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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1303, August 24, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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THE PHOTOGRAPHIC NEWS. 539 with the portions C D and D E respectively, when it will be found that the focal lengths are at H and K respectively. Now use the whole of the line ABODE, and then no sharp focus will be obtained. This shows that, with oblique excentrical rays, the position of focus varies accord ing to the part of the lens used to form the image. Astigmatism.—Next take an ordinary convex lens of as long focus as possible, and use it so that its principal axis is in a line with the sun. Focus an image of the sun on a piece of paper, and it will be fairly sharp. Next twist the lens so that the axis forms a considerable angle with the line joining it and the sun ; it will be found that it will be impossible to obtain a sharp image ; if sharp in one direc tion, it will be out of focus in the direction at right angles to it. Again, if you take a photographic lens, covering a considerable angle, using a large aperture, and focus win dow bars on the ground glass, it will be seen that, if the horizontal bars are in focus near the top of the plate, the vertical bars will be fuzzy, or vice versa. This really opens out one of the most intricate problems in common optics, when entered into with great nicety ; but it can be ex plained in a simple manner. Suppose A B C D be a perspective view of a lens (con vex), and let parallel rays of light strike it at some angle with the axis. It will be seen that a slice of the whole bundle of rays may be taken, which will be in the same plane as the axis and the distant source of light (distant Fig. 27. because the rays are supposed to be parallel). Such a slice of light would cut the lens in B D. Another slice of the beam may be taken at right angles to the first slice, entering the lens in A C, and these may be considered directions which would give the greatest alteration in focus. Draw a line MOE parallel to the rays through O, the optical centre. Since it passes through the optical centre, it is a secondary axis (see Lesson III.). The ray refracted at D will cut this axis at E, and that from B at 0 (see fig. 26), whilst the rays from A and C, being symmetrical, will cut the axis at one point, G. Thus, whilst the image of the object of light will be in focus at G for all parts parallel to the line A C (as the cross section of the vertical bars), it will be out of focus for all others, particularly in the parts of the object vertical to this (the vertical windows bars). This optical defect is called “ astigmatism,” and we shall now see that it can be very largely mitigated, though never entirely eliminated, by using a diaphragm or stop. The next figure shows the same incident parallel rays as Fig. 28. before ibut now we have inserted a stop S S behind the lens. It will be seen that for oblique rays only a portion of the lens near the margin is used, and these rays are brought to a focus near G. If wo now take the focus of direct parallel rays, shown as dotted lines, we shall see that the marginal rays, whose focus is about N, are cut off, and the central rays with a focus about M alone used. If a plane surface be placed at right angles to the axis of the lens and at M, both the oblique rays and the central rays will form fairly sharp images ; the rays which would give fuzziness to them being cut off by the stop. Thus, if the stop were away, a distant point lying in the direction A A, B B, would be represented by a patch of light, of which G L would be the greatest breadth. By using a stop, the astigmatism is not done away with, but only diminished to a passable extent, so as to give the appearance of focus. In the above figure, a stop or diaphragm has been shown between the focussing screen and the lens. It can be (and is usually) placed in front of the lens, so as to give the same result. In fact, it will be found that that is a better position for it to occupy. The student may re-draw fig. 28, and place the diaphragm in it. Whatis a Sharp Image ?—Now comes the question—What amount of fuzziness is allowable, and will give the appear ance of a sharp image? This is a thoroughly practical question, and has been settled in a practical manner. The area that the image of a point is represented by is what is usually called the disc of the least confusion,* and if the diameter of that disc is about 86 of an inch, it subtends such an angle at the distance at which an eye usually ex amines a photograph as to make the disc appear as a point. That is, the images of all the points of light coming from an object focussed may be represented by discs of light of 11a of an inch in diameter. It may be looked atthis way : supposing no diffraction phenomenainterfered, the image of a distant landscape formed by a pinhole m3 of an inch in diameter would appear to the eye absolutely sharp. Let us see how this affects a view of the same sub ject taken on a quarter plate and on a whole-plate, using lenses in both cases to embrace the same field of view. In both plates the discs of confusion need only be of an inch, and therefore the quarter-plate lens need only bo stopped down to half the amount as the whole-plate lens, for if equally stopped down the diameter of the discs of confusion would be proportional to the diameter of the plates; and if on the whole-plate they were 230 inch in diameter, in the quarter-plate they would be 250 inch in diameter. This is a point well worthy of attention in photography. Many people take small views to be subsequently enlarged, and they focus, as they think, sharply; whilst, when en larged, the image is, more often than not, ill-defined, because the diameters of the discs of confusion also are enlarged. The photographer should focus sharply with as big a stop as he can, and then subsequently put in a smaller stop, the size of the latter evidently depending on the size to which an enlargement is to be carried. BRUSSELS INTERNATIONAL PHOTOGRAPHIC EXHIBITION. We subjoin the list of jurors and their awards, as tele graphed to us by our Special Correspondent:— Jury.—Dr. Stolze, Berlin ; Capt. W. de W. Abney, London; M.M. L. Warnerke; Davanne, Paris; De Blochouse, Brussels ; A. Geruzet, Brussels ; De Pitteurs, Zepperen ; L. Montefiore, Paris ; L. Dommartin, Brussels; E. Smits, Brussels. Grand Diploma of Honour (hers concours). — MM. Goupil and Co., Paris. Diplomas of Honour.—Dr. Stolze, Berlin (hors concours) ; MM. Geruzet freres, Brussels (hors concours) ; “ Photo graphic News,” London ; Socit Beige de Microscopie, Brussels ; L’Association Beige de Photographie, Brussels ; Campo and Colard, Brussels ; Gauthier-Villars, Paris ; Srczniewski, Petersburg ; Leon Vidal, Paris. * Every pencil of convergent concentrated rays has a disc of least confue sion, and is that orca where the rays are, so as to give the greatest intensity.
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