the lens (perpendicular to the lens axis). For example, when the subject is moving straight towards or straight away from the camera, it may appear as though it is hardly moving and a fast shutter speed is not required to produce a sharp image; however, when that same subject moves at the same speed across the field of view of the camera, the speed of the subject appears much faster. A faster shutter speed is required to stop the action in this case. The camera-to-subject distance also affects the amount of image movement at the film plane; for example, a car moving across your field of view at 55 mph from a distance of 700 yards appears to be moving slowly. The same car moving at 55 mph and only 15 feet away appears to be moving very fast; therefore, the closer a moving object is to the camera, the faster the shutter speed must be to capture a sharp image. When the subject is moving diagonally across your angle of view, movement is more apparent than when moving straight away or toward the camera, but less apparent than  when  moving  straight  across  the  field  of  view. Remember,  long-focal-length  lenses  exaggerate  the effects of camera and subject movement, and short- focal-length  lenses  reduce  the  effect. Experience and common sense are your best guides for  determining  shutter  speed  that  will  minimize  image movement, but the following can be used as a guide to help  make  these  determinations: Double the shutter speed when the subject speed is doubled. Halve the speed when the speed of the subject is halved. Double the shutter speed when the camera- to-subject  distance  is  halved. Halve  the  shutter  speed  when  the  camera- to-subject  distance  is  doubled. Double the shutter speed when the focal length is doubled. Halve the shutter speed when the focal length is halved. When in doubt, use the next higher shutter speed. There  are  mathematical  formulas  used  to  determine appropriate shutter speeds for subjects moving at all speeds  when  photographed  with  various  lenses,  but  the use of these formulas is not practical. Table 4-2 shows stop motion relationships when a 50mm lens is used. This table is not intended to be memorized but is only intended to provide a better understanding of the relationship  of  subject  motion,  distance,  and  direction. COMBINING  APERTURE  AND SHUTTER   SPEED So far three camera controls have been discussed separately: focus, aperture, and shutter. Focus is the most straightforward because it is used to produce a sharp image of the subject. Aperture and shutter each affect the image in two distinct ways. They both control the amount of light that makes the exposure, and they both affect image sharpness. The aperture alters depth of field, and the shutter controls the image movement or blur. The light-sensitive material must receive the correct amount of light to produce a quality photograph. Under most lighting conditions, it does not matter whether you use a wide aperture with a fast shutter speed or a small aperture   with   a   slow   shutter   speed.   When   the combination is correct, both provide the same amount of  exposure. Aperture and shutter speeds each have a doubling and halving effect on exposure. This doubling and halving relationship of aperture and shutter allows you to combine different f/stops and shutter speeds to alter the image, while, at the same time, admitting the same amount of exposure to the light-sensitive material; for example, you have determined that the correct camera settings for your subject is 1/125 second, at f/16. Instead of using this combination of shutter speed and f/stop, you could double the shutter speed (to stop action) and halve the f/stop. In this example your new camera setting could be 1/250 second at f/11, 1/500 second at f/8, or 1/1000 second at f/5.6, and so on. Or when you need more depth of field, 1/60 second at f/22 or 1/30 second at f/32, and so on, can be used. These shutter speed and f/stop  combinations  are  called  equivalent  exposures. Equivalent exposures are used to control depth of field and to stop motion. Table 4-3 shows some equivalent exposures  of  a  typical  situation. Each of the combinations in table 4-3 produces the same exposure; however, the amount of depth of field and  image  blur  are  different  in  each  image.  The combination of shutter speed and f/stop is used to best capture the subject and effect you want to create. You should use a light meter for most of the photographs you take. The light meter provides you with a number of f/stop and shutter speed combinations; however; depending on the situation, the level of light alone can determine the camera settings. For example, 4-16