LIGHT FILTERS 309 



by breathing upon them and rubbing the surfaces with lens tissue, after making certain 

 that all the grit has been removed from the surfaces. Solid glass filters can be cleaned 

 by rubbing the surface with lens tissue dampened with denatured alcohol. This 

 procedure is not recommended for cemented filters as alcohol is a solvent for Canada 

 balsam; moreover the alcohol may cause the gelatin to swell if it reaches the cemented 

 edges, thereby destroying the desirable optical properties of the filter. Under no 

 circumstances should cemented gelatin filters or gelatin films be washed in water, 

 alcohol, or other liquid. Gelatin films may be cleaned by laying one surface down on a 

 clean sheet of paper placed on a hard fiat surface and rubbing the upper surface with a 

 soft material, such as flannel or preferably lens tissue. The surfaces of gelatin film 

 scratch very easily, and there is, therefore, a hmit to the amount of cleaning which 

 may be done by this method. Should it be necessary to cut gelatin film, the film 

 should be protected by placing it between two sheets of thin paper, and the three 

 thicknesses should be cut simultaneously with sharp scissors or a sharp edge such as a 

 razor blade. If dull scissors are used, the gelatin may chip and flake, especially if it is 

 brittle from age. 



Mounting of Filters. — Filters can be used in several positions on the camera, the 

 position determining the method of mounting. They may be used before the lens, 

 behind the lens, or immediately in front of the film. Although filters of mediocre 

 quality can be used if placed in this last position, this form of mounting has the dis- 

 advantages of requiring a filter as large as the film on which the latent image is 

 formed, and defects in the surface of the filter or specks of dust on its surface show up 

 on the negative. Filters behind the lens do not have these disadvantages but are 

 inaccessible. Before-the-lens filters are accessible for change and need be only slightly 

 larger than the lens barrel diameter. This form of mounting is almost universally 

 used at the present time. 



The position of the filter affects the focusing of the lens on the film, and this fact 

 must be taken into consideration where sharp negatives are required. If the filter is 

 placed before the lens, the plane of sharp focus will be slightly moved back from where 

 it is without the filter. For many cases, such as landscape photography, this change 

 in focus is not important, especially if the filter is very thin, as a gelatin film. But 

 where the sharpest images are required, as in copying line drawings, focusing should 

 be done with the filter in place. Since filters always cut down the amount of light and 

 since it is often difficult to focus with colored light, the use of "dummy" filters is 

 frequently resorted to. Dummy filters have no appreciable selective absorption, but 

 alter the image rays through the camera in the same way the usual filter does. There- 

 fore focusing can be done with such a dummy with ease, and the desired filter can 

 replace it when the photograph is being made. 



When the filter is placed on the back combination of the lens, the plane of sharp 

 focus with the filter in place is moved forward by about one-third the thickness of the 

 filter, so that focusing must be done with the filter in place. If the filter is placed 

 immediately in front of the photographic plate, the shift in focus will be negligible 

 when a gelatin film is used. But if a thick glass or cemented filter is used in a holder 

 in front of the plate, the bellows extension must be shortened by the thickness of the 

 filter. If this is not done, the image will be properly in focus on the filter and not on 

 the photographic plate. 



Filters may be mounted on the camera lens in two general methods. A verj'' 

 convenient way, especially for the amateur photographer having a small number of 

 filters of the same size, is to have the fUters mounted in circular metal cells which slip 

 on the camera lens. However, when a filter is to be used with several lenses of differ- 

 ent size, it is usually more satisfactory to use square filter plates and to support these 

 in a frame or holder fitting over, or held just in front of, the lens. Circular cells, as 



