Aerial Photography 
Cameras: Color aerial photography for mapping requires 
a precision cartographic camera with the highest quality wide- 
angle color corrected lens. This same equipment is needed, of 
course, for the best quality panchromatic photography but 
color is somewhat more demanding with regards to evenness of 
illumination over the entire focal plane. In addition to color 
correction, the lens must provide good illumination at the cor 
ners as well as at the center. Otherwise, the star filter, that is 
the anti-vignetting filter, that must be used to eliminate the 
hot spot at the center of the photograph, will have to be so dense 
that there will be too little light for adequate exposure of the 
film. Underexposure of the film can be compensated for in the 
laboratory much more readily in the case of panchromatic pho 
tography than with color. While we have not tested other 
cameras, exhibits and manufacturers statements show that 
several manufacturers are now making cameras with lenses that 
meet these general specifications and are adequate for color 
photography. 
In color photography, the filter must also cut out the ultra 
violet and part of the blue light. The peach shading of the 
optical glass of these filters serves to screen out the ultraviolet 
radiation while also depressing a portion of the blue spectrum. 
We use two types of these filters; one cuts off wave lengths 
below 380 millimicrons and the other eliminates wave lengths 
below 420 millimicrons. With the color films available today, we 
are able to use the “420” filter for about 90% of our color pho 
tography but use the “380” for early morning and late afternoon 
photography. Our photography for nautical charting is partly 
over water areas and we have to pay careful attention to sun 
spots which I will discuss in another section. For this reason, 
we tend to avoid photography in the middle of the day. 
In reference to the requirement for geometric fidelity, I 
want next to discuss what we might call “differential color 
magnification”. In addition to the stability of the film base, the 
metric quality of the aerial photograph is affected by an in 
escapable lens defect frequently referred to as differential color 
magnification. This effect is present to the same degree in both 
panchromatic and color photography but it can be corrected for 
through the use of three color lens distortion compensation 
curves when the color of the object is known as it is on color 
photography. This effect is small and would not deserve men 
tion except for the fact that the layer structure of color film has 
been a source of suspicion in some photogrammetrists’ minds. 
Actually, the order of coating of color film emulsions happens 
to be such that the thickness of the emulsions reduces or com 
pensates for differential color magnifciation, giving a net im 
provement of metric quality when color film is used. This may 
also account for the greater ground resolution found with 
modern color films. 
Color Films: Figures A and B show the spectral sensi 
tivity curves for the two color films that we are using today, 
Ansco FPC-289 and Eastman MS Ektachrome. 
Exposure control is critical in all aerial photography and 
especially so with color. On the basis of experience, we have 
developed 4 separate exposure tables, or ASA charts, ranging 
from ASA 125 to ASA 250. The aerial photographer is told 
which chart to use with each roll of color film. Table I shows 
the chart for an ASA rating of 250. 
(Evaluation Density 0.70 Net) WAVE LENGTH 
Figure A 
SPECTRAL SENSITIVITY 
EASTMAN MS EKTACHROME 
Figure B 
COAST AND GEODETIC SURVEY 
EXPOSURE CHART—ASA 250 
DeJur Meter Readings—Camera 1/250 Second 
Lens Apertures 
5.6 
6.3 
8 
11 
16 
Light Water (Shoal) 
50 
75 
100 
200 
400 
Dark Water (Shoal) 
15* 
35 
75 
150 
300 
Medium Deep 
25 
40 
5C 
100 
200 
Deep Water 
15* 
20 
30 
60 
120 
Beach 
50 
75 
100 
200 
400 
Light Trees 
35 
50 
65 
130 
260 
Dark Trees 
20 
30 
40 
80 
120 
Farm Land 
50 
75 
100 
200 
400 
City 
35 
50 
65 
130 
260 
Note: Readings obtained using a Weston meter 
should be halved. 
* Meter readings lower than 20 result in under 
exposure. 
TABLE I 
The photoelectric cell component of the exposure meter is 
mounted outside of the aircraft body in a vertical position as 
near as possible to the lens of the camera. The light interrogator 
is pointed at the target covered by the center crosshairs of the 
3