FILTERS Two  main  reasons  for  using  filters  in  aerial photography are in use today—to control haze and to get a pure infrared photograph. Aerial  views  photographed  without  a  filter  may appear  flat  because  of  atmospheric  haze.  This  haze  may become  evident  in  aerial  photos  even  when  the  pictures are  made  on  "clear"  days.  Haze  has  the  effect  of r e f l e c t i n g    a n d    s c a t t e r i n g    t h e    s h o r t e r wavelengths—ultraviolet radiation and blue light. Since   all   films   are   sensitive   to   these   shorter wavelengths, they record as a veil over the scene when a filter is not used. This veiling of the image becomes more pronounced as the altitude of the aircraft is increased. This is because the mass of haze (water vapor and dust particles) between the aircraft and the ground increases. Filters used for controlling haze in aerial work range in color from light yellow to red. When you use these filters, the photographic image is recorded by light with a longer wavelength (green or red) that is not appreciably scattered by haze. Filters, such as Kodak Wratten No. 2B or 2E, absorb ultraviolet radiation  and  reduce  the  effects  of  haze  without affecting  the  monochromatic  rendering  of  visible colors. When greater haze control for black-and-white photography is required, deeper yellow or red filters should be used. However, when these deeper colored filters are used, the tonal rendering of colors is affected. The amount of haze control in black-and-white aerial photography  increases  with  the  use  of  the  following filters in this order: No. 8 (yellow), No. 15 (deep yellow), and No. 25 (red). The greatest penetration or control of haze for black-and-white aerial photography can be obtained by using a black-and-white infrared sensitive film with a suitable filter, such as a No. 25, No. 29, No. 70, or No. 89B. Haze filters for conventional color film are different from those normally used with black-and-white film because all colors of light must be used to obtain correct (true color) results. Filters used with color film are usually colorless or pale pink, such as a No. 1A Skylight filter, or one of several densities of pale yellow (No, 2B and 2E, for example). These filters are not dense enough to require additional exposure. On bright, clear days when haze is minimal and you are taking verticals or low obliques from altitudes below 2,000 feet, good color results may be obtained without using a filter. However, when haze is apparent or when you are taking high obliques, the use of a UV16, or 1A Skylight filter is recommended. From higher altitudes or when haze is a problem, consider using a No. 2B and No. 2E filter. Keep in mind that different types of color film may require different filters. Filter requirements are listed with each package of film. NOTE: Haze should not be confused with mist or fog, which affects film as a white or gray area. Haze penetration  filters  have  no  effect  on  mist  or  fog. Atmospheric haze is always present, but it is especially noticeable in distant scenes and from high altitudes. DETERMINING  EXPOSURE Determining   proper   exposure   for   aerial photography  can  be  more  complicated  than  determining exposure for ground photography. You can make exposure readings with your meter before leaving the ground to determine the proper exposure for ground photography. (Be sure to allow for the filter factor correction.) In most cases, for low level air-to-ground photography  and  for  air-to-air  photography,  you  should use about the same exposure in the air as you would use on the ground. Once you are airborne and before taking air-to-ground pictures, take a light meter reading of the ground from about the same altitude that you plan to work When you are at a fairly low altitude with little or no visible haze, the exposure reading should be very similar to the ground exposure. So set the camera about halfway between the two readings. However, if you have noticeable haze or if you are working from a high altitude or if your air-to-air subject is far away, your airborne  meter  reading  may  be  significantly  higher  than the ground reading. This is because your meter is affected by the large area of sky and the amount of light reflected by the haze. In this situation, the camera setting  should  be  determined  by  the  substitution method. The substitution method is also an effective way of determining exposure. The gray wing of an aircraft or a suitable substitute may be used to determine basic exposure. In any event, bracket your exposure by at least   one   f/stop   whenever   possible.   It   is   more economical to take a wide range of exposures than to refly a mission. High altitudes, the sky in high obliques, and  high  levels  of  haze  reduce  subject  contrast  and increase  exposure  latitude.  These  factors  oftentimes cause overexposure. SHUTTER SPEED AND LENS APERTURE Since depth of field is insignificant in air-to-ground photography, you can concentrate on the problem of objectionable image motion. Set the focus at infinity, and set the aperture wide open. There is no need to stop 4-17