A lens that has met with con siderable favor with half-tone en gravers is the Steinheil Wide Angle Aplanat, Series VI. (See Fig. 371.) It gives perfect flatness of picture and sharpness of image, together with considerable field.
The diaphragms or stops, as more familiarly spoken of, are an important adjunct of the lens, and considerable judgment is neces sary in the use of the several sizes, which can only be acquired by experience. It should be kept in mind that they are related to each other by regular gradation, and that the diameter of each is a proportion of the length of focus of the lens. The gradation followed as a rule gives a value double--i.e., admitting double the amount of light—that of the smaller one immediately preceding.
One or two general rules regulat ing their use may be laid down for easy remembrance : ist. In copy ing a picture and making a con siderable reduction, use a small stop, commensurate with the amount of reduction. zd. A small stop gives greater detail in the shadows, sending the light rays more directly, or in straighter line, through the screen. 3d. The larger stops allow the light to spread, and so have relatively a greater influence in filling up the high lights. Keeping these rules in view, it will be readily comprehended that the use of too large a stop will fill up the high lights of the picture before sufficient detail could be got in the shadows, and the inference naturally follows that when a lack of detail is observed a smaller stop should be used. The character of the picture to be copied has also to be judged, and as a rule, should it contain much of dark shadow, a small stop and longer time of exposure is indicated, so as to get enough of detail. It is a good method and will bring out the best results to use several stops—say, two or three- -and proportion the time with each as to give detail in the shadows by so much given with the small stop, so much more given with the next larger, and possibly a shorter exposure with a still larger for the high lights. It is well to aim for getting the high lights right, the detail in the shadows being more easily got by giving more exposure with the small stop.
Diaphragm Apertures.—The stops or diaphragms sent with lenses as purchased usually have their apertures round in form ; but recent investigation has shown that other forms may be used with advantage. Square apertures, or square with the corners cut out, also diagonal, as follows, are each seen to have a varying influence on the shape of the dot, and through it on the picture resulting. Every square in the screen acting as a separate lens causes the dots to partake of the shape of the apertures used. This investigation has been very fully made by Dr. J. M. Eder, of Ger many, and Mr. Levy, of Philadel phia. The latter gentleman gave the result of his investigations in a very interesting article in the Paper and Press, and has invented a form of diaphragm in which the shape of the aperture may be modified or changed, as shown in Figs. 373-376. The elaborate writing of Dr. Eder has been translated and published in the Process Photogram, of Lon don, England, and from it we ex tract the following paragraph, as presenting the gist of the whole : " With square diaphragm aper tures the preparation of half-tone negatives is rendered easier and more certain than with round ones ; the reason of this lies in the fact that the corners of the square nega tive dots in the high lights beauti fully coalesce, by which means the dot remains open, so that the square stops—even if their ratio aperture may not be so great as that of the round ones--it is easier to obtain undulated half-tone negatives. Square stops with the corners cut out I especially recommend for half-tone work ;
the beginner should commence his experience with such a stop. The influence of the diagonal aperture is to present the dots in lines, and this will be more readily observed if one-half the time allotted to this form be given the one way, and the diaphragm turned round so as to give it in the other direction. These special diaphragms may be made in cardboard and blackened." The Copying copying board should be so placed as to be conveniently shifted to accommodate any reduction or enlargement that may be required, but at the same time retain its being in position perfectly vertical to the camera holder. This is secured by being erected on the same platform as the camera, and run ning in the same grooves, as explained in the foregoing diagram. The copy may be affixed in any way convenient, only where special means have to be adopted to keep it flat, perhaps the simplest way is to place a piece of plate glass over it, for which it is as well to make provision, so it may be readily fixed and at the same time impinge with some pressure on the copy. With a couple of catches of thin brass at the bottom, to sup port the weight of the glass, into which a wedge of folded paper may be pushed to keep it tight, and pieces of spring brass at sides and top, secured with a screw at end, so that it may turn in any direction and hold, it will be found all that is needful (see Fig. 377.) In the use of a plate glass cover, however, it should be seen that no reflections proceed from it, and if there are, means must be taken to obviate them. And this leads us to the next important consideration, that of you have access to the skylight of a photographer's gallery, or can erect such, you are all right, and have at hand a most potent assistance in securing suitable negatives with greater certainty and with less perplexing judgment. But you may be dependent on a side light from a window, with every variation of cloud and sunshine, rain or clear weather, while volume and intensity will also vary with the season, and it will call for the exercise of much study and waste from many trials on plates, and patience, to secure the exact conditions of a good neg ative. A northern light will give the most steady and have less variation, but more frequently one has to make the most of what he can get, and then it lies for the man of inventive parts to bring out good results from circumstances as he finds them. Direct sunlight may even be used, and good negatives secured by it with very much shortened time of exposure.