, is essential to your success as a photographer.
About the author
Steve Barnes is a professional portrait photographer and co-owner of Hayley Barnes Photography in League City, Texas.
Please visit his website at http://www.hayleybarnesphoto.com
See also: MicroStock Photography | Photography Tips