TABLE 1. --Exposure table for conversion of foot-candles of available light 



to f-stops 



that is to be used in the actual work. Cali- 

 bration of the readout meter in footcandles 

 permits rapid f-stop computations (table 1), 

 Interpolation for various shutter speeds is 

 accomplished by reading the requisite number 

 of f-stop columns to the left or right in the 

 table. 



FILM 



Ambient light levels below 3 footcandles 

 dictated the use of a black and white negative 

 film with an ASA daylight index of 500. Forced 

 processing boosted this to effective indexes 

 of 1,000 and 2,000. 



Higher light levels will permit the use of 

 color films, permitting greater detail to be 

 observed. When color films are used, proper 

 underwater color filtering is necessary.' 



TITLING 



Titling or indexing of each film roll was 

 accomplished with the use of surplus L,M-I-- 

 (1) titler (fig. 24). Alternatively, a data card 

 can be held in front of the lens, and the camera 

 operated while on shipboard. 



RESULTS OBTAINED 



Several thousand feet of usable film were 

 obtained during midwater gear and fishing 



'Paul J. Kruse, Jr., 1962. An underwater photographic 

 surveillance system, paper given at the 91st meeting of 

 the Society of Motion Picture and Television Engineers', 

 May 1, 1962. 



trials. The footage proved invaluable in two 

 distinct but closely related phases of the study : 

 (1) determining the efficiency of the gear in 

 action and (2) studying the reaction of the fish 

 to the trawl. 



Gear Results : 



The use of metric photography is a new 

 tool in the study of trawl mechanics, and 

 the underwater photographic system can be 

 used to measure underwater distances ac- 

 curately. The following formula permits ac- 

 curate measurements to be made from the 

 image on a film or from a TV monitor: 



(F X Os) 



D = Is_ + 24% 



12 



Where: 



D = True underwater distance (corrected 

 for underwater magnification) in feet. 



F = Focal length of camera lens (inches). 



Os = Object size (inches). 



Is = Size of image on film (inches). 



Example: 

 It was necessary to find the horizontal 

 spread of a trawl mouth at a given speed. 

 Footage taken across the trawl mouth showed 

 an 8-inch diameter float on the far side.' 

 The float image is measured, under low 

 magnification, and found to be 0.0333 inches 

 in diameter. The lens has a focal length of 

 0.5 inches. 



(0.5x8 ) 



D= 0.0333 + 24% 



12 



12.5 feet 



13 



