The international photographer (Jan-Dec 1935)

November, 1935 T h INTERNATIONAL PHOTOGRAPHER Twenty-seven tion, producing the utmost results. When used at the full diaphragm opening they will produce sharp results. There is, however, an optical law which should be kept in mind — the larger the aperture of the lens, the shorter is the depth of focus. As the diaphragm of the lens is closed down, the depth of focus will increase. When working under adverse lighting conditions, the fast lens is kept wide open, but in daylight it is ordinarily not required. Despite this, many amateurs are in the habit of using their lenses at large diaphragm openings, preventing overexposure by using fast shutter speeds. This may be a wise procedure when making action photos. For ordinary subjects, fast shutter speeds such as 1 /300th sec, l/200th sec, etc., are not necessary. It would be wiser to close the lens down to get a greater depth of focus and to use a slower shutter speed. Some photographers may raise the point that the miniature camera is easily moved when the shutter is tripped, making a fast shutter speed advisable to get sharp pictures. However, about 1 / 60th sec. should be fast enough for this purpose, and many of us can keep the camera steady at slower speeds. Soft Focus: Many miniature camera photographers have often experimented with soft focus, especially in portraiture, and the interest in this type of photography is particularly keen at this time of the year. The long evenings allow a considerable amount of time for indoor work. One point must be cleared up in the minds of many amateurs, and that is soft focus does not mean out-of-focus. The latter produces distinctly unpleasant results. In soft focus the image is sharply focused on the film, but instead of the crisp images produced by the usual miniature camera lens the lines are softened; there is greater depth and more plasticity to the photograph. Detail is suppressed, the subject being treated in masses. This effect can only be truly obtained with a soft focus objective or its equivalent. Some photographers have resorted to the use of gauze and similar material to soften the sharp lines produced by their lenses. The gauze can be placed in front of the lens when the picture is taken, or before the enlarger lens when the print is made. The latter method is to be preferred, for the photographer will still have a sharp negative for making ordinary prints. The effects thus produced, however, cannot be compared with those made with a soft focus lens. BY AUGUST WOLFMAN Leica owners have available the Thambar soft focus lens which produced superb results. A new product has just been announced which owners of other miniature cameras can avail themselves o-'. It is the Duto auxiliary lens, supplied in two powers, No. 0 and No. 1, for stronger and weaker lightings. It is designed to give pleasing soft effects. More detailed information can be obtained from Burleigh Brooks, 127 West Forty-second Street, New York City. Chrome Alum Bath: A chrome alum bath for hardening miniature negatives has been recommended in this department from time to time. Some photographers have been hesitant to use such a bath, fearing a sludge will form that will settle on the film. After the film has been developed and before being treated with the chrome alum bath, the film should be rinsed in running water for a minute or two. When the chrome alum bath is added, agitate the tank for a few seconds. Then again we can prevent the formation of a sludge by acidifying the chrome alum bath so that it will neutralize the alkaline developer. This can be done by adding sodium bisulphite to the chrome alum bath. A last precaution is to wipe the film carefully when it is hung up to dry. HISTORICAL BACKGROUNDS (Continued from Put/e 17) degree of curvature of the field, remains the same whatever the separation of the two elements. Obviously, therefore, by employing a negative and positive lens separated from one another, it is possible to correct curvature of field with a choice of glasses of almost unlimited refractive index variation. Starting with this fundamental elementary conception, Taylor found out that in order to correct distortion it was necessary to divide the positive element into two, placing one positive element each side of the negative (dispersive) elements. By suitable choice of glasses chromatic aberration was corrected. Spherical aberration and coma were offset by a suitable choice of curves and astigmatism was taken care of by placing the stop near the dispensive element so that it was nearer that element than either of the two positive elements. No fundamentally new principle of photographic lens design has been discovered since then, although obviously the fundamental triplet idea has been modified and extended by splitting the functions of any one or more of the three original elements between sveral glasses. One such extension of the previously known properties of photographic lenses was made in 1920, when Taylor, Taylor & Hobson introduced the F 2. type of lens, which was first introduced to the kinematograph studios as the "Kinic" lens, and later was completely redesigned as the F 2. "Speed Panchro." After some preliminary hesitation about using lenses of 72. aperture, the superior definition and quality of these lenses won acceptance under the most riqid tests and the use of really fast lenses came into general use in the studios for the first time. Shortly after the introduction of sound, as you know, combination of incandescent lighting and panchromatic film came in'o general use. For the first time a new factor entered into the correction of photographic lenses, or rather a modification of the regular chromatic correction had to be considered. Formerly all lenses had been corrected so that the blue and yellow rays focused at a common point, and the other rays as close as possible. In other words, except by extremely complicated design it has not been possible to correct the chromatic aberration so that all rays focus at exactly the same point, so the compromise was made by focusing the two principal colors and getting the others as close as possible. Inasmuch as incandescent light gave off a preponderance of red and yellow light, and as panchromatic film was so sensitive to red, it was obviously necessary to recognize the importance of where the red rays focused. Formerly, owing to the correction for the blue and yellow, the red rays had focused quite a way off but that had not been of any special importance. It then became necessary to change the chromatic correction to focus the blue and red rays. The Kinic formula as mentioned above was further modified — the illustration shows the first form of this lens — to give the F 2 speed with the critical correction of the chromatic error for the blue and red rays. In making this correction, the blue G Fraunhofer and the red C Fraunhofer lines were focused at one point. (The Fraunhofer blue G lines were used in preference to the blue F line formerly employed in old lenses which were corrected for the blue F and yellow D lines.) By this is meant that the maximum error of focus for the yellow was only .001 -inch even for the 3-inch F2 lens, as illustrated in the figure. This was so close that the lens could almost be termed apochromatic As such these lenses are very effectively corrected for color processes such as bi-pack color process or something like the Kodachrome process if and when that would be introduced in the 35 mm. field. As is generally known, Taylor Hobson have also modified the Speed Panchro type of correction to give the special lenses used by Technicolor in their three-color process. In these lenses, however, the corrections are complicated due to the fact that all corrections are carried through the split-beam prism which is part of the Technicolor system, but the fundamentally precise color corrections in combination with the exquisite balance of all the other corrections has given Technicolor such sharp original negatives to work from, that they have been able to carry this sharpness all the way through subsequent processes — you are all familiar with the vast improvements in the definition of the latest Technicolor results. It is hoped that this brief analysis will acquaint the cameraman with some of the facts entering into the development and perfection of the lenses which he uses every day and upon which he depends so much for obtaining the results he desires. 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