The photographic news

• Bandzählung: 35.1891
• Erscheinungsdatum: 1891
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
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• Sammlungen: Fotografie
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• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

42 THE PHOTOGRAPHIC NEWS. [January 16, 1891. focussing; and farther, to all intents and purposes, in con nection with hand-camera work, it means that the lens should display these properties whilst working with a large angular aperture—say f /8 at least. Strictly speaking, this property does not exist at all; but, in the case of some lenses, so near an approach to it exists that none of the planes in any subject likely to be photographed will be perceptibly, or at least offensively, out of focus. A mistake, however, that seems sometimes to be made, is to suppose that the qualified “ fixed focus ” that I now describe can be got by the adoption of some particular form of lens, whereas it depends solely on the focal length and the aperture. In a communication on “ Depth of Focus ” that I sent to the PHOTOGRAPHIC News some time ago, the optical part of this question was fully discussed. Here I shall only give a short description of the way in which a hand- camera should be adjusted so as to get the best effect with a fixed focus. A distant object should be focussed for, and the camera should then be extended till it is seen that this distant object is just out of focus, or, if the full aperture of the lens be f/8, as it most commonly is, the camera should be extended 1th to 1th of an inch beyond the position of best focus. To give an idea of the practical result of adjusting a camera in this way, I may give the following figures; taking a set of lenses such as are likely to be used for hand-cameras, the assumption being that all are used atf/8. With a focus of 4 ins., the object in best focus will be about 17 ft. d'st. 5 G 7 8 10 26 „ 38 o 67 ", 100 ,, Whilst objects at half these distances will be only just perceptibly out of focus from being too near the camera. It may be said, roughly, that no adjustment is needed for quarter-plate cameras; that adjustment is not needed in the case of 5 by 4 cameras, for the majority of work ; and that a very considerable deal of work might be done, even with half-plate cameras, without adjustment. It is to be noted that there is no necessity for achroma- tisation in the case of lenses used with fixed focus, if an adjustment of the camera be made corresponding with the difference of the visual focus, and of the focus of the chemical rays. A deal of ingenuity has been spent in so designing hand-cameras that the fact that they are photographic gear is concealed. They are made in the form of boxes, hand-bags, parcels; like watches, to go within the waist coat, the lens protruding through a button-hole ; and in a multitude of other forms besides. There can be no doubt that this concealment may be useful for some kinds of work, although it always gives rise to suspicion of that kind of detective work that was condemned at the beginning of this chapter; but it is surprising how much the greater part of the work commonly done by hand cameras, specially made, can be done with an ordinary small camera of the best make, with a good lens and in stantaneous shutter. All that is necessary is to know that everything is absolutely light-tight. In exposing, the camera is held in the hand, and * ‘ aiming " may be done by any simple sighting arrangement fixed to the camera, or even by looking along one of the edges of the base- board. As an example of the way in which a ommon camera may be used for hand work, I may be allowed to describe the arrangement I use myself. This is simply a quarter-plate camera of what I consider the best English make, with a rectilinear lens working at a maximum aper ture of No. 2 U. S. (a little more than//6). The camera was first of all adjusted in the following way: A distant object was focussed with the greatest sharpness under an eye-piece, and from the position of the camera thus fixed, the lens was racked out 1’ inch. A peg was fixed so that, without removing it, this was the farthest that the camera could be racked in. The camera is now so ad justed that, used with the full aperture of the lens, the distance will be just perceptibly out of focus—an object at a distance of about fifty feet will be in the sharpest focus (the focal length of the lens being about six inches), and everything up to within a little more than twenty feet off will be in at least fairly good focus. A set of marks have also been made so that, with smaller stops, by adjusting to the different marks, the con dition of always having the distance just a little out of focus shall be fulfilled. The marks are made so that the lens is racked out from the position of sharpest focus for a distant object, the following distances for the following stops. The list includes the full aperture, No. 2, already mentioned :— [ when an) ( will be in} ( will still object z 50 ft. off < best focus { 25 ft. off t be in about ) ( and one ) ( fair focus. ,, No. 4, , ,, n 38 ,, ,, 19 ,, , „ No. 8, k „ „ 25 „ 13 „ „ „ No. 16, | „ 19 „ „ 10 „ » „ No. 32, | „ „ 13 „ „ 7 „ „ Stereoscopic Cameras.—I entered—if not fully, at least sufficiently to give a general idea of the subject—into stereoscopy generally in the series of articles on optics that I communicated to the PHOTOGRAPIIC News some time ago, and almost anything that I could say here would be little more than repetition of what I said before. The greatest distance commonly met with between the human eyes is about 24 ins., and theoretically there should be no greater distance than this between the lenses of a stereoscopic camera, and the length of the plate should be limited to 5} inches. In practice, however, a larger size than this is always used, the lenses being cor respondingly separated. The largest size of plate, so far as I know, ever used with the common stereoscopic camera is 8 by 5, involving a distance of four inches between the lenses. I think, however, that such a distance should be avoided, as, at least in the case of any near objects being included, an effect of exaggerated stereoscopy is liable to be produced ; 7 by 5 inches is a better size, and 6} by 43, or 6} by 4 j, probably better still. Many stereoscopic cameras are so made that, removing a central division, and substituting a front with one lens flange for that with two, the camera can be used for ordi nary photography on the whole size of the plate. In doing stereoscopic work with modern dry plates, a shutter for giving an exposure simultaneously with both lenses is almost an essential, and this shutter should be capable of giving a “ time ” as well as an instantaneous exposure, otherwise, in addition, a flap uncovering and covering both lenses simultaneously will be needed for giving exposures of from, say, half a second upwards. White mackintosh with a matt surface is proposed as a screen for the lantern ; its chief advantages are its opacity, thinness, and freedom from liability to crack when rolled,