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Photographic lighting is the illumination of scenes to be photographed. A photograph simply records patterns of light, color, and shade; lighting is all-important in controlling the image. In many cases even illumination is desired to give an accurate rendition of the scene. In other cases the direction, brightness, and color of light are manipulated for effect. Lighting is particularly important for monochrome photography, where there is no color information, only the interplay of highlights and shadows. Lighting and exposure are used to create effects such as low-key and high-key.
The main sources of light for photography are:
- Daylight, which varies with the weather and the time of day. Different techniques are necessary to take best advantage of, say, brilliant sunshine, and an overcast evening.
- Continuous artificial light, which may be normal lighting, or produced by special photoflood lights. The properties of different light sources vary; household incandescent lighting, fluorescent lighting, sodium discharge street lighting, etc., are very different and produce different results, and require different correction if a subjectively neutral rendition of colors is required.
- A bright and very brief photographic flash from a single position (usually very close to the camera) or, in a studio environment, from several.
- For special purposes lightning, electric sparks, fireworks, moonlight, or other light sources may be exploited.
Perceptual cause and effect
Lighting creates the 2D pattern of contrast the brain interprets to recognize 3D objects in photographs. In an in-person viewing experience the brain relies on stereoscopic vision, parallax, shifting focal in addition to the clues created by the highlight and shadow patterns the light on the object creates. When viewing a photo the brain tries to match the patterns of contrast and color it seen to those other sensory memories.
The baseline for what seems "normal" in lighting is the direction and character of natural and artificial sources and the context provided by other clues. In the example the photographer added a warming gel on the flash of the woman standing in a field in late afternoon light. The viewer knows the time of day from the angle of the shadows and neutral color balance would have seemed odd in that context. But similarly the image of the woman if masked out and put on a plain white or neutral gray background would seem abnormally yellow.
The goal in all photographs is not to create an impression of normality. But as with magic, knowing what the audience normally expects to see required to pull off a lighting strategy which fools the brain or creates an other than normal impression. Light direction relative to the camera can make a round ball appear to be a flat disk or a sphere. The position of highlights and direction and length of shadows will provide other clues to shape and outdoors the time of day. The tone of the shadows on an object or provide contextual clues about the time of day or environment and by inference based on personal experience the mood of person.
A skilled photographer can manipulate how a viewer is likely to react to the content of a photo by manipulating the lighting. Outdoors that can require changing location, waiting for the ideal time of day or in some cases the ideal time of year for the lighting to create the desired impression in the photo or manipulating the natural lighting by using reflectors or flash. In a studio setting there is no limit to options for lighting objects to ether make them look "seen by eye" normal or surreal as the goals for the photograph require. But more often than not the reaction on the part of the view will be from the baseline of whether the lighting seems normal/natural or not compared to other clues. Mistakes less skilled photographer often make when mixing flash and natural lighting is not matching with the flash the highlight and shadow clues seen in the ambient lit background. If the background is illuminated by the setting sun but the face in the foreground appears to have been photographed at noon it will not seem normal because the clues don't match.
The natural light baseline
The sun hitting the front of objects facing the camera acts as "key" light creating highlights and casting shadows. Detail in shadows can be seen because the sunlight reflects off water vapor and dust the atmosphere creating omni-directional "fill". In open shade 3D objects will also usually cast shadows because the downward vector of skylight is usually stronger than the sideways vectors illuminating the sides. When a photographer puts the sun behind an object its role in the lighting strategy changes from modeling the front of the object to one of defining its outline and creating the impression of physical separation and 3D space a frontally illuminated scene lacks. To differentiate that role from that of "key" modeling when a modeling source moves behind the object it is typically called a "rim" or "accent" light. In portrait lighting it also called a "hair" light because it is used to create the appearance of physical separation between the subject's head and background. In natural lighting the tone of the background is influenced by its reflective qualities and whether it is illuminated by the sun directly or skylight indirectly. So either the sun or sky, or a combination of both can be the "background" lighting.
Creating natural looking artificial lighting
Artificial lighting strategies which seem most "natural" duplicate the same contrast pattern clues seen on 3D objects is various lighting conditions. A typical studio lighting configuration will consist of a fill source to control shadow tone, a single frontal key light to create the highlight modeling clues on the front of object facing the camera over the shadows the fill illuminates, one or more rim/accent lights to create separation between foreground and background, and one or more background lights to control the tone of the background and separation between it and the foreground.
There are two significant differences between natural lighting and artificial sources. One is the character of the fill and the other is more rapid fall-off in intensity. In nature skylight fill is omni-directional and usually brighter from above. That "wrap around" characteristic is difficult to duplicate with a directional artificial source. In a fixed studio location it is possible to bounce fill backwards off a white wall to flood the space with indirect reflected light similar to how the sun reflects off the atmosphere. Another way is to supplement a fill source from the direction of the camera with reflectors placed near the sides of the foreground subject.
The Inverse-Square Law describes the predictable way a point light source radiates and changes in intensity with distance. As the distance from a source doubles the area of the footprint of light increase by a factor four, the square ^2 of the distance. Because the same number of photons are spread over four times the area when distance is doubled the intensity at any point will be 1/ distance ^2 or 1/4 the strength. Photographic light sources are not point light sources so the law does not strictly apply but it explains why distance of artificial sources affects the character of lighting and lighting strategies in ways not seen in nature.
According to the inverse-square law if the distance of a light source is changed in the following distance increments 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64 the intensity will decrease by one f/stop. In practical terms it means if one face in a group portrait is 4m from the "key" light and another is 5.6m away the face further from the light will be one f/stop darker. In an outdoor portrait of a group of 200 people taken on an overcast day the lighting all the faces will be equal. The same group photographed indoors would be far more difficult to light evenly. The simplest strategy requiring the least flash equipment would be to get above the group with the camera, have them look up and bounce the lighting off the ceiling so like an overcast day every face is as equidistant as possible to the apparent source of the light.
Even something as basic as a head and shoulders portrait must take inverse-square fall-off into account by posing the front of the subject's face as close or closer to the "key" light than the shoulder or any other body part if the goal is to make the front of the face the most strongly contrasting focal point on a darker background. The position of the fill source relative to the face will also affect whether the nose shadow is the lightest (when fill is centered near camera) darkest (when fill is placed to the side) one on the face. The distance of the key and fill sources to the face will affect the rate at which the shadows transition from light to dark on the face.
Creating surreal lighting
Natural and surreal are just different sides of the same cause and effect coin. Understanding what makes lighting seem natural makes it easier to understand how to create other desired reactions. Natural light usually comes from above, so strategies which place the key light below the face will appear to be unusual or unnatural. The brain adapts color perception in a way which makes color balance seem neutral on white clothing and faces. The eyes also adapt to brightness as they scan and usually perceive a full range of detail in most environments. Lighting a scene with a tonal range or color cast which is out of context with what would typically be expected will cause the viewer to notice the environment and make other than normal assumptions about it. It is also possible to create the impression of environmental context where none is seen in the photograph, such the look of a person standing under a streetlight at night by using a gridded flash attached to the ceiling of the studio with no fill source.
Three light setup
The three light setup is a standard method used in photography and, although is not the only method used, it is the most common. This method uses three separate positions that help the photographer illuminate the subject. It is formed by; the Key or main light, the fill light, and the back light.