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The photographic news
- Bandzählung
- 29.1885
- Erscheinungsdatum
- 1885
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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. 1385, March 20, 1885
- Digitalisat
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Zeitschrift
The photographic news
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Band
Band 29.1885
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- Register Index III
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Band 29.1885
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MARCH 20, 1885.J THE PHOTOGRAPHIC NEWS. 183 line of view, but at different distances ; Di is the image of these points on the plate ; E is another point in a different part of the view ; and Ei is its image on the plate. First, in reference to the points D, I wish you to observe that the further they are from the pin-hole, the smaller is their image upon the plate ; secondly, that the further the plate is from the pin-hole, the larger will be the image of any point in the view ; but the image of D, will increase in more rapid ratio than the image of D, or D,. Secondly, I wish to draw your attention to the fact that other points on the plane Ct will have images of about the same pro portionate size as D P For example, the point E is a greater distance from the pin-hole H than is the point D,. It has conse quently a more acutely diverging cone of rays passing through H. In a corresponding degree its image, Ei, is also further from H, and the longer continuance of the divergence compensates for the smaller degree. In like manner the cone of rays meets the pin-hole obliquely, and, as a consequence, the perforation only permits the passage of a band of light narrower than its diameter ; but this narrow band meets the sensitive plate obliquely also— thus it is spread out over the same area as the band from D, which meets both B and A at right angles.* In the next diagram I endeavour to show that two points C in the view, having a distance from one another equal to the diameter of pinhole IS, will give over-lapping images on the plate A ; and that these images overlap more as the distance of the points from the pinhole increases as drawn at A,B, and C 2 ; con sequently a sign board would give you a more legible image with your pinhole camera 20 yards distance, than if the camera were moved to 50 or a 100 yards from the same board. The following diagram shows that the two points in the view C, the images of which overlap when the plate is placed at A, give images which do not overlap when the distance of the plate from the pin-hole is increased so much as to A., and the lumin osity of any point on the focusing screen or sensitive plate is diminished by the decreased number of points in the view which throw light upon it. In this diagram I have placed the points in the view at a dis tance from one another greater than the diameter of the pin hole. Had I not done so, no increased distance of the plate from the pin-hole could have freed them from this overlapping. The blurring of the picture—the “focal aberration,”if I may use such a term where there is no such thing as focus in the question—will always be proportionate to the size of the pin-hole, divided by the distance of the pin-hole from the plate. At the December meeting I was asked if perfect roundness and smoothness of the pin-hole were not essential to the produc tion of a picture. My reply that " the shape and smoothness • The narrower band of light, however, having to cover an equal area on be plate necessarily illumines it to a lower degree. of the aperture were, theoretically, of no importance,” is con firmed by a picture which I now exhibit, of the same view taken with an oblong perforation in a thin sheet of copper, placed vertically. The perforation is about seven times as long as it is wide, and the effect—as might be anticipated—is to make the letters in the name of the street rather taller than they should be, to dull the horizontal lines in the roof, &c., and rather to sharpen the vertical lines. I was also asked if the pin-hole views were free from the per spective distortions common to views taken with lenses. As I cannot pretend to have an eye for perspective so keen as to detect errors in any ordinary photographic views, I have simply endeavoured to give a practical reply to this question by taking the same view with an ordinary rapid rectilinear lens, and will leave it to those who see, or think they see, faults in photos of this description, to compare the pin-hole with lens picture. I should expect them to be identical, and their correctness far beyond the criticism of an eye under any ordinary training. This subject was commenced with no thought of anything beyond a passing curiosity to see what sort of picture I could obtain without a lens ; but I think we may reasonably look for some educational advantages from such experiments, and the present instance has considerable bearing upon the art of focussing, and the value of small stops. With the view of illustrating this, I have taken another nega tive of the same street corner with the pin-hole diaphragm in conjunction with the lens, but the lens which has an eight-inch focus was sixteen inches distance from the plate, so that until the diaphragm was inserted, each point in the view occupied a disc on the plate about equal in size to the area of the lens, with the well-known result that no picture was visible beyond the simple fact that you could discriminate between earth and sky ; but after the insertion of the pin-hole diaphragm each point in the view occupied a disc on the plate equal in size to the area of the pin-hole, and the landscape at once showed its broad details, and had about the same degree of clearness as obtained when the pin-hole was used without the lens. It is worth while to note, in passing—though it is precisely what we might predict—that it was found impossible to focus the view with the pin-hole diaphragm and lens in their places. In the diagram you have represented roughly the contrast between the two cases. In A the pin-hole stop is in, the plate A being double the focal distance. It would be difficult to see where the lines of light cross ; the difference midway between the lens L and the plate A differs so in sharpness from points an inch or so on either side of correct focal distance, that the eye fails to appreciate it; but with the full aperture of the lens as drawn at B, the point at which the rays cross is most clearly marked; the slightest movement of the plate backwards or forwards gives a most palpable blurring of the image, hence the common rule to focus with the largest aperture, and expose with the smallest which your light and other circumstances will permit. (To be continued.) Eotes. The annular eclipse of the 16th inst. was, it appears, well observed at some of the transatlantic stations. Was it not Rej lander who said that the portrait photo-
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