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The Camera Obscura and Pinhole Photography 36

aperture, image, screen, distance, objects, 0-4 and perspective

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THE CAMERA OBSCURA AND PINHOLE PHOTOGRAPHY 36. The Camera Obscura. The camera obscura (Fig. 12) appears to have been known at a very early date.' In one of his undated manuscripts, the celebrated painter, engineer, and philo sopher, Leonardo da Vinci, who died in 1519, describes this phenomenon in the following way, without giving any indication that it was either a recent or personal discovery : " When the images of illuminated objects enter a very dark room through a very small hole and fall on a piece of white paper at some distance from the hole, one sees on the paper all the objects in their own forms and colours. They will be smaller in size and will appear upside down because of the intersection of the rays. . . . A suitable hole can be made in a very thin plate of iron." Outside the room each illuminated point, scattering the light in all directions, sends through the aperture a beam of light in the form of a very narrow cone. This cone has its apex at the object point in question, and its base is that of the aperture. Thus it illuminates the scattering or translucent screen on which it is received by a small spot, which is thus the image of the point object.

Within certain limits, the spot formed by the projeqtion of the aperture on the screen will become smaller, and consequently the whole image sharper, as the aperture itself becomes smaller and as the material in which the aperture is made is thinner. The image will also become sharper as the aperture is moved farther away from the screen. In fact, under these conditions, the sizes of the individual spots increase much less quickly than the dimensions of the image. 2 The camera obscura was much improved in the second part of the sixteenth century by fitting a biconvex lens at the aperture. In the early part of the eighteenth century it was developed into a portable instrument similar to our present-day cameras, and was frequently used by artists as a means of making sketches from Nature.

37. Identity of the Camera Obscura Image with an Exact Perspective. In 15b8 D. Barbaro recommended the use of the camera obscura for automatically making perspective drawings.

Suppose that a sheet of glass is put in front of the camera at the same distance from the aperture as the screen on which the image is projected is behind, and parallel to, this screen. The perspective formed on this surface with the aperture as viewpoint will be accurately formed by the intersection with the plane of this glass of all the rays which, after passing through the aperture, go to make up the images of the external objects. The exact identity of this image and of the perspective obtained can be easily shown (Fig. 13). It is due to the fact that the traces of the points of intersection of the lines in a beam of concurrent straight lines, with two parallel planes symmetrically placed relative to the meeting point of the lines, are superposable one on the other.

38. Pinhole Photography. Although not much practised in recent years, pinhole photography can give very useful results in the case of inanimate objects ; it even yields images under conditions in which it would be impossible to get comparable results with the objectives now available (Milieux, 1886).

In order to obtain an image of sufficient sharpness it seems to be an advantage to use an aperture of the smallest possible diameter in a very thin plate.' A simple experiment, such as forming the image of a luminous filament of an electric lamp, shows that with each distance of the object from the camera there corresponds, for a given diameter of the aperture, a distance from the aperture to the receiving screen (e.g. a matt glass) at which the greatest possible sharpness of the image is obtained. Actually, the phenomenon of the diffraction of light modifies, sometimes in one way and sometimes in another, according to the circumstances, the effective diameter of the spot, as ascertained in accordance with the laws of the rectilinear propagation of There is quite an appreciable latitude in the all distances being expressed in millimetres. For example, with an aperture of 0-4 mm. diameter, the distance F should be between the two limits calculated as follows 625 X 0-4 X 0-4 = 'co mm. ; 1,250 X 0-4 X 0-4----- 200 mm.

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