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Lighting for Portrait Photography (Part 2): Controlling Exposure within the Camera


By Steve Barnes

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. There are many methods employed to accomplish this. In Part 3 of this article, several such methods will be 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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