The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Distortion
short focus, compared to the size of the plate, have been used. In such a case near objects look exaggerated in dimension compared to others more distant, when the prints are examined at the distance of normal vision ; whereas if the pictures are viewed from a distance equal to the focus of the lens this is not seen; hence the distortion is rather relative to the point of sight than absolute.
(d) Distortion of parallel lines. Whilst this is strictly speaking a distortion of perspective, it is better to consider it as a separate subject, because it is solely due to a misuse of the
Fig. 45-
camera. When a lofty object has to be taken and the camera is tilted upwards, unless the swing back be used, what should be vertical parallel lines in the print will appear to- converge as in fig. 45. It is quite obvious that if the photograph of a building which is distorted, as shown by fig. 45, is rephotographed, the dis-torted original may be so tilted (the top being nearer to the camera than the bottom) as to make the sides parallel in the reproduc-tion ; a method of correction which is often resorted to. Writing in The Amateur Photographer for November 28th, 1901, Mr. W. Piper points out that such a method over-corrects the heights, and he gives the following rules for such accurate correction as may be necessary when architectural measurements are to be taken from the photograph. Mr. Piper says : - " To correct con-vergency and vertical distortion together, both negative and copy must be inclined to certain definite angles, which can be found from formulae that I shall presently give. Three factors must be known to determine the angles, and these are -
(1) " The tilt of the camera when taking the original negative;
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