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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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Zeitschrift
The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
- Ausgabe No. 1270, January 5, 1883 1
- Ausgabe No. 1271, January 12, 1883 17
- Ausgabe No. 1272, January 19, 1883 33
- Ausgabe No. 1273, January 26, 1883 49
- Ausgabe No. 1274, February 2, 1883 65
- Ausgabe No. 1275, February 9, 1883 81
- Ausgabe No. 1276, February 16, 1883 97
- Ausgabe No. 1277, February 23, 1883 113
- Ausgabe No. 1278, March 2, 1883 129
- Ausgabe No. 1279, March 9, 1883 145
- Ausgabe No. 1280, March 16, 1883 161
- Ausgabe No. 1281, March 22, 1883 177
- Ausgabe No. 1282, March 30, 1883 193
- Ausgabe No. 1283, April 6, 1883 209
- Ausgabe No. 1284, April 13, 1883 225
- Ausgabe No. 1285, April 20, 1883 241
- Ausgabe No. 1286, April 27, 1883 257
- Ausgabe No. 1287, May 4, 1883 273
- Ausgabe No. 1288, May 11, 1883 289
- Ausgabe No. 1289, May 18, 1883 305
- Ausgabe No. 1290, May 25, 1883 321
- Ausgabe No. 1291, June 1, 1883 337
- Ausgabe No. 1292, June 8, 1883 353
- Ausgabe No. 1293, June 15, 1883 369
- Ausgabe No. 1294, June 22, 1883 385
- Ausgabe No. 1295, June 29, 1883 401
- Ausgabe No. 1296, July 6, 1883 417
- Ausgabe No. 1297, July 13, 1883 433
- Ausgabe No. 1298, July 20, 1883 449
- Ausgabe No. 1299, July 27, 1883 465
- Ausgabe No. 1300, August 3, 1883 481
- Ausgabe No. 1301, August 10, 1883 497
- Ausgabe No. 1302, August 17, 1883 513
- Ausgabe No. 1303, August 24, 1883 529
- Ausgabe No. 1304, August 31, 1883 545
- Ausgabe No. 1305, September 7, 1883 561
- Ausgabe No. 1306, September 14, 1883 577
- Ausgabe No. 1307, September 21, 1883 593
- Ausgabe No. 1308, September 28, 1883 609
- Ausgabe No. 1309, October 5, 1883 625
- Ausgabe No. 1310, October 12, 1883 641
- Ausgabe No. 1311, October 19, 1883 657
- Ausgabe No. 1312, October 26, 1883 673
- Ausgabe No. 1313, November 2, 1883 689
- Ausgabe No. 1314, November 9, 1883 705
- Ausgabe No. 1315, November 16, 1883 721
- Ausgabe No. 1316, November 23, 1883 737
- Ausgabe No. 1317, November 30, 1883 753
- Ausgabe No. 1318, December 7, 1883 769
- Ausgabe No. 1319, December 14, 1883 785
- Ausgabe No. 1320, December 21, 1883 801
- Ausgabe No. 1321, December 28, 1883 817
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Band
Band 27.1883
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October 12, 1883.] THE PHOTOGRAPHIC NEWS. 653 The distortions of two single lenses mutuanly correcting eathothera The axis of the emergent ray is parallel to that of theimmergent one, ana ru projection is the result. such an instrument, however, astigmatism may, and some times does, exist to a very large amount. , Another astounding statement that Captain. 4e makes in the same paper, is this: he says, W hen diaphragm is behind the lens, the image of a point ly g rays from an object not lying in the axis of the lens fall upon a marginal portion of the lens, and are bent towards the thicker edge of that marginal portion, which thicker edge is of course towards the centre of the lens. The farther Fig. 1. Distortion by a single lens with the diaphragms in front. The distance from P toP’is the amount of the inward displacement of the emergent ray causing barrel-shaped distortion. removed from the axis are any rays of light, the nearer the edge of the lens will these rays fall, and the greater will be the inclination of the two surfaces —the inner and outer—of the part of the lens in action ; that is to say, the more wedge-shaped will it be, and the more will the resulting part of the image be bent inwards towards the axial line, or distorted. When a stop is placed behind the lens, the same effect of bending marginal rays Fig. 2. Distortion by single lens with diaphragm behind. The outward disnlaee- ment of the rays from P to 1 causes pincushion-shapea distortion. towards the thicker edge of the portion of lens in action comes into play ; but as the image is, in this case, formed by a part of the lens not on the same side of its axis as the image itself, but on the opposite side, this bending of the marginal rays produces a distortion of an opposite character, and objects towards the side of a picture are represented farther from its centre than they should be. By placing a diaphragm between two such lenses, these disturbing causes neutralize each other, and a non-dis torting doublet or rectilinear objective is produced. In Ftg. 3. near the direction of the axis will be less proportionally dis placed than the image of a point lying further away from the axis; whilst, when the diaphragm is in front, the image of the first point would be more proportionally displaced than that of the latter.” There is, in fact, no such contrast as Captain Abney asserts to exist between the proportional displacement of the rays near to and distant from the axis in the two cases of a diaphragm in front of and behind the lens. In both cases the image of a point lying near the direction of the axis will be less proportionally displaced than the image of a point lying further away from the axis. This is shown even in the familiar diagrams which Captain Abney makes use of to illustrate barrel and cushion-shaped distortions; where, as is evident, the further the bounding lines of the figures are from the axial point, the more they are affected in their curvature. On one point in which Mr. Wheeler assumes to correct Captain Abney, he is himself in the wrong. The late Mr. Thomas Grubb demonstrated some years ago that it was wrong in principle to suppose that depth of focus could be gained by the sacrifice of absolute sharpness associated with spherical aberration. Mr. Wheeler says: “ A common practical test for telescopes is that of coming sharply to focus. With a given aperture or stop, this quality is pro portional to the accuracy with which the aberrations have been corrected, and depth of focus is obviously in an inverse ratio." There is no “obviously” at all in the matter, and the fact, indeed, is rather the other way. If the difficulty of coming to an exact focal point is due to smallness of aperture, then with the small aperture there will be some “ depth of focus” and loss of light, of course; but with lenses of equal aperture, the difficulty of coming to an exact focus indicates inferior definition at the focal point, without any gain whatever, but on the other hand some loss, of definition in the planes not focussed upon. W. E. DEBENHASr. AFAR-OFF PHOTOGRAPHY. Sir,—My attention was attracted by the account contained in the Liverpool Mercury t cotcerning the " triumph in photography ” supposed to have been achieved by Mr. Banks—viz, the tuccess attending his efforts to secure a photograph of the “hills in the Isle of Man,” which are discernible from Blackpool only at sunset, and are distant some sixty miles. Being a lover of the photographic art, and eager to in crease my knowledge by every available means, I lost no time in procuring a “print” from the wonderful and oft- enlarged negative. Picture my surprise and disappointment when, becoming possessed of the coveted treasure, I found I had fallen a victim to a piece of artifice; in short, the first glance assured me that the negative had been secured, not from nature, but from a painting, more remarkable from its display of “buoys” and “sea-gulls” than from any idea of perspective. Seeing in your last issue a reference to this “ photo graphic curiosity,” I at once concluded you could not have seen the result of Mr. Banks’ endeavours to secure a view of these hills from Blackpool, and that you were innocently lending your assistance in carrying out apiece of deception which is more calculated to afford profit to Mr. Banks than satisfaction to the general public. I therefore beg to en close you a copy, in order that you may see and judge for yourself as to whether it is worth while making any com ment on this subject in your next issue.—I am, yours, &c., L. F. [Our correspondent is quite right, and we thank him for the correction. The picture sent us shows across the sea a series of Monts Blanc, which do duty for the Isle of Man. The sketch is what the artist thinks the island should be like, not what it is in reality.— ED. P. N.]
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