OUTLINE OF PHOTOGRAPHY 5 



dimensional subject as might be seen with one eye of the observer were it placed at 

 the same position as the camera lens. 



Briefly, it may be said that photographic lens systems differ from other lens sys- 

 tems in that they cover a wide angle of view. Telescopes, microscopes, and other 

 objectives cover a field of view of perhaps 5 or 10°. A photographic lens system 

 usually covers a field of view of 45 to 60°; in wide-angle photography the field of view 

 may extend to 90 or 135°, and an extreme case has been recorded in which the field 

 of view was as great as 180°. Throughout this large range of view, the lens system 

 must be properly corrected for all forms of distortion, and all spectral colors must be in 

 focus on the same plane simultaneously. Moreover, provision must be made for 

 focusing the image on the photographic plate, although, in the simpler and less expen- 

 sive cameras for amateur use, the lens is fixed once and for all to take pictures with 

 reasonable sharpness of focus from infinity to some distance (usually between 6 and 

 20 ft.) near the camera. 



Shutter. — The shutter in cameras consists of an aperture of some sort through 

 which light from the image and lens passes on its way to the photographic plate. 

 The function of the shutter is to act as a light chopper, i.e., to open for a length of 

 time sufficient to permit a definite quantity of light to fall upon the plate, thereby 

 producing a latent image. The amount of time during which the shutter is open and 

 permits the passage of light to the film depends upon the intrinsic brightness of the 

 illuminated subject, the "speed" of the lens system, whether or not there is motion 

 taking place in the subject being photographed, and, if so, the speed of the object, 

 the distance of the object from the lens, and the direction of relative motion with 

 respect to the lens, as well as upon the speed or sensitivity of the photosensitive mate- 

 rial. The exposure is definitely associated with the "speed" of the lens, the intrinsic 

 brightness of the illuminated subject, and the duration of time for which the open 

 shutter permits light to pass through the lens to the photographic plate. 



Various types of shutters are made at the present time, and each of these has its 

 own peculiar characteristics, advantages, and disadvantages. There is no single 

 shutter which, under all conditions, will provide ideal light-chopping conditions. 

 Therefore, under certain circumstances, distortion of the photographed image will be 

 apparent, or various portions of the photographic plate will not be uniformly exposed, 

 thereby producing distortion by improper rendition of tone values. 



Photographic Sensitive Materials. — The most important component of the entire 

 photographic process, and the one which alone makes photography possible, is the 

 sensitive plate or film; all other equipment, no matter how useful, is supplementary and 

 of no avail without the photographically sensitive material. The characteristics of 

 photographic sensitive materials will be discussed in greater detail in Chap. VI. 

 If true tonal rendition is to be achieved in monochromatic photography, the amount 

 of silver deposit on the negative should be exactly proportional to the intensity of 

 illumination of the subject being photographed. The brightness range of most 

 ordinary subjects is not usually greater than 100 to 1 and this range can generally be 

 accommodated by average photographic negative materials. Where the silver deposit 

 of the negative is not proportional to the illumination intensity of the subject being 

 photographed, some tonal distortion will occur. Tonal distortion may also take 

 place in printing the paper positive from the negative, so that for accurate tone 

 reproduction both the negative and the print must have their exposure factors timed 

 (other factors also enter into the matter but will be discussed in detail later) properly to 

 produce a final result (print) having the same luminous visual intensity variations as 

 the original subject. 



Camera as an Integrated Unit. — The camera shown diagrammatically in Fig. 1 is 

 the device which contains the various essential mechanical and optical elements for 



