International photographer (Jan-Dec 1934)

Ten The INTERNATIONAL PHOTOGRAPHER May, 1934 Miniature Camera Photography \ILTER TALK: With summer and the much thought-of vacation almost upon us, a check-up of accessories to add to the equipment should be taken, notably filters. Miniature camera users represent the advanced type of amateur photographer, and it is safe to say that at least 90 per cent of baby camera owners possess one or more filters. The necessity of the filter for successfully recording certain subjects is well recognized by photographers, but it is taken in a matter-of-fact way. They employ yellow filters to compensate for the extreme blue sensitivity of the film they are using, enabling blue skies, sea scenes, etc., to be photographed in a successful manner. A clear understanding of the manner in which the filter functions, or the correct use of filters of different color, or density, is lacking. The usual yellow filter is quite prevalent amongst miniature owners' equipment, but few photographers will be found to have a series of yellow filters of different densities, or perhaps the new green filters, or such filters as the 23-A, 29-F, or 72-Gamma, etc. As its name implies the filter allows certain wavelengths (colors) of light to enter the lens while retarding others, in this manner compensating for the unbalanced sensitivity of the emulsion being used, except in cases where special effect filters are employed, which produce exaggerated results. The filter holds back a certain percentage of the actinic light necessitating a longer exposure in order to correctly expose the film, hence the filter factor. The deeper the color of the filter the more light it holds back, and the greater is the filter factor. The latter is also dependent upon the color sensitivity of the emulsion. The more sensitivity the film possesses to the color of light the filter admits into the lens, the shorter will be the required augmentation of the exposure. In order to easily understand the action of the filter it is advisable to refer to the three primary colors of light — red, green and blue-violet. Yellow light is a combination of red and green light. The filter allows the light of its own color to enter the lens and retards the other colors. If we slip a yellow filter over the lens it will allow red and green light to pass through it, since these are the components of yellow, and it will retard a portion of the blue-violet light according to the density of the filter. Thus a Kl filter cuts off only a portion of blue and violet light, whereas the denser K2 filter eliminates all violet light, and a large portion of blue light. Now let us assume we employ a red filter, which according to light theory allows red light to pass through it, cutting off the other two primary colors of light — blue-violet and green. By referring to an actual transmission graph of a red filter, such as the F filter, we find that it eliminates all violet, blue, green and yellow light, allowing only orange and red light to pass through it. When such a filter is employed to photograph the sky the latter will appear practically black in the finished print. Normally the sky is blue in color and since blue will not pass through the F filter the sky will reproduce in the manner described. Correctly used a filter of this type produces artificial night effects. Lately there have appeared green glass filters which are classed as being suitable for both orthochromatic and panchromatic films. In filtering for an orthochromatic I ;:3ft. By Augustus Wolf man emulsion we are mainly concerned in eliminating blueviolet light. The usual orthochromatic film possesses a large excess of sensitivity to blue-violet, and is also capable of recording yellow and green light to a lesser degree. The green filter cuts off the blue-violet and red light. It serves its purpose with orthochromatic films because of its elimination of blue-violet. Since ortho films are not sensitive to red the elimination of that color in this case is of no concern. Most modern super speed panchromatic emulsions usually exhibit an excess of sensitivity to red, therefore the new green filters are suitable in this case since they eliminate a portion of red light besides blue-violet. Green glass filters also represent an improvement over the yellow glass filters commonly employed. To understand this point let us digress for a moment to consider briefly the manufacture of filters. The first type of filter to be commonly used was made by cementing a piece of dyed gelatine between two pieces of glass. Even though perfectly plane parallel pieces of glass were chosen strains were set up by the cement, and the result was that the finished filter was far from being plane and parallel, which is naturally productive of unsharp results. Such defects were overcome only when glasses of unusual thickness were used, as in the case of the Wratten A filters. The precision necessary in miniature photography obviates the use of poorly made cemented gelatine filters, with the result that yellow filters made from optical glass colored yellow in the melt appeared on the market. In this way good plane parallel filters could be produced by the careful grinding of the glass. However, yellow pot glass filters present a specific disadvantage. Dr. Phil H. M. Kellner in an article in the September, 1933, "American Photography," clearly explains the advantage of green glass filters. Much of the following information is obtained from that article. Yellow glass filters are made from cadmium sulphide glass which is a so-called colloid glass, and there are therefore variations in the quality and density of the color, since it is not so dependent upon the quantity of cadmium sulphide which is added to the glass, as on the temperature used in making the glass. In the same melt may therefore appear parts which are yellow and strongly absorbent, and parts which are brownish and do not absorb as well as the yellow parts. If the filters are not inspected such variations will appear in them. Because of the variation of the density of the color it is practically impossible to produce a series of filters of equal density, necessitating the setting up of a wide tolerance for each grade of density. Thus it may happen when purchasing two yellow glass filters of different density, that the designated lighter one will approach the upper limit of tolerance, whereas the designated darker one will be at the lower limit of tolerance. 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