Volltext Seite (XML)
December 31, 1880.] THE PHOTOGRAPHIC NEWS. 625 Pgotograpkir 8ehs. Vol. XXIV. No. 1165.—DECEMBER 31, 1880. CONTENTS. page On Focussing for a View 625 At Home 625 At the Opening of the Bristol International Exhibition 627 Paget Prize Competition 628 Correspondence .... 630 To Correspondents 630 Index 631—636 ON FOCUSSING FOR A VIEW. The usual method in which a photographer sets about to focus when he is going to take a view is pretty much as follows:— Having settled the point from which he is going to take it, and having established his camera and determined what lens he is going to use, he places his head under the focussing cloth and adjusts the distance between the lens and the ground-glass until some part of the view appears quite sharp. This is done either with the lens’s full aperture, or with a large stop. Most photographers focus for whatever they consider to be the principal object in the view, which is generally in what may be termed the “ middle distance.” When this much has been done, there comes the question, “ What stop shall I use ? ” There are several factors to be taken into consideration in answering this question. If the photographer be a wise one, he knows that the use of the largest stop which will give definition will give the best picture—the picture with most atmosphere in it—and he tries to find out the largest stop that he may use ; but, as he gradually inserts a smaller and smaller stop into the diaphragm slit, the light on the ground glass becomes so small that it is quite impossible for him to tell whether he has really got definition in the foreground and distance or not. At last he probably trusts either to former ex perience, or in despair he inserts a stop so small that he knows “it must give definition everywhere." The whole operation has probably taken some considerable time. Now, it seems a pity to us, that this process should be gone through, when there is a means of finding out at once, and with absolute certainty, the largest diaphragm that may be used, and this with no more trouble than is involved in making a mental calculation so simple that any child could do it. Here is the method. Let the photographer focus, with full aperture or a large stop, for a point which he considers to be about twice as far from the camera as is the nearest point which he wishes to be included in the view. Now let him estimate roughly how far the point, for which he has focussed, is from the camera. If he have a good eye, it will be quite sufficient to guess it; if not, he can generally “ pace ” it in a few seconds, and this will be quite exact enough. This distance is to be divided by the focal length of the lens. The figure which is arrived at will represent, in hundredths of an inch, the largest diaphragm which may be used. Let us take an example. The lens is (say) a rapid rectilinear of eight inches equivalent focal length ; the result would be the same what ever the lens, provided the focal length be the same. The nearest pointin the foreground is about fifteen or sixteen feet away, so the photographer focusses for a point about twice as far away—that is, ten yards. The result of this is, that the focus is evenly divided betivcen the foreground and the distance. Ten yards is forty-five times eight inches, therefore a stop may be used whose diameter is 14% of an inch— practically half-an inch—or the next smaller. This may sound complicated, but let any photographer try it; he will soon find that he can do this simple calculation five or six times over during the time another photographer is discovering, by the usual means, what diaphragm he may with safety use, or is finding out that he cannot discover it; and the former will have the satisfaction of knowing that he has put in that stop which will give him the very best, and, at the same time, the most rapid results compatible with good definition. This rule will be found specially useful where “instantaneous” results are aimed at, and where it is of particular advantage to be able to use the largest possible aperture. In such a case, of course, it is left entirely to the discretion of the photographer, whether he will not use a somewhat larger diaphragm than the one indicated by his calculation, thereby attaining greater rapidity by the sacrifice of some of the definition in the foreground and the background. A case where special difficulty is experienced in dis covering the largest admissible diaphragm, is in the case of a badly-lighted interior. Here it is often only after long straining that the photographer can manage to focus with full aperture, and when he begins to reduce this, he finds that he can see nothing whatever on the ground glass. It would be possible to give an exact rule for the largest diaphragm that might be used in such a case, but it would involve a formula far too complicated to be of any practical use. The following plan, however, will keep the photographer well within the mark. Let him focus full aperture, then divide the focus between the nearest and the most distant objects which are to be in the picture, as nearly as he can, by focussing first for one and then for the other, and turning his focussing screw to a point which he estimates to be half-way between these two points. Now guess or pace the distance of the nearer of the two objects. Divide by the focal length of the lens, and the result will be in fiftieths of an inch the diameter of the largest stop which may be used. It often happens, especially in churches and cathedrals, that there is in the most distant part of an interior a window, which is the only object which can be seen with sufficient distinctness on the ground glass to focus. In such a case, focus for the window with full aperture, then estimate the distance of the nearest object which is to be part of the picture, divide the distance by the focal length as before, and the result will be the largest permissible stop in hundredths of an inch. These last two rules only apply to interiors where the greatest distance is considerable, say, at least, forty times the focal length of the lens used. Let photographers try this method of focussing, and they will find it surprisingly much easier than they think, and they will no longer be seen with their heads under the focussing cloth, and with one arm wildly flourishing about, as if asking assistance for its distracted owner, but really searching for “ that confounded diaphragm slit,” which would almost seem to have dropped out of the lens. The above remarks apply only to the use of lenses which are able to give definitionwith full aperture to the margin of the plate used when photographing objects in a plane at right angles to the axis of the lens. If it be necessary to “ stop down ” merely to give definition at the edge of the plate, entirely new factors are introduced, and no rule can be given. o at ome. MR. W. HARVEY BARTON AT LLAN HOUSE, BRISTOL. “ A Bristol Mansion in the Olden Time.” The picture is so familiar, we need scarcely describe it. A lofty narrow building with peaked roof, storey above storey projecting; queer bay windows of mullioned glass, panels and quaint wood-work everywhere, and a prevailing air of antiquity