Optics of photography : on a new process for equalizing the definition of all the planes of a solid figure represented in a photographic picture / by A. Claudet.

  • Claudet, A. (Antoine), 1797-1867.
Date:
[1866?]
Catalogue details

Licence: Public Domain Mark

Credit: Optics of photography : on a new process for equalizing the definition of all the planes of a solid figure represented in a photographic picture / by A. Claudet. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    cusing or ground glass, any one of these segments, and all in. succession, may be brought into focus. If we focus upon No. 4, for example, we sec that the segments before and behind gra- dually lose their sharpness, in a greater or less degree, according to the quality of the lens; and from that experiment we may judge empirically of what is called the depth of focus of the lens. By comparing at the same time the photographic image with the image we had on the ground glass, we see if the visual and che- mical foci agree, or to what extent they differ. But our present object not being to test whether the chemical and visual foci agree, we will take a lens in which we know that they coincide. Now, supposing that we focus upon No. 1, we shall find that the photographic image of that segment will be very well defined, No. 2 a little less. No. 3 and all the others until No. 8 gradu- ally losing their sharpness, so that No. 8 will be the most indis- tinct. In the same way, if we take a portrait so that the nose is on the plane of No. 1, this part of the face will be well defined ; the eyes, which are on the plane of No. 2, will be a little less well defined; the ear, on the plane of No. 3, still less defined; and if the body is obliquely turned, the shoulder, which corre- sponds with the plane of No. 8, will be considerably confused. Experimenting again upon the focimeter, let us suppose that, after having operated with No. 1 in focus, we move the frame holding the plate to a point previously marked on the camera- board where No. 8 is in perfect focus. If we then expose the plate a second time, or rather continue the exposure, we shall find that upon the first confused image of No. 8 a new image well defined has been impressed, and at the same time a confused image of No. 1 will have been impressed upon the first image of No. 1 which was well defined. In examining the result, we shall find it better than if the second impression on both segments. No. 1 and No. 8, had not been taken. In the middle of a confused image of No. 1 and No. 8 we shall have one perfectly defined, the whole having the appearance of the shadow of a pin not quite in contact with the surface; that shadow being slightly blended from dark to light, but still sufficiently defined to show the exact form and size of the pin. Now what has been done for the two extreme segments of the focimeter Nos. 1 and 8, can consecutively be done for the inter- mediate segments Nos. 2, 3, 4, 5, 6, 7, and 8; and in fact it is un- avoidably done during the movement of the plate from No. 1 to No. 8; and the result is that every segment has the image of any small spot delineated upon it as if that spot was seen through a thin vapour. This being well understood, let us apply the same mode of C. i. { .