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

Advanced Photography Course






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