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