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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- http://digital.slub-dresden.de/id1780948042-18910000
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- Bandzählung
- No. 1717, July 31, 1891
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The photographic news
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Band 35.1891
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Band 35.1891
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546 THE PHOTOGRAPHIC NEWS. [July 31, 1891. less distance from the lens), the brightness of the axis of the lens is greater within the average focus than without. If the distance becomes greater from without inwards, the brightness of the axis of the lens is greater without than within the average focus. Working this out with all due care, I find that, for any amount of spherical aberration at all likely to be used in practice, or even for much more, there is no appreciable difference in the brightness of the points on the axis of the lens between the point where the extreme central rays intersect, and that where the extreme marginal rays intersect. It has to be admitted that, if we consider the case of violent spherical aberration, there is a slightly greater illumination of the axis without the average focus than within. Thus, with the enormous spherical aberration given by a plano-convex lens with an opening of one-third its focal length, the intensity of the light may be greater by about ten per cent, for the point where the central rays intersect, than for that where the marginal rays cross. With any spherical aberration useful in prac tice, however, the illumination of the axis between the two points just mentioned is so uniform that any differ ence there is in brightness between the extremes, if worked out, only shows in the third or fourth decimal figure. The deduction from these calculations being that, so far as brilliancy of light along the axis of the lens between the point where the central rays intersect, and that where the marginal rays intersect, may be taken as a measure of concentration of light; the concentration is equal between the two points. I thought it best to make a set of actual experiments, with a lens having adjustable spherical aberration, using more care than I had in any previous set of experiments. The lens used was J. II. Dallmeyer’s “4a Patent” lens—to be quite exact, No. 42,020. All the experiments involving spherical aberra tion were made with the back cell unscrewed as far as possible. A few rough preliminary trials were made, and then the first definite experiment was made in the following manner:—Opposite to the camera, and at a distance of some twenty yards, there was fixed a white stake. At distances in front and behind this (just so far off line that all might be visible) were fixed two other stakes at dis tances equivalent to equal distances behind and in front of the focus of the lens when focussed for the middle stake. (Mr. Dallmeyer, in his communication to the Photo graphic News, already referred to, assumes—somewhat unnecessarily, I think—that I do not know that such equivalent distances will not be equal; that, in the case under consideration, for example, the distance between the middle stake and the nearer one will be less than the distance between the middle stake and the farther one.) On each stake there were painted circular discs in black of different sizes. The middle stake was focussed for, the back cell of the lens being fully screwed home. The want of definition in the case of the other two stakes was now naturally equal. The cell was then unscrewed to its limit, the middle stake was again focussed for, and the result was observed as well as possible. It seemed in this case that, whereas the definition was distinctly worse than before in the case of the more distant post, it was, if anything, better in the case of the nearer. This result was, so far as the farther stake was concerned, distinctly confirmed by exposing plates, and this especially if the exposures were fairly ample. In such cases the black discs sometimes entirely disappeared, whilst they were quite distinct in the case of the nearer stake, and did not disappear with the same exposures, using the lens without spherical aberration, and focussed for the middle stake. This experiment is not by any means so conclusive as it would appear to be, for one of the first things that I discovered was that, working in this way, it would be very easy to prove anything that I wanted to prove, because of the uncertainty of focussing when a considerable amount of spherical aberration is introduced. Doing one’s level best, there are limits some distance apart between which one is doubtful where to focus ; thus, focussing once, marking the place of focus, putting the image out of focus, and focussing once again, the chances are, that there will be a very appreciable difference of distance between the two planes focussed for. Again, I found that there is a tendency to focus for different planes for different kinds of subjects even at the same distance; thus, there is a tendency to focus in two different planes at some distance apart for two different subjects, one of which is of the nature of dark spots on a white ground, the other of which is of the nature of light spots on a dark ground. The thing, however, brought out by this part of the investigation which I consider most curious is, that there seems to be a personal equation in the matter; thus, placing a well-defined object opposite to the camera, whilst the back cell was fully unscrewed to give maximum aberration, and asking different people—none of whom knew anything of the object of the experiments—to focus (always racking the lens quite out of focus after an experi ment was finished) it was found that one person would incline to focus farther forwards or backwards than another. No one of them would always focus for exactly the same plane, but the average focussing of one would always be farther forwards or backwards than that of the other. Seeing, then, the absolute unreliableness of experiments in which one trusts to trial alone as to what plane to focus for with a lens exhibiting spherical aberration, I con sidered it advisable to find some more definite way of focussing, and, after some consideration, I came to the conclusion that it would be fair to focus for a plane situated half way between that in which the central rays intersect, and that in which the marginal rays intersect. To discover the position of this plane, an opaque disc was prepared, a little less in diameter than the lens; it was fixed in front of the front combination, and a large lamp flame at a distance of about twenty feet was focussed. The disc was then removed, and there was inserted the smallest diaphragm that would allow of decisive focussing. The result was that it was found necessary to rack the lens just about one-third of an inch farther from the lens to get the image sharp. The effect was then tried of racking the lens equal distances forward and backward of the plane half way between these two. The result was that the flame distinctly retained a fairly well-defined outline for a greater distance racking forwards (bringing the plane of focus nearer to the lens) than racking back wards. Forward racking represents foreground, backward racking distance. These results were corroborated by taking photographs of the flame. The destruction of the form of the flame behind the average focus, when the lens was racked forwards, was particularly emphasised if the exposure was somewhat prolonged. Another experiment here suggested itself. For the lamp flame—that of a lamp with a large circular wick, and a flame-spreader, giving a flame about three inches in diameter—there was substituted an “artificial star” made by replacing the lamp flame by the bulb of a mercurial
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