308 HANDBOOK OF PHOTOGRAPHY 



Supplementary Lens Attachments. — Ordinary supplementary lens attachments 

 are not filters at all, although their construction and mechanical use makes it desirable 

 to consider them along with filters, since they are used in the same manner as filters. 

 Such supplementary lenses usually fall into two classes: (1) those intended to increase 

 the effective focal length of the lens system thereby, at the same time narrowing the 

 field of view; and (2) those which decrease the focal length and increase the angle of 

 view. The first class of supplementary lenses reduces the speed of the lens system since 

 the focal length is increased for any given aperture. Conversely, the second class 

 of supplementary lenses increases the speed of the lens. 



Supplementary lenses which increase the focal length of the lens system are some- 

 times called portrait lenses; those which decrease the focal length are sometimes 

 referred to as wide-angle, copying, or reproduction lenses. A set of supplementary 

 lenses usually costs but a small fraction of the cost of the lens system in a camera, and 

 their use provides a lens system having several focal lengths and angles of view. 



These supplementarj'' lenses are usually mounted in metal cells which slip over the 

 camera lens, or are held on the camera lens by means of a metal-spring holder. 



Special-effects Equipment. — A wide variety of lens accessories is available under 

 this name including diffusion disks for giving a soft diffused effect without making the 

 image out of focus; fog plates for making negatives possessing the appearance of being 

 taken in heavy fog; duplicator disks, in which one-half of the disk is opaque and the 

 other transparent and permitting (by rotation of the disk) exposures of half of the film 

 at a time for trick effects, etc. 



CARE, MOUNTING, AND CEMENTING OF FILTERS 



Care of Filters. — When properly constructed, surfaced, and polished, filters are of 

 as high quality as optical lenses, and should be given the same careful consideration 

 accorded to a good lens. When not in use, the filters should be kept in a substantial 

 case or other suitable container which will keep the individual plates separated from 

 one another and in their proper place. By keeping filters in their proper container, 

 the chance of getting them dirty, scratching the surfaces, or otherwise marring their 

 optical properties will be minimized, and the plates will always be ready for use. 

 Filters should be kept in a dry place at normal room temperatures and should be pro- 

 tected from strong light, ultraviolet, infrared, heat, and similar radiations. Although 

 colored-glass filters are not ordinarily harmed by these radiations, the above pre- 

 cautions are especially necessary for certain gelatin and cemented gelatin filters which 

 are subject to change in their transmission characteristics with aging and which are 

 frequently unstable to heat and strong light. Excessive heat is likely to soften the 

 Canada balsam in cemented filters, and, if this occurs, the definition of the filter will 

 most probably be ruined. Plain gelatin films, without protective glass, should be kept 

 flat by placing the films in a clean white envelope or between sheets of soft paper and 

 pressing between parallel surfaces, as the leaves of a book. 



Cleanliness is necessary in handhng filters as well as in handUng lenses, and this is 

 especially true in the case of gelatin films because of the difficulty of cleaning the sur- 

 faces should they become soiled. Filters not mounted in cells should be handled only 

 by holding them by their edges or corners; they should not be grasped with the fingers 

 covering a portion of their surface. Some filters are protected at their edges with 

 binding tape, and this provides a satisfactory, if somewhat narrow, means of holding 

 the plates. 



Cleaning Filters. — ^Like lenses, filters should be cleaned by brushing their surfaces 

 with light, fine tissue paper, lens tissue, or a camel's-hair brush. If the filters become 

 so dirty that brushing does not suffice to clean them, they may be moistened slightly 



