A speed-rating method for aerial film is known as Aerial Film Speed (AFS). This speed-rating system is adopted by the American National Standards Institute (ANSI) and is used to establish the speed of aerial film. Effective Aerial Film Speed (EAFS) is used to describe the actual aerial film speed that results from processing film through any process other than the one specified by ANSI. Aerial Film Speed and Effective Aerial Film Speed should not be confused with ISO speed or exposure index (EI). They are NOT equivalent. When using ground pictorial films that are assigned an ISO speed, such as Vericolor, Ektachrome, or Technical Pan, you should conduct tests to determine which film speed settings for your camera or light meter will produce optimum results.

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 reflecting and scattering the shorter 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. IA 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 UV 16, or IA 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.