that a film which has been either slightly over or under exposed will 

 produce a picture of poorer quality than a film which has been properly 

 exposed . 



Assuming a film with a particular emulsion speed rating or exposure 

 index, the following three factors affect the exposure of the film: 

 (a) the light intensity of the scene to be photographed; (b) the f-stop 

 or lens opening of the camera; and (c) the shutter speed or the amount 

 of time that the light from the scene is allowed to reach the film. 

 Since photographs are produced by the light reflected from the scene to 

 be photographed, the intensity of this light becomes the important factor 

 in properly exposing the film and it is measured with an exposure meter. 

 The f-stop or lens opening determines the area over which light is 

 allowed to enter the camera, small f-stop values indicating a relatively 

 large area for light entrance and large f-stop values indicating a 

 relatively small area for light entrance. F-stop values on most czuneras 

 are arranged so that increasing the f-stop one stop halves the light 

 entering, the camera (see Table 1). Here it should be noted that f-2.5 

 in the U. S. numbers and f-6.3 in the English system are not full stops. 

 Also, some cameras may start with f-4.5 from the Continental system and 

 skip to the f-6.3, 5-11, etc., of the English system, or start with the 

 f-4 of the English system and skip to the f-6.3, 5-9, etc., of the 

 Continental system. Thus, at some point in the lower range of the f- 

 stop values there is more than one full f-stop between the graduations 

 on the camera scale. The shutter speed determines the time that the 

 light is permitted to reach the film. Of the four factors affecting 

 film exposure, the one over which the photographer has the least control 

 is the reflected light intensity from the scene. Hence, the proper 

 film exposure is usually obtained by varying the three other factors in 

 such a way that the proper amount of light reaches the film. 



In order to properly expose a film, the ASA rating of the film 

 being used is set on the proper scale on the light meter. The light 

 intensity of the scene to be photographed is then read with the light 

 meter by pointing the photo cell of the meter at the scene and slightly 

 downward to eliminate most of the light from the sky. The light value 

 obtained is then set into the proper scale on the meter. The f-stop 

 scale and shutter speed scale of the light meter will then show 

 combinations of f-stop openings and shutter speeds which may be used 

 to obtain the proper exposure. In determining which combination of 

 f-stop opening or shutter speed which will be used, it is usually 

 better to use a shutter speed of l/50th of a second or faster and to 

 try to stay somewhere in the middle of the range of the f-stop openings. 



