CAMERAS 69 



hand, more expensive shutters may have numerous speeds, and the actual periods of 

 opening may correspond fairly closely with the markings placed on the shutter by the 

 manufacturer. 



The shutter may be an iris type or of the focal-plane type. These are described 

 more fully in the section on shutters. 



The enclosure between lens and film must be lighttight and in the focusing types 

 of camera must be flexible. The simplest enclosure is that of a box camera. It is 

 rugged and rigid and enables the lens to be maintained in a fixed position with respect 

 to the sensitive material. In other types of camera the bellows is a molded or metal 

 section into which another molded or metal section turns. With lenses of short focal 

 length the variable lens-film distance is not very great, but the necessary accuracy of 

 adjustment may be increasingly great. 



The most common form of lighttight enclosure is the leather or composition 

 bellows. If images are to be photographed natural size, the bellows must extend to a 

 length equal to at least twice the focal length of the lens. When the bellows is not fully 

 extended, it tends to sag and to cut off the edges of the picture. Therefore hooks are 

 usually provided to hold up the center of the bellows when the lens is focused on 

 distant objects, or when the camera is closed. These hooks engage with eyes on the 

 camera frame automatically when the camera is closed and disengage when the 

 bellows is extended beyond the point where support is needed. 



The scene or object to be photographed is located in a view finder in the smaller 

 cameras and upon a ground-glass screen placed in the focal plane in larger cameras. 

 View finders are of several types as described on page 80. 



CAMERA TYPES 



Pinhole Camera. — The earliest, and simplest, form of camera uses a pinhole 

 instead of a lens. Although it is capable of producing very beautiful landscapes of 

 great softness, the pinhole camera is of academic interest only at the present time. 

 A minute hole is punched in a sheet of metal or other opaque material and is placed in 

 front of a lighttight enclosure at the rear of which is a screen on which the ima.ge is 

 allowed to fall. The sensitive material may be placed upon this screen. 



The pinhole has some advantages over the best of lenses. It suffers no distortion. 

 It has infinite depth of field. It will cover a very wide angle, 125° compared to the 

 75 to 90° covered by a modern wide-angle lens. Photographs made with the pinhole 

 have apparent depth that often compares most favorably with stereoscope camera 

 pictures. Furthermore the pinhole camera is cheap! 



The disadvantages are the excessively long exposures necessary and the fact that 

 "wire sharpness" is not possible. 



In Fig. 1 will be seen a side view through a pinhole camera. Point sources of light 

 are not brought to a point focus as with a converging lens. A point source produces a 

 cone of light, the dimensions of a cross section of the cone depending upon the size of 

 the aperture. If the pinhole is circular, the cone will be circular; and if the hole 

 is square, the point source will become a small square of light when it falls upon the 

 screen. These circles (or squares) of confusion increase in size as the screen is moved 

 away from the hole; but, since the image is enlarged at the same time and to the same 

 degree as the enlargement of the circle of confusion, the relative sharpness of the 

 image for a given pinhole is independent of the aperture-screen distance. The size 

 of the image is increased by increasing the distance between pinhole and screen. The 

 time of exposure will be directly proportional to the square of the distance between 

 pinhole and screen. 



Increasing the size of the hole will increase the illumination but will also increase 

 the circles of confusion so that the sharpness of the picture will suffer. Decreasing 



