Since a yellow light filter removes the ultra-violet and much of the blue-violet light, it necessarily increases the exposure, because if we remove those rays to which the film is most sensitive, we must compensate for it by exposing the film for a longer time to the action of the remaining rays, and the amount of this increased exposure will be dependent both on the proportion of the violet and the blue rays which are removed by the filter and also on the sensitiveness of the film for the remaining rays (green, orange and red) which are not removed by the filter.
The number of times by which the exposure must be increased for a given filter with a given film is called the "multiplying factor" of the filter, and since the factor depends both upon the depth of the filter and upon the color sensitiveness of the film, it is meaningless to refer to filters as "three times" or "six times" filters without specifying with what material they are to be used.
It is always desirable that we should be able to give as short an exposure as possible; what is required in a filter is that it should produce the greatest possible effect with the least possible increase of exposure, so that a filter will be considered most efficient when it produces the maximum result with the minimum multiplying factor. To a certain extent the multiplying factor depends upon the result that is wanted; thus in order to get exactly the same proportional exposure when using a Kodak Color Filter with Kodak NC Film, as that obtained without it the necessary increase of exposure is ten times, but in fact the Color Filter is generally used for distant landscapes where haze is to be cut out, and for clouds against the sky, and under such conditions an increase of three times the normal exposure that would be correct for an ordinary landscape will give the most satisfactory results.
For many purposes, however, the Kodak Color Filter is too strong; the exposure when using it is so prolonged that it is not practical to use the Kodak without a tripod, and to meet these difficulties the Kodak Sky Filter has been introduced. (Fig. 116.)
Fig. 116. Kodak Sky Filter.
a. Made without a filter.
b. Made with a Wratten G filter.
In this filter only half the gelatine, which is cemented between the glasses, is stained with the yellow dye, the other half being clear, and the filter is placed on the lens with its stained half on top so that the light from the sky will pass through the stained half and the light from the landscape through the clear half of the filter. In this way the yellow dye reduces the density of the sky in the negative without greatly affecting the exposure of the foreground and enables us to get a rendering of clouds in a blue sky by cutting out a part of the very strong light that comes from the sky, while the exposure necessary is increased only to a small extent.
The sky filter is not suitable for the cutting of haze since its colored half does not cover the landcsape, which is the part of the field where the haze occurs. Its use is confined to that suggested by its name.
When it is desired to make blue photograph somewhat darker than can be done with the Kodak Color Filter the Wratten K2 should be used, and for recording still more contrast, which is sometimes wanted in pictures of extremely distant landscapes that are under haze, the Wratten G filter is very valuable. Thus, distant mountains and all other distant landscape scenes (Figs. 117a and 117b) may be photographed through a strong yellow filter by giving the necessary increase of exposure, with a Kodak mounted on a tripod. The K2 will require an increase of exposure of about twenty times and the G of one hundred times on the Kodak Film.
a. Original Definition.
b. Definition after screwing up tightly in cell.
In order that filters may not spoil the definition it is important that the glasses between which they are cemented should be of good optical quality. This is very carefully controlled in the case of the Kodak and Wratten filters, which are all measured by an instrument specially built for the detection of optical errors introduced by filters. The filters have to be mounted in the cells so that they cannot be strained by pressure being put upon them, since if they are squeezed the balsam with which they are cemented together will be displaced and the definition will be spoiled. (Fig. 118.)
Filters should be treated with care equal to that accorded to lenses. When not in use they should be kept in their cases and on no account allowed to get damp or dirty. With reasonable care in handling they should never become so dirty as to require other cleaning than can be given by breathing upon them and polishing with a clean, soft piece of linen or cotton cloth. A filter should never be allowed to become wet under any circumstances, because if water comes into contact with the gelatine at the edges of the filters it will cause the gelatine to swell and so separate the glasses, causing air to run in between it and the glass.
The dyes used for the Kodak and Wratten filters are quite stable to light, and no fear of fading need be felt. The filters, however, should be kept in their cases when not in use in order to protect them.