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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- F 135
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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. 1284, April 13, 1883
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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
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Band
Band 27.1883
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- Titel
- The photographic news
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April 13, 1883.] THE PHOTOGRAPHIC NEWS. 229 share in the honour? So we must end where we began, by saying that the proper recognition of inventors is rarely met with, merely adding a word of caution, that in these circumstances it behoves one to be very careful in assigning honours to this or that claimant. NOTES ON PHOTOGRAPHY. BY E. HOWARD FARMER. Lecture XVIII.—Optics—Continued. Depth of Focus.—This depends upon two factors: one, the diameter of the stop ; and two, the focal length of the lens. It is inversely proportioned to the diameter of the stop— i.e., a stop Of two inches diameter will only give half the depth of focus given by a one-inch stop ; and inversely proportioned to the focal length—i.e, a lens of twelve inches focus will only give half the depth of focus given by a lens of six inch focal length. Compound Lenses.—These consist of two lenses either of the same or different focal lengths, mounted at some distance apart, and with a stop between them. They are employed in two principal cases : 1, when absolute freedom from distortion is required; and 2nd, when rapidity of exposure is necessary. By combining two lenses in this way the distortion and aberrations produced by one lens can be corrected by the other, enabling a much larger stop to be used, and giving an image free from dis tortion. The extra rapidity is obtained, however, at the expense of the angle of view, and as the rapidity increases, so does the angle which can be included in the picture diminish. Apparent Distortion (Incorrect Perspective').—When a photograph is taken including a very wide angle, the resulting picture appears distorted, or the foreground appears exaggerated, and the distance dwarfed. This is due to the want of accommodation possessed by the eye ; if the eye lens could include a wide angle for viewing wide angle pictures, all difficulties would vanish ; but, un fortunately, the angle it can readily do so is not more than 55 or 60degrees. Now, in order for pictures to appear in correct perspective, it is necessary that they should be viewed at such a distance that the angle they subtend with the eye is equal to the angle they subtended with the lens in the camera, and this distance is obviously about equal to the focal length of the lens; hence the rule : every picture should be viewed at a distance equal to the focal length of the lens with which it is to be taken. Remember ing that the eye only includes an angle of about 55 degrees, a picture subtending a wider angle cannot be viewed at the proper distance, because the eye cannot then include it ; and when it is moved further off, so as to subtend a smaller angle, apparent distortion is pro duced. It may and does frequently occur, that of two pictures taken from the same point of view, and including the same angle, one taken on a quarter plate will appear distorted, while another taken on a 7} by 5 plate appears all right. This is also due to the eyes’ want of accommoda tion. Supposing the angle of view to be 55° in each case, the quarter plate picture will have to be held about four inches, and the 7} by 5 about seven inches, from the eye ; but the eye has not the power of seeing distinctly objects so close to it as four inches, or, in other words, has only a limited range of focus; in consequence, while the 7} by 5 picture can be viewed at the right distance for correct per spective, the quarter plate has to be held further off, and so appears distorted. This explains why carte pictures are best taken with a lens of seven or eight inches focal length. Focal or Equivalent Focal Length.—The focal length of a lens is the distince fron its optical centre to the point where parallel rays passing through it are brought to a focus; in the case of compound lenses this distance is called its equivalent focal length, meaning that it is equiva lent to a single lens of the same focal length. There are several methods of ascertaining the focal or equivalent focal lengths of a lens. 1. Focus the sun or other distant object on a screen, and measure the distance from any point on the lens to the screen ; turn the lens round, focus, and again measure the distance from the same point. The mean of these two distances is the focal length of the lens, and the point in its centre (optical axis) this distance from the focus is its optical centre. 2. Focus with a lens of known focal length an object on a screen, and measure the length of the image ; focus the same object with the lens whose focal length is required, and also measure the image formed ; then what ever proportion the second image bears to the first, so will the focal length of the second lens be to that of the first one. 3. Focus a plane object on the ground glass so that the image is exactly the same size as the object, and measure the distance between the object and the ground glass. One-fourth of this distance is the focal length of the lens, and the point midway between is its optical centre. 4. To calculate the equivalent focal length of a com pound lens :—Multiply the focal length of one lens by that of the other, and divide by the sum of their focal lengths, less the distance they are separated. Angle of View.—To find the angle of view included in a pictureDraw a line equal to the length of the picture, and erect a perpendicular on its centre equal to the focal length of the lens employed. Draw lines from this perpendicular to the ends of the base-line. The angle they include measured with a sector gives the number of degrees included in the picture. N.B.—This only holds when the picture is much smaller than the object. FRENCH CORRESPONDENCE. M. Thiebaut’s Pellicle Paper—Photographic Society of France. M. Thiebaut’s Pellicle Paper.—I have lately had occasion to make some experiments on M. Thiebaut’s reversible pellicle. The preparation is really excellent, and I do not think it possible to be made better. Paper thus pre pared is used like an ordinary gelatino-bromide plate. The operations of development, fixing, and washing, do not present any difficulty, and the immense advantage gained is that the dry plate can be easily detached from the support or frame, and no danger need be feared on that account. Although the pellicles may be used on either side, they are very solid. Nothing is more simple than the process of transferring the pellicle afterwards to glass, and the photographer’s luggage is considerably lightened. Banquet of the Photographic Society of France.— At the banquet of the Society, held on March 17th, a toast was proposed in the name of the photographic press, French and foreign. This is another proof of the good feeling existing between contemporaries of all countries. Leon Vidal. • — A NEW DANGER. Whether the Times was hoaxed or not when it printed that astonishing narrative of the West-end curate who was coaxed into kissing a young lady of considerable attractions, and maliciously photographed in the act, e ben trovato. Who is safe ? Dynamite is nothing to your new “ Pocket Patent Instantane- ous Photograph ! ” The peace of a family, the reputation of a house, may “be made to jump,” as the French say, in two seconds, without even the alarm of an explosion I The rays of the sun are sufficient to work silently this dismal ruin. Your lady’s-maid surreptitiously gets a patent Hash of you (who make up rather well) with your wig off and your teeth out. The valet takes his ducal, or even Royal, master under circumstances and at times when he is not posing at all as the hero of the
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