DEFECTS IN NEGATIVES AND PRINTS 617 



being off center from center of camera, and (h) using lens which will not cover the 

 entire negative area. 



3. Prevention. — Since these defects are inherent in the lens system itself, there is 

 usually nothing that the photographer can do which will prevent or correct these dis- 

 tortions without changing his lens system. Every lens contains some defects and 

 distortions although in most cameras these have been reduced to the point where they 

 are of no consequence for the ordinary run of work. In certain classes of work, where 

 high precision, sharp definition, and absolute freedom from distortion are required, 

 the best possible lenses will be required. Under (g) the photographer may be able to 

 align the lens and camera so as to produce even illumination if the defect is caused 

 simply by improper alignment. In (h) the difficulty may be due to use of a lens of 

 insufficient covering power for the plate or film used, rather than an inherent lens 

 defect. Replacing by lens of greater covering power, or using a smaller plate which 

 the lens will cover, will correct this trouble. 



4. Remedy. — The remedy for a defective optical system in a camera consists in 

 obtaining another but satisfactory lens system, or returning the defective lens to the 

 manufacturer for whatever grinding, polishing, or other corrections may be required. 

 Items ig) and {h) may be corrected by the photographer as enumerated under 

 Prevention, 



Fogged Negatives 



Fog may be defined as a uniform darkening of the negative due to a deposit of 

 silver which does not form a part of the image and which may partially or completely 

 obliterate the image. Fog tends to increase the density and decrease the contrast of 

 the negative. Fog may be general, in which case the entire negative is more or less 

 uniformly darkened, or it may be local, in which case only a part of the negative is 

 darker than it should be. 



Aerial Fog. 



1. Distinguishing Characteristic. — General veiling of the negative or portions of the 

 negative exposed to air during development. 



2. Cause. — Exposure to air during development. The tendency toward aerial fog 

 is especially marked with certain freshly prepared developers or by developers contain- 

 ing formaldehyde. Aerial fog is produced only with hydroquinone under ordinary 

 processing conditions, but developers such as pyro, glycine, and para-aminophenol 

 produce aerial fog in the presence of traces of copper or tin salts. Even slight traces 

 of salts of copper produce decided aerial fog. Fogging increases as the proportion of 

 alkali and hydroquinine is increased in metal-hydroquinine developers. It is especi- 

 ally bad in some methods of processing motion-picture film. 



3. Prevention. — Aerial fog may be reduced by the use of desensitizers, by the 

 addition of sulphite or bromide to the developer, or by the use of partially oxidized 

 developers. Under ordinary conditions, the addition of 2 parts of pinakrytol green 

 per 1,000,000 parts of developer will prevent aerial fog. A predevelopment bath of 1 

 part of pinakryptol green to 25,000 parts of water is also effective in preventing aerial 

 fog. 



4. Remedy. — Sometimes reduction may be resorted to, but frequently there is no 

 remedy. 



Dichroic Fog. 



1. Distinguishing Characteristic. — Two-color fog which appears as green or reddish- 

 green sheen by reflected light and red by transmitted light. 



2. Cawse.— This type of fog is produced by a deposit of finely divided particles of 

 silver, the size of the particles determining the color as seen by transmitted light. It 



