As  explained  previously,  camera  exposures  are controlled by the shutter speed and aperture. The shutter speed controls the time light is permitted to reach the film. The illuminance (or intensity as it is sometimes called) is controlled by the aperture of the camera. The term illuminance means the amount of light reaching the film plane. By adjusting these controls, you allow the correct amount of light to reach the film. The correct amount of light varies, depending on the film speed. Correct exposure for negative films is defined as the exposure  required  to  produce  a  negative  that  yields excellent  prints  with  the  least  amount  of  difficulty. Correct exposure for color reversal film produces color images  in  densities  that  represent  the  appearance  of  the original  scene. FACTORS  THAT  AFFECT  EXPOSURE 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 4-18