You must consider four major factors that affect exposure when you are taking photographs. These factors are as follows:

Film speed (ISO)

Reflected properties of the subject

Lighting conditions

Bellows extension

Film Speed

As explained in chapter 2, ISO is a system of rating film speed or sensitivity to light. ISO numbers are arithmetic; that is, an ISO number that is twice as high as another ISO number is twice as sensitive to light. Each time an ISO film speed is doubled, the exposure should be halved. When the ISO is halved, the exposure should be doubled; for example, if the correct camera setting is 1/250 second at f/16 with ISO 100 film, the same subject photographed with ISO 200 film would require only half the exposure or 1/500 second at W16 or 1/250 second at f/22, and so on.

Daylight Conditions

The two primary considerations for determining your exposure under daylight conditions are the intensity and the direction of daylight.

INTENSITY. From early morning until later evening, even on a clear day, the intensity of daylight is constantly changing as the sun rises, moves across the sky, and sets. Although the intensity of daylight varies throughout the day, the time between about 2 hours after sunrise until about 2 hours before sunset is considered a time when the light intensity for the same geographical location remains constant for exposure purposes.

Daylight conditions for camera exposures can be divided into the following five intensity conditions.

Bright Sun on Light Sand or Snow. Bright sun is daylight that is not affected by any apparent atmospheric interference. Because of the amount of reflected light from sand or snow, the intensity of light in these scenes is greater than that of a scene with average reflectance. This greater intensity of light requires a higher f/stop or a faster shutter speed to provide approximately one half of the exposure required for the basic exposure with bright or hazy sun.

Bright Sun. This type of daylight illumination is produced on a bright, sunny day where distinct shadows are present. Bright sun is the condition that determines the BASIC EXPOSURE for an average scene.

Cloudy Bright. A weak, hazy sun is the result of a heavier or thicker haze or cloud cover as compared to the bright sun condition. The condition causes a decrease in the daylight intensity and an increase in the diffusion of daylight. This lighting condition produces shadows that are soft or indistinct. A lower f/stop or slower shutter speed is required to approximately double the basic exposure to compensate for this decreased daylight intensity.

Cloudy. Cloudy conditions are the result of a layer of clouds that further reduce the intensity of daylight and diffuse the light completely. This condition occurs on an overcast day when the position of the sun can be located only as a bright area in the clouds. Shadows are not present under this lighting condition. The scene brightness range is low and therefore photographs made during this condition usually lack good contrast. An increase of four times (two f/stops) from the basic exposure is required to compensate for the decreased intensity of light.

Heavy Overcast or Open Shade. This condition exists when the position of the sun cannot be located. The scene brightness range is low and therefore photographs made during heavy overcast conditions usually lack good contrast. An increase of eight times (three f/stops) to the exposure is required horn the basic exposure to compensate for the decreased intensity of light.

DIRECTION. The direction of the sun or light source illuminating your subject also affects your basic exposure. The camera settings recommended for films

Figure 4-18. Lighting directions.

exposed during bright sun on light sand or snow, bright sun, cloudy bright, and cloudy conditions are for scenes that are front-lighted only. The direction of the light source from heavy overcast or open shade conditions does not affect exposure because it is extremely diffused and the direction is not apparent.

The amount of light reflected from the scene changes, as the direction of the light changes. As the lighting direction is changed from in front of the subject to behind the subject, the amount of light reflected from the subject is reduced; therefore, depending on the direction from which the light is falling on your subject, you may have to compensate the camera exposure. There are three basic lighting directions with which you must become familiar. These lighting directions are as follows: frontlighting, side lighting, and backlighting (fig. 4-18).

Frontlighting. Whenever light originates from behind the camera and illuminates the front of the subject, it is called frontlighting. A subject appears brightest and reflects the most light toward the camera when the subject is front-lighted.

Side Lighting. As the camera is moved in an arc away from frontlighting, less light is reflected from the subject into the lens. Whenever the light source has a 90-degree relationship with the camera, the incident light on the subject is called side lighting. In side lighting situations, part of the subject is in shadow. Photographs of side-lighted scenes usually require two times (one f/stop) more exposure than frontlighted subjects when you want detail in the shadows.

Backlighting. When the light source is directly behind the subject and aimed toward the camera, it is called backlighting. In back-lighted situations, the subject is in shadow and the light reflected from the subject toward the camera is decreased greatly. A silhouette effect (no shadow detail) of a back-lighted scene is produced by closing down one f/stop from the basic exposure. If shadow detail is desired, an increase of four times (two f/stops) from the basic exposure is required.

Reflection Properties

Otherwise the intensity and direction of light falling on the subject, the texture of the surface, and the colors and shades of the scene also have an effect on film exposure.

SURFACE TEXTURE. Smooth, glossy surfaces scatter or diffuse reflected light very little; therefore, these objects reflect a large percentage of light to the lens. Rough surfaces greatly scatter and diffuse light. Less light from rough surfaces is reflected to the lens.

COLORS AND SHADES. Not all light that falls on the surface of a subject is reflected. A brilliant white object reflects a high percentage of incident light, and a black object reflects very little of the light. Between these two extremes are the numerous tones of gray and colors of various hues and brightnesses. Each colored or gray object in a scene reflects a specific amount of light. A scene that consists primarily of light-colored or light-toned objects usually requires an exposure compensation to decrease the exposure as compared to the basic exposure for an average scene. A scene that consists primarily of dark-colored or dark-toned objects usually requires an exposure compensation to provide more exposure as compared to the basic exposure for an average scene. The primary reason light scenes and dark scenes require less exposure and more exposure, respectively, as compared to the average scene, is to maintain detail in the highlight of the light scenes and detail in the shadow areas of the dark scenes.

The color quality of a light source also has an effect on the amount of light reflected from an object; for example, a blue object does not reflect as much light when illuminated with a red light source, as compared to the same object being illuminated with a blue light source. This difference in reflectance is caused by the blue object absorbing the reddish light and reflecting the bluish light.

Any man-made light is an artificial light source. This light may be a tungsten lamp, a fluorescent lamp, a mercury-vapor lamp, and so on. The same factors that affect exposures for daylight apply to artificial light as well. Artificial light has some advantages. Distance, direction, and color temperature can be controlled using these light sources.