The Dictionary Of Photography

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

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Divergent Rays                                  Dodging Negatives
"Let N denote the angle of inclination of the negative, and C that of the corrected copy. The other signs are those already described.
11 Correction is secured so long as
and
"It is obvious from these formulae that C is always greater
(c) Distortion of marginal lines or aber?'ation of the thickness of le?ises. If a square ruled on paper be examined with an ordinary reading glass it will be found that the lines instead of being straight will be curved. When a landscape or single lens is used with a stop in front of it, lines falling near the margins will be curved outwards, as with the stop in this position the lower part of the lens forms the image of the upper part of the object; with the stop behind the lens, the reverse is the case, and the distortion is pincushion shape. From this it is obvious that by placing a lens on either side of a diaphragm we get the two distortions curing one another, and hence the rectilinear or doublet lenses.
Other photographic distortions. See Anamorphoscope, Ana-morphosis and Shutters (p. 573.)
Divergent Rays in optics are those which continually recede farther and farther from one another, being the opposite to con-vergent. (See Lens.)
Dodging Negatives. There are few negatives which will not be improved by some after treatment. Under the heads of Intensification and Reduction will be found instructions for these processes which have for their general purpose the increase of, or reduction of, density respectively, but in many cases it is found that one portion of a negative is so dense in proportion to the rest that it will not print: to improve such a negative we may use the method of Harmonising Harsh Negatives (q.v.), or we may locally reduce it or intensify it. To locally intensify a negative
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