CHAPTER IV 



CAMERAS 



By Keith Henney 



BASIC ELEMENTS 



Any camera must have the following parts: a lens to form the image upon the 

 sensitive material; a holder for the sensitive material; a lighttight enclosure (fre- 

 quently a tube or bellows) to cover the space between the lens and the sensitive mate- 

 rial; a shutter to open and close the lens aperture for the desired exposure time; a 

 finder to show what is being photographed. 



The simplest camera, e.g., the popular box camera, has these elements in exceed- 

 ingly simple form. The lens is fixed focus; the shutter is a simple flip-flap arrange- 

 ment that makes an exposure with each push of the shutter lever whether up or down; 

 the film runs over a pair of rollers as it is taken from the unexposed film spool and is 

 wound up on the take-up spool. 



To these basic elements other accessories and convenient adjustments may be 

 added. The lens may have an adjustable diaphragm so that the amount of light 

 admitted to the film or plate in unit time and the depth of focus may be controlled. 

 The lighttight enclosure (the bellows) may be extensible so that the lens-film dis- 

 tance may be varied as required for focusing images at greater or lesser distances 

 from the lens; the shutter may have various speeds so that exposure may be controlled 

 independently of the aperture opening, and a spirit level may be added; the front 

 board carrying the lens may be adjusted up or down and sideways or swing hori- 

 zontally and vertically from some median position; the back which holds the film 

 or plate may swing about a vertical or horizontal pivot ; the bellows may have double 

 or triple extension for photographing objects very near the lens and so on. 



If the camera is to be focused, there must be a focusing scale or some other method 

 must be provided for determining the correct distance of lens-to-film for a given 

 object-to-lens distance. 



Focusing is accomplished in several ways. The lens-film distance may be adjusted 

 by extending or closing the bellows, or by screwing the lens and shutter into, or out of, 

 a helical mount. The front lens only may be adjusted as to lens-film distance. 

 Finally, the front lens may be removed completely for close-ups. The focal length 

 of a camera lens may be increased or decreased by using either the back or front ele- 

 ments singly (if so designed) or by the use of accessory clip-on lenses. 



A variable diaphragm opening may be provided by a simple series of holes in a 

 metal slide which may be moved in front of the lens, or it may be continuously variable 

 in diameter as in an iris. 



The speeds at which the shutter may be opened and closed may be few or many; 

 or the speed may be continuously variable over a wide range. Inexpensive hand 

 cameras for amateur use seldom have shutter speeds slower than .^5 sec. because of 

 the impossibility of getting snapshots at slower speeds without accompanying move- 

 ment of the subject or camera during exposure. These cameras seldom have shutter 

 speeds greater than Koo sec. In cheap cameras the actual periods of time during 

 which the shutter is open may vary widely from the marked speed. On the other 



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