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NEW INVESTIGATIONS OF CURVED FOCAL PLATES 
A. Frey Samsioe, Dr. Techn.. and Percy H. Tham, Dr. Tech., Stockholm. 
At the 6th International Congress of Photogrammetry, held in the Hague 
in 1948, the lens distortion problem was thoroughly discussed. Since then, the 
steadly increasing requirements for accuracy, especially with regard to the use 
of photogrammetry for large-scale surveying, have emphasized the importance 
of this problem. 
Present-day compensation methods must therefore be carefully considered. 
Porro-Koppe’s principle as developed in the plotting machines as well as the 
principle with compensating plates have certain obvious disadvantages. Thus, 
when employing Porro-Koppe’s principle, is is not possible — without special 
arrangements — to use new cameras with the older type of plotting machines. 
Further, it is not possible to design Multiplex equipment, which is used very 
much in Sweden, for this principle, owing to its special projection system. 
Compensating plates on the other hand as a compensating device is an 
indispensable accessory to all cameras. In view of the extremely high accuracy 
required, modern objectives also need compensating plates. Their distortion 
may be only 10 /¿or less, yet the influence of this upon the stereo-measurement 
cannot be disregarded in present-day surveying. Furthermore, it is well known 
that, for lenses marred by distortions amounting to 100 /¿, the compensation 
cannot be made by merely one plate — thus, for instance, a Topogon lens with 
a compensating plate will give residual parallaxes of about 10 /¿. 
It would thus seem advisable to give closer consideration to this very com 
pensation. The author’s principle, i.e. the curved back plate, gives a correct 
image without any compensation. An image from lens distortion may be used 
at once and for all purposes. The one objection that may be raised against the 
design is that the film must be deformed in three dimensions. But for a modern 
objective, say the Wild Aviogon or the Bausch and Lomb Cartogon, the curve 
will have a total aplitude of only 0.05 mm. All tests in Sweden have clearly 
demonstrated that the design comes up to every expectation. The plate is easy 
and cheap to manufacture, and makes possible the use of more luminous wide- 
angle objectives with a larger aperture, which are cheaper than the present 
complicated lenses. Having arranged a test field of photogrammetry in Sweden 
this year, the use of the curved plate will be checked thoroughly, by means of 
the control points. The method is described in Photogrammetric Engineering, 
1950. 
To-day, one camera with curved back plate is in function in Sweden, one 
is rebuilt for Norway and two cameras will be rebuilt for the United States, 
one for the Government and one for another Nation.