520 HANDBOOK OF PHOTOGRAPHY 



2. Cause. — Presence of developable grains of silver halide in the unexposed por- 

 tions of the negative. Probably due to (a) contact of negative with fogging materials, 

 (b) deterioration of negative material with age, or (c) as a result of manufacturing 

 operations. 



3. Prevention. — (a) Store so as to be free from light, undue heat, or other radiations 

 or from gases and fumes, (b) Use materials within the expiration date established 

 bj' the manufacturer. Store in cool, dry atmosphere, (c) There is no remedy in this 

 case, although the amount of emulsion fog due to manufacturing processes should 

 never be appreciable with products made by a reliable concern. 



Light Fog. 



General fog may be produced by light as well as by chemical action. Light fog is 

 the more common tj'pe of fog, especially if a properly compounded, fresh dcA'eloper of 

 chemical purity is used, 



1. Disti7iguishing Characteristics. — General fog or darkening of the negative; also 

 local fog which may appear as streaks, often with straight edges. 



2. Cause. — In general light fog is caused by light striking the negative prior to 

 fixation in the hypo bath. The manner in which the light may strike the photosensi- 

 tive material varies considerably, but among the possibilities may be mentioned: 

 {a) fog produced wdthin the camera due to imperfect, old, or loose construction which 

 permits passage of light through various parts or through joints; (6) by the improper 

 use of camera equipment, such as inserting the slide of the plateholder by one corner 

 instead of by the entire edge; (c) use of camera and accessory fittings not intended to 

 be used together, such as incorrectly fitting plateholders; (d) loading or unloading the 

 photosensitive material in light which is too strong; (e) by using too strong a safe 

 light in the darkroom or a safe light with filter unsuited for the emulsion or by permit- 

 ting the negative to be exposed too long to the darkroom lamp; (/) by chemical emana- 

 tions from the finish of camera parts. 



3. Prevention. — In general, the best prevention for light fog is to handle and process 

 the photosensitive material in sufficientlj^ subdued light (or in the case of panchromatic 

 emulsions, in complete darkness), so that light radiations can produce no fog. Pre- 

 ventive measures for items (a) to (e) above will suggest themselves when the true 

 cause of the fog has been determined. In the case of (/), exposure of camera parts to 

 strong sunlight and painting interior of the camera parts with a weak solution of potas- 

 sium permanganate have been found helpful. 



4. Remedy. — Reduction may sometimes be useful in remedying general fog. 



Local Fog. 



1. Distinguishing Characteristic. — Fog or uniform darkening of parts of the nega- 

 tive in a manner having no relation to the photographed image. 



2. Cause. — Same as Light Fog (see above). 



3. Prevention. — Same as Light Fog; see above. A frequent cause for local light 

 fog is pinholes in the bellows of the camera. In this case, the pinholes may be 

 detected by placing a lighted electric lamp inside the bellows, and watching for the 

 .spots of light in a darkened room. The pinholes may be repaired by cementing a piece 

 of thin black opaque leather or cloth over the hole. 



4. Remedy. — Local reduction of the more dense portions of the negative may 

 sometimes be helpful. 



Incorrect Contrast 



It is sometimes difficult for the beginner to distinguish between incorrect density 

 and incorrect contrast in negatives, especially since a given negative may possess both 



