CAMERAS 



95 



CD will have a height of XZ. The ratio of XZ to XF will be equal to the ratio of 

 BA to EC. A long-focal-length lens will focus the two objects in the plane X'Z', and 

 the ratio of the two image heights will be X'Z'/X'Y', which, by similar triangles, is 



Fig. 26. — Effect of moving close to object to increase image size. Viewed at normal 

 distance (10 in.) a print made under these conditions will appear distorted in perspective. 



equal to BA/EC. Therefore the perspective will be exactly the same in the two 

 cases. The images produced by the long-focal-length lens will be larger than the 

 images produced by the short-focal-length lens, but the angle subtended at the lens by 

 the two objects will be exactly the same in the two cases. The print made with the 

 long-focus lens should be viewed at a distance OX', while the print made with the lens 

 of short focal length should be viewed at a distance of OX. If the image XYZ is 

 enlarged so that it has the same dimensions as X'Y'Z', then it may be viewed at 

 the distance OX', and so far as perspective is concerned there will be no difference 

 between the two prints. 



The objection to the lens of short focal length is the natural tendency of the user 

 to move up close to the subject in order to get a large image. This is sure to 

 produce an exaggerated perspective. In Fig. 26 the relative sizes of the two 

 images on the final print will be XZ/XY = X'Z'/X'Y' = AB/EC when made with 

 the two lenses from the same viewpoint. If the short-focal-length lens is moved 

 closer to the image (Fig. 26), the ratio of the two images will be X"Z"/X"Y" = 

 AB/FC, with the result that the nearer object will be larger, when compared to the 

 farther object, than it appears in the print made from the longer focal-length lens. 



Example. — The relation between image distance, object distance, and focal length of lens is 



- = ^ 

 O ~ d- F 



where O = size of object; 



/ = size of image; 



d = distance of object from lens; 



F = focal length of lens. 



Assume two poles in the ground, 10 m. apart and 10 m. high. The camera is first placed 20 m. from 



the first pole. The focal length is 6 cm. (0.05 m.). On the print the nearest pole will have an image 



height determined by the above formula of 1000 cm./(L/0) = ^""^oo = 2.5 cm. The pole farther 



away will have a height of 1.66 cm. These two images will have a ratio of 2.5: 1.66 or 1.5. The image 



of the nearer pole will be 1.5 times as high as the one farther away. 



Now move closer to the poles so that the negative is made at a distance of 10 m. from the nearer pole. 



In this case the two image heights will be, respectively, 5 cm. and 2.5 cm., or the nearer pole will be 



twice as high as the farther pole. 



Proper Viewing Distance. — Prints must be viewed at the proper distance if the 

 perspective is to be natural. Consider two prints, one made with a short-focal-length 

 lens and the other with a long-focus lens. The short-focus lens was moved closer 

 to the object when the exposure was made to secure an image more nearly equal in 

 size to that of the other lens. If the print made with the short lens is held at the 

 same distance as the print made with the longer lens, the perspective will not be 

 natural, but if the smaller print is moved closer to the eyes, the perspective will seem 



