When two flash units of equal intensity and at equal distance from a subject illuminate the same area, the exposure for one unit should be determined and then the exposure should be halved because twice the intensity of light is reflected from the subject. OPEN FLASH Flash pictures can be made without the camera shutter  and  flash  being  synchronized,  using  a  technique called  open  flash.  In  the  open-flash  method,  the  camera shutter is set at T or B, the shutter is opened, the flash unit  fired,  and  the  shutter  closed.  The  open-flash technique is sometimes used when the level of light over a large scene is very low or at night. This method of flash photography allows the photographing of large scenes that ordinarily are quite difficult to illuminate with artificial light. The photographer can walk into a scene with the flash unit and illuminate sections of the scene or the entire scene. Any number of flashes can be used during the exposure while the shutter remains open. A silhouette of your body can be recorded if your body gets between the flash and the camera. To arrive at the exposure for an open-flash picture using  a  manual  flash,  determine  your  flash-to-subject distance and f/stop. Keep the distance equal to the objects being illuminated when using manual flash; for example, when the f/stop for the scene is f/5.6 based on a  flash-to-subject  distance  of  10-feet,  every  flash  within the scene should be 10 feet from that section of the scene being illuminated, When an automatic flash is used, the flash automatically shuts off when the proper amount of light is reflected from the subject, providing the object is within its distance range. When you are using a manual  flash,  the  exposure  for  open  flash  is  determined as previously discussed. This is true unless two or more flash units with equal intensities are used at equal distances, or two or more flashes from the same unit at the  same  distance  are  used  to  illuminate  the  subject. MULTIPLE  EXPOSURES  WITH ELECTRONIC FLASH Interesting multiple exposures can be made with only one or two electronic flash units. Multiple exposure pictures,  besides  being  artistic  and  interesting,  are  often used to study subject motion and position. This can be accomplished  by  the  following  procedures: 1. Darken the room and position your subject against  a  black  background. 2. Allow enough background area for the number of different exposures you intend to make. When you are using a ground glass camera, mark off on the glass, with grease pencil, the areas where the subject should be for each different exposure. If not using a ground glass camera, make a pencil sketch to help you position the  subject. 3. Set up the electronic flash lights so the minimum amount  of  illumination  falls  on  the  background  itself. 4. Turn off all room lights and make your first exposure. Then, without advancing the film, move your subject to the next position for the second exposure. Repeat this procedure for each image you want to record on the film. PHOTOGRAPHING  ACTION  WITH ELECTRONIC FLASH Action of any kind, no matter how slight, can add interest to most pictures. Each type of action requires a different camera technique, but because of the short duration of light from electronic flash, it is ideal for recording  any  action  ranging  from  a  fleeting  expression to a sports triumph. Most electronic flash units have a maximum  flash  duration  (the  length  of  time  the  light  is on) of about 1/800th second, and a minimum flash duration as short as 1/20000th second, thus you can “freeze”  almost  any  action  with  the  flash. Indoors, where there is little existing light, you have no problem because the electronic flash itself stops the action;  however,  outdoors  in  daylight,  you  may encounter ghost images. Ghost images can occur when existing light and a slow shutter speed are used in conjunction with electronic flash. A ghost image appears as a blur when one image is recorded by the existing light and a second sharp image by the electronic flash. ELECTRONIC  FLASH  AT  NIGHT Flash photography, outdoors at night, can produce very underexposed photographs if not taken properly. Outdoors, flash does not carry very far; therefore, it can be difficult to light objects from a distance; however, this limited coverage also gives you great control. Indoors, part of the output of a flash unit is reflected from the ceiling and walls back to the subject. Rarely do you  find  such  reflective  surfaces  outside,  so  some  light is lost. To compensate for the light lost, you must open up your aperture when photographing objects at any distance. Because so much light is absorbed in these large areas, it may not be uncommon to open up your aperture two or three f/stops. Tests should be conducted before  shooting  in  large,  indoor  areas,  such  as gymnasiums and hangar bays or outdoors at night, to 5-36