The polarizing filter may be thought of as a screen, with an optical grid or slots, that stops all light that is not vibrating in a plane parallel to the axis of the grid lines. As the filter is rotated, the amount of polarized light can  be  controlled.  When  the  rodlike  crystals  are perpendicular to the vibration direction of the light, the polarized light is greatly absorbed. When the rodlike crystals are parallel to the vibration direction of the polarized light, the polarized light is almost totally transmitted. Because  polarizing  filters  are  colorless,  they  can  be used as neutral density filters. Even when polarized light is not present in a scene, polarizing filters can be used to reduce the intensity of light. When two polarizing filters are used, their combined densities can be varied considerably. In  color  photography,  the  only  way  you  can reproduce the sky darker without affecting the other colors in the scene is to use a polarizing filter. You can achieve various effects from light sky to dark sky by rotating the fiter to various positions. You can see this effect by viewing the scene through the viewfinder of a single-lens reflex (SLR) camera or by viewing the scene through the ground glass of a view camera. To see how much  reflection  control  you  are  getting,  rotate  the  filter as you are viewing the scene. Getting  the  maximum  effect  with  a  polarizing  filter depends on your angle to the subject as well as the rotation  of  the  filter.  When  the  reflection  cannot  be completely  eliminated,  try  changing  your  camera  angle to the subject. The maximum control of unwanted surface  reflections  and  greatest  reduction  of  light intensity occurs when two polarizing filters are used with their optical grids perpendicular to each other. This arrangement can be either two filters in tandem in front of the camera lens or one filter in front of the light source and another filter in front of the camera lens. You cannot control  reflections  from  bare  metal  surfaces  because  the reflected  light  is  not  polarized. SKYLIGHT  FILTERS By absorbing ultraviolet radiation, a skylight (1A) filter adds warmth to a scene recorded on a color transparency film. It does this by reducing the bluish cast prevalent in distant scenes and in scenes photographed on heavily overcast days or in open shade. A skylight filter is used primarily with daylight color reversal film exposed  under  the  above  conditions.  A  skylight  filter  is light pink in color. FILTER  FACTORS Filters function by absorbing a portion of the light reflected from the subject to the camera. To compensate for this absorption and the loss of light, you may have to increase the exposure to compensate for the light absorbed by the filter. A numerical value is assigned called a “filter factor” or multiplying factor. This numerical factor is based on several variables that include the color sensitivity of the film, density of the filter, color of the filter, and color temperature of the light source. As these variables change, the filter factor also   changes   to   produce   the   correct   exposure consistently. Filters are often identified as “2 X yellow” or “4 X orange.” That implies that the filter factor is 2 and 4, respectively. Remember, the filter factor does not always  remain  constant  when  conditions  change. For example, a blue filter used with panchromatic film  exposed  with  daylight  requires  a  smaller  filter factor than when the same film and filter are used with tungsten  light.  The  reason  for  this  is  daylight  has  a  higher content of blue light that is readily transmitted by the blue  filter.  Thus,  with  the  same  film  and  filter combination  and  with  the  same  camera  shutter  speed and f/stop, more exposing light is available at the film plane  with  daylight  as  compared  to  tungsten  light. A  filter  that  absorbs  a  great  amount  of  illumination from a given light source is assigned a larger filter factor. A filter that absorbs a lower amount of illumination from the  same  light  source  is  assigned  a  smaller  filter  factor. To obtain the necessary light at the film plane for correct exposure with a filter, you must increase the original  calculated  exposure  (without  a  filter).  This increase in exposure is determined with a filter factor. When a filter has a factor greater than 1, an adjustment to the exposure must be made. There are three general methods of using filter factors  to  determine  the  exposure  increase  required: 1.   Divide the ISO speed by the filter factor, and use the  product  as  the  effective  film  speed. Example:   If the filter factor is 2 and the IS0 speed of the  film  is  100,  the  effective  film  speed  is  50 (100 + 2 = 50). Thus setting a film speed of 50 on your light meter produces  the  equivalent  of  1  f/stop  of  additional exposure. 2.  Determine  the  required  exposure  without  the  use of a filter; then multiply the unfiltered shutter speed by the  filter  factor. 3-9