2 
structed at the Old Delft Works in Delft, Holland, with an equivalent focal 
length of 12 inches (30 cm). A 70 mm non-perforated film is employed resulting 
in an image size of 2.5 X 2.5 inch 2 (6.3 X 6.3 cm 2 ). 
Resolution. 
It will be clear that the mirror camera here described with its outstanding 
correction of both monochromatic and chromatic aberrations will not appear to 
its full advantage when used with very sensitive films of poor resolving power. It 
was expected that optimum resolution of the combination optics-film would be 
attained by fulfilling the „balance of sharpness” condition i.e. that situation 
where the contributions of both optics and film to the total unsharpness are of 
the same magnitude. An extensive investigation in this direction was carried out, 
the results of which have been partly published elsewhere recently 2 ). A very good 
resolution was obtained with a film of appr. ten times lower speed than the well- 
known Kodak Super XX, in this case the Ilford Pan F film. Collimator photo 
graphs on this film showed a resolving power of over 100 lines per millimetre 
for high contrast targets (log contrast = 2), and 45 lines per millimetre for low 
contrast targets (log contrast —0.2). Photographs taken with this camera appeared 
to contain not less information on 2.5 X 2.5 inch 2 format as those taken with a 
high quality lens camera 9X9 inch 2 format. 
Field. 
The field of the mirror camera is limited by the required film curvature and 
this makes the camera not very practical for low altitude photography. However, 
an increase in ground coverage may be obtained by 
several methods, one of which seems particularly 
promising. The principle of this method is indicated 
in figure 2. The camera is mounted perpendicular to 
the direction of flight, with its axis horizontal. A 
flat mirror in front of the camera is rotating around 
a horizontal axis in the flying direction. This mirror 
is moved into a number of positions successively cor 
responding to slightly overlapping photographs. In 
this way a strip is photographed perpendicular to the direction of flight. 
The necessary number of mirror positions depends on the size of the camera- 
field and the required ground coverage. For the camera here described and an 
angular field of 60°, six mirror positions are sufficient. Assuming a flying speed 
of 500 miles per hour and a flying altitude of 40.000 feet, the requirement of an 
overlap of 60°/o of the successive strips results in an available time of appr. 3 / 4 of 
a second for one photograph and subsequent movement of the flat mirror into 
the next position. 
There is a small shift in the direction of flight depending of course on the 
time necessary to photograph each strip. A complete compensation of this shift is, 
if necessary, easily obtained. 
Film-cassette and shutter. 
The construction of the film-cassette and shutter differs somewhat from 
2 ) Bouwers A. “Resolving power in Photography”, Applied Scientific Research, Vol. B2, 1951.