In addition to the question of photographic quality there arises the matter of shrinkage and distortion. These are negligible with plates, but are a more or less unknown quantity in film. Irregular shrinkages of as much as two per cent. are found on experiment. This defect, of course, would be an obstacle only in exact mapping work.
Positype Paper.—The need sometimes arises in military operations to secure prints ready for examination within a few minutes after the receipt of the negatives. Even the 15 or 20 minutes within which a negative can be developed and a wet print taken may be considered too long. While such occasions are probably more apt to occur in popular magazine stories than in actual warfare, it is important to have available methods of producing prints with an absolute minimum of delay. This need is met to some degree by a direct print process, commercially exploited under the name of “Positype.”
In this process the exposure is made directly on a sensitized paper or card, which is developed, the image dissolved out, the residue exposed, and again developed; thus furnishing a positive picture (reversed right and left). The time necessary to develop a print ready for examination need not be more than three minutes. Only a single print is available, but this is all that would be called for under the extreme conditions suggested. If later, copies are desired they may be made by the same process.
Plates and Films Found Satisfactory for Aerial Work.—The following plates and films have been found particularly good for aerial photography. The list is not intended to be complete. Furthermore, it may be expected to be soon superseded, as the efforts of various manufacturers are directed toward developing special aerial photographic plates.
Among orthochromatic plates: The Cramer Commercial Isonon, the Jougla Ortho.
Among panchromatic plates: The Ilford Special Panchromatic, the Cramer Spectrum Process.
Film: Ansco Speedex, Eastman Aero.
CHAPTER XIX
FILTERS
The Function of Filters in Aerial Photography.—The use of color screens or filters has been very common in ordinary landscape photography, for the purpose of securing approximately correct renderings of the brightnesses of colored objects. Plates of the non-color-sensitive type have their maximum of sensitiveness in the blue of the spectrum (Fig. [105]) and in consequence blue skies photograph as white, while other colors are likewise reproduced on a totally wrong scale. Filters for correct brightness rendering are calculated for a given color sensitive plate so that the resultant reaction to the light of the spectrum copies the sensitiveness of the eye, which is greatest in the yellow-green. Such filters for use with the common orthochromatic plates are of a general yellow color.
Filters for aerial work are meant to serve quite a different purpose. Correct tone or color rendering is of quite secondary importance to another use of filters, namely, to cut or pierce aerial haze. It is quite a matter of accident that the same general color of filter is called for both to give correct color rendering and to pierce aerial haze, namely, yellow. Yet on closer analysis it is found that quite different types of yellow filter are demanded, spectroscopically considered.