720 HANDBOOK OF PHOTOGRAPHY 



Most operators consider that the pilot, who has the more comprehensive view, is in a 



better position to determine this than the photographer, whose head is down looking 



through the view finder. 



As the flight proceeds down the strip, the photographer should watch through his 



view finder to pick up any variations in drift. Frequently he is able to detect this 



before the pilot is and should have a signal by means of which he can notify the pilot 



in case the crab changes abruptly. The photographer 



i^j^ 1/ne'^'^ "" "* must also watch the level bubbles on his camera, must 



frequently check the time interval if he is shooting by 



stop watch, or must constantly watch his finder, if he is 



shooting in accordance with the image travel between 



two predetermined lines on his finder. 



Previous to the flight, the pilot should arrange a 



sight by means of which he can determine where the 



Level bubble -->^^!^ ^ center line of the next adjacent strip may fall. One 



I simple way of arranging this is to cut out a wooden 



T^ o o- Ui- ui 1 block with a straight side which can be placed against 



Fig. 8. — Sighting block. ^ , , • . • i i , • i • -in 



the window of the ship (provided the window is vertical) 



with one edge of the block angling away from the window along which he can sight 



(Fig. 8). This overlap block can be laid out as follows: 



If a picture 9 in. wide is to overlap the picture of the adjacent strip by 30 per cent, 

 we previously determined that the strip spacing is 6.3 in. at the scale of the picture. 

 Assume now that a camera of 8.25-in. focal length is being used, then 8.25 divided 

 by 6.2 equals the proportion of the base to the altitude of the triangle along which the 

 pilot will sight. In the illustration (Fig. 8) a simple form of block is shown in which A 

 represents the focal length and B represents the strip separation. 



The pilot should endeavor to line up sights on the airplane itself, so that he does 

 not have to use the block continuously. For example, he may possibly be able to 

 sight across the window sill to a certain point on the tire or the wheel. He will find 

 it helpful if he frequently makes this sight, as he goes down a strip and remembers 

 these points as the center line of the next adjacent strip. Such a procedure will prove 

 almost indispensable in determining the amount to move over at the end of the strip, 

 unless he is so fortunate as to have country with an abundance of detail and excellent 

 maps so that he may select the starting point of each strip by comparing the map with 

 the ground. 



The work should be planned so that, insofar as possible, it is not necessary to 

 change rolls of film in the middle of a strip. It is usually economical to change rolls of 

 film at the end of the strip, even though 10 or 15 exposures may be wasted. The value 

 of this wasted film is usually less than the cost of operating the airplane while the pilot 

 circles around waiting for the photographer to change the film. Furthermore, there 

 is always a strong possibility that the pilot will not be able to pick up the same line 

 with sufficient precision to give an acceptable connection. 



Laboratory Operations. Developing the Film. — Many different formulas are used 

 in the development of film, depending upon the operator, type of country, and the 

 purpose for which the film is to be used. In country which is tremendously con- 

 trasty, such as alternate green fields and very light-colored soil, a soft developer, 

 such as a borax, is preferable. On the other hand, if the country has a monotonous 

 character, such as desert or timber, the film should be developed with a maximum of 

 contrast, such as can be obtained with a pyro developer. 



Making the Prints. — In making the prints from the film negatives, it is advisable 

 to have available the full range of contrast in contact papers, as frequently films in the 

 same flight will gi^^e the best results on widely varjdng contrasts of paper. 



