Lighting unevenness commonly occurs when using hard lighting from an undiffused spotlight or flashgun placed too close to the subject. This results in the elements closest to the light source being significantly brighter than those farther away. When you double the distance of a near-point light source, the illumination at the subject reduces to one-quarter of its original brightness.
For instance, with a still life setup 1 meter wide, side-lit from a position 1 meter away, the illumination will be four times brighter on one side than the other. For completely even lighting, refer to the section on copying, where two lights of identical intensity are placed equidistantly on opposite sides of the subject.
To minimize uneven lighting without compromising quality, simply move the light source farther back in a direct line from the set. At 2 meters, the variation across the set reduces to one-and-a-quarter stops, and at 3 meters, it decreases to only two-thirds of a stop. Alternatively, you can add an additional light source, diffuse the light, narrow the set, or position the darkest, least reflective objects closest to the light source.
Most light sources used in photography emit ‘white’ light, which is a mixture of all colors and is said to have a continuous spectrum. This spectrum can vary greatly, from an ordinary domestic light bulb, which is rich in red and yellow but weak in blue, to electronic flash containing relatively more energy in blue wavelengths than red. Most sources can be assigned a ‘color temperature’; the higher the Kelvin (K) value, the bluer the light.
When shooting color, particularly color slides, it is crucial to match the color temperature of your lighting to your film. This is typically 5500 K for ‘daylight’ film or 3200 K for ‘tungsten light’ film. Alternatively, you can use a tinted correction filter, such as an 85 B or 80 A, to align the light source with the film. If all your subject lighting is the same color temperature, you can use an adjusting filter over your camera lens. For mixed lighting scenarios, such as daylight and studio lamps, you can place a filter over one source to match the other and the film.
Some light sources, such as sodium street lights and lasers, do not produce a full range of wavelengths and cannot be filtered to yield a white light result. Cameras often feature ‘auto white balance,’ which functions similarly to a video camera. It samples the color content of the surrounding light and adjusts the CCD’s color sensitivity to ensure that a white object appears white, regardless of the lighting’s color temperature. Within certain limits, further color corrections can be made later using digital manipulation software.
