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of exposure in a real camera. Lack of flatness cannot be deduced from measurements
on the negative, since the results must then include other random errors due to film
processing and other causes.
Cameras having a register glass have some advantages in this connexion, since
by using a réseau engraved on the glass some indications of the combined flatness and
processing errors can be obtained. Robinson 10 [11] has described a routine test carried
out on Ordnance Survey réseau cameras. At the conclusion of each sortie, five or six
extra frames are exposed. These are immediately processed and the positions of
réseau crosses are measured; the sortie is then rejected if the crosses have gross
position errors.
An earlier method, largely for research purposes, used gratings of fine transparent
lines on the register glass which were exposed to a multiple light source. Lack of
contact between film and grating is shown by thickening of the lines in the negative.
Areas of bad contact are quickly seen as dark patches, and semi-quantitative results
are possible by measuring the thickness of the line images.
The Royal Aircraft Establishment, Farnborough, has devised a technique
whereby interference figures can be formed between the film and the register glass.
By high-speed photography, the whole history of the film during an exposure can be
investigated. Typical results show that considerable time is needed for the film to
“flatten” and that there are residual movements for minutes after the pressure is
applied. The flatness at exposure is therefore variable from frame to frame. This
method is clearly a very powerful tool for the investigation of different camera
pressure-pad mechanisms for research purposes, although it cannot be used as a
routine check. It is hoped that a paper on the method will be presented at the 1964
Lisbon Congress.
For cameras with suction backs, no method has yet been described to measure
the film flatness by other than mechanical means. Usually, measurements on the
suction plate are made by mechanical gauging, and it is assumed that the film will
be in good contact and will be of uniform thickness.
Hallerfi 121 * quotes some measurements made upon seven makes of 9^-inch
aerial film. Variations in thickness of about 10/x were found, usually with the
variation periodic along the film at some 40-mm. period. These were presumably a
function of the machinery used for coating the film. These variations superimposed
on any residual departures from flatness of the backing plate could reach significant
values.
There is still a need for some simple method of determining flatness as a routine
check on cameras whether they have register glasses or not. The method should not
require the camera to be partially dismantled, i.e. special register glasses to be
fitted or lenses to be removed, and it should not require elaborate equipment either
to carry out the test or to measure the results.
10. Plate Flatness
Hallerfl 123 has described methods of measurement of plate flatness and given
some typical results. These and earlier results quoted by Hothmer suggest that
plates as supplied will have local areas with deviations of 20 ¡jl or more from mean
flatness. This at 45° incidence will lead to distortion errors of 10 /x. The method of
supporting plates in the focal plane is important as considerable flexures can occur,
even under their own weight. Helming 1133 has detected errors due to this cause in
photo-theodolites, while Oswal [14] has discussed the theory of flexure and shown how
various alternative methods of support compare.
383