camera   in   manual   mode,   you   must   understand   the relationship between the f/stop and shutter speed. Shutter  speeds  are  indicated  so  that  each  marked shutter speed admits one-half or two times the adjacent marked speed. Since both the lens aperture and shutter speed represent “full stop” changes in exposure, either can be moved as long as the other is moved a similar number  of  stops  to  compensate.  A  basic  exposure  of 1/125" at f/16 can be changed to 1/500" (two stops less light   transmitted)   at   f/8   (two   stops   more   light transmitted), and the result will be the same total amount of light transmitted to the film. Shutter   speeds   cannot   be   set   between   marked “stops.” If an exposure is calculated to be 1/40" at f/8, using the closest shutter speed available on your camera, 1/30"  or  1/60"  will  not  result  in  an  exposure  error because  of  the  exposure  latitude  of  the  film.  An alternative is to set the shutter speed and an equivalent “half-stop”  of  lens  apeture,  such  as  1/30"  at  f/9.5,  or 1/60" at f/6.3. With  a  selection  of  possible  combinations,  which should be used? Does it matter which is used? Why does the  manufacturer  put  so  many  combinations  on  the camera? Before  these  questions  are  answered,  you  must understand the correlation of lens apertures and shutter speeds. Think of the lens aperture as a water pipe (the larger the diameter of the pipe, the more the water can flow). Extending this further, think of the film sensitivity in terms of a bucket that has to be filled and the light intensity as the water pressure. If a bucket can be filled in 1/30" with a pipe 8 square inches in area, how long would it take to fill using a pipe 4  square  inches  in  area?  Obviously,  twice  as  long  — 1/15". If the exposure is calculated at 1/30" at f/11, how long   an   exposure   is   required   at   f/16   (the   aperture one-half the area of f/11)? The answer is 1/15". What  happens  if  the  water  pressure  increases?  It takes less time to fill the bucket. If we use a larger bucket (lower ISO film speed), it takes more water (exposure) to fill it. Shutter Speed Considerations Generally, the shutter speed is chosen according to the  amount  the  subject  moves  or  how  much  of  the movement  you  desire  to  show.  If  the  subject  moves slowly, a slower shutter speed can be used; if the subject moves rapidly, a faster shutter speed must be used to stop  the  movement  and  prevent  blurring  the  image. Movement of the camera and photographer also must be considered.  Therefore,  the  use  of  a  tripod  or  similar brace  is  advisable  when  using  a  shutter  speed  slower than the reciprocal of the lens focal length; for example, 50mm lens (1/60"), and 200mm lens (1/250"). To stop the movement or action in a picture, you must consider the following three factors: l The relative movement of the subject . The subject’s direction of movement .  The  camera-to-subject  distance THE   RELATIVE   MOVEMENT   OF   THE SUBJECT.—  The faster the movement, the faster the shutter speed required. The term  relative movement  is used because if the motion of the subject is followed, that is, the action is “panned” with the camera, a slower shutter speed can be used than if the camera were held stationary. THE   SUBJECT’S   DIRECTION   OF   MOVE- MENT.—  A  subject  traveling  at  a  right  angle  to  the camera/lens axis requires a faster shutter speed than one traveling at a diagonal. Conversely, a subject moving toward  or  away  from  the  camera,  parallel  to  the  lens axis, can be “stopped” with a slower shutter speed than movement in other directions (fig. 11-21). THE  CAMERA-TO-SUBJECT  DISTANCE.— The  closer  the  action  is  to  the  camera,  the  faster  the shutter speed must be. A car traveling 60 miles per hour across the lens axis at a distance of 100 feet would be “stopped”  by  a  shutter  speed  of  1/1000"  (or  perhaps 1/500").  However,  if  the  camera-to-subject  distance were increased to 500 feet, the action could be “stopped” with a shutter speed of “1/250" or” 1/125." If the car was a half mile away, 1/60" should be sufficient to stop the movement. DEPTH OF FIELD Selection of a f/stop is done mainly for the desired depth of field. “Depth of field” is defined as the distance between  the  nearest  and  farthest  points  of  acceptable sharp focus of the scene photographed (fig. 11-22). Control of the depth of field is a valuable tool in photography.  Depth-of-field  charts  are  given  in  all camera  instruction  books  as  well  as  in  photographic reference manuals, but many photographers fail to use them to their own advantage. Simply stated, depth of field increases as the focal length of the lens decreases (a shorter focal-length lens is used), as the lens aperture decreases (gets smaller in size)  and  as  the  distance  focused  on  (focal  point) increases, or both. Inversely, depth of field is less for long-focal-length   lenses   than   for   short-focal-length 11-24