EXAMPLE: You are photographing a document with a camera that has a 5-inch lens and the bellows are extended  7  inches.  Your  light  meter  indicated  an exposure of 1/30 second at f/4. The new exposure time is determined as follows: To adjust the aperture, use the following formula: EXAMPLE: A 4-inch lens is extended to 4 inches beyond one focal length. The original camera settings are 5 seconds at f/11. Using the above formula, the problem is solved as follows: f/16  RULE You  should  use  a  light  meter  for  most  of  the photographs that you take in the fleet. These light meters are either built into the camera or are separate hand-held models; however, there may be times when your light meter does not operate properly, or you do not have time to use it in order to “grab” an awesome shot. The f/16 rule of exposure allows you to determine basic camera exposure settings for both black-and-white and color photography  without  the  aid  of  electronic  devices. The f/16 rule states: The basic exposure for an average subject in bright frontal sunlight is f/16 at: 1 film  speed Therefore,  to  calculate  the  BASIC  exposure  for bright, sunny conditions, set f/16 on the camera lens and use the IS0 speed of the film for the shutter speed; for example, when you use IS0 125 film, set the shutter speed at 1/125 second and the lens aperture at f/16. For IS0 64 film, set the shutter speed at 1/60 second and the lens aperture at f/16, and so on. When the camera does not have a shutter speed corresponding to the IS0 of the film, use the shutter speed that is closest to the IS0 of the film. The f/16 rule is based on the correct exposure for an average subject under bright, sunny conditions. If the sun goes behind a cloud, however, then the lighting on the subject is decreased and you must change the basic 4-21 exposure.  The  aperture  settings  for  different  daylight intensities  are  as  follows: Bright sun on light sand or snow-f/22 Bright    sun-f/16 Cloudy   bright-f/11 Cloudy-f/8 Heavy overcast or open shade-f/5.6 For  each  of  these  different  daylight  intensity situations, you begin with the ISO speed to determine the shutter speed, set the aperture to f/16, and open up or  stop  down  the  aperture  for  the  lighting  conditions. After calculating the exposure, you can change the setting to any equivalent exposure; for example, if you determine the required exposure to be 1/500 second at f/5.6 but you wish to use a small aperture for greater depth of field, you can change the setting to 1/60 second at f/16. Remember, the f/16 rule provides you with a basic exposure for front-lighted subjects only. When the subjects are side-lighted or back-lighted, you must double  or  quadruple  the  exposure,  respectively. Because many cameras are fully automatic, you may wonder why you need to know basic exposure. There  are  three  good  reasons  for  knowing  and understanding  the  basic  principles  of  exposure.  First, you want to control the depth of field and stop action instead of the camera controlling it. Second, a light meter cannot think All a light meter does is respond to the light it receives. You must know when to override the camera; for example, when the subject is side- lighted or back-lighted. Third, meters are mechanical and  can  fail.  They  can  be  inconsistent,  consistently wrong, or fail altogether. When you can workout in your head, roughly what the camera exposures should be, you will know when the camera or light meter is wrong. Knowing when a light meter is giving incorrect readings could  make  the  difference  between  success  or  failure  of an  important  photographic  assignment. LIGHT  METERS The correct use of a light meter greatly increases the accuracy in determining your camera exposure. You should also understand that the incorrect use of a light meter can result in consistently unacceptable results. To assure  consistently  acceptable  exposures,  you  must become  thoroughly  proficient  with  the  correct  operation of a light meter.