Photographic optics and colour photography : including the camera, kinematograph, optical lantern, and the theory and practice of image formation / by George Lindsay Johnson.

  • Johnson, G. Lindsay (George Lindsay), 1853-1943.
Date:
1909
Catalogue details

Licence: In copyright

Credit: Photographic optics and colour photography : including the camera, kinematograph, optical lantern, and the theory and practice of image formation / by George Lindsay Johnson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    22/348 (page 4)
    If /, be very much larger than /2, which it usually is, it may be omitted without any perceptible error, and the equation becomes simplified to Then in the above case d = \/0,25 x 6M or 0,45 mm. We may therefore give the following rule :— Multiply the distance between the screen and pinhole, expressed in decimals of a metre, by 0,81, and the square root of the product will give the diameter of the pinhole in millimetres. d The intensity of light is proportional to j (/2 being the distance of the screen from the hole), but it must be remem- bered, as D. J. Carnegie first pointed out,1 that the thickness of the perforated plate always reduces its effectivity, so that the exposure must be increased in practice by about 30 per cent. This is more especially the case as the plate distance is diminished below 10 in. The size of the picture is always the ratio between Distance of object from pinhole Distance of screen from pinhole so that if the object is 20 ft. away, and the camera exten- sion = 6 in., the image = ~ of the object. Pinhole pictures have the advantage of being invariably orthoscopic, but the exjfesure is enormously greater than with a lens. Compared with a portrait lens working at ^, a pin- hole made with a No. 5 needle in. diameter), with the plate 15 in. distant, the exposure will be f-^-^ = H62, or about 13,400 times as long. Moreover, a pinhole does not give rise to point images, but to circular patches of light, or rather, diffraction rings, which are not sharply marked out, but fade away from the centre to the edge of the patch; the rate of fading depending on the distance of the screen and the diameter of the hole. We have mentioned the pinhole camera first on account of its extreme simplicity, but in point of importance the following 1 British Journal of Photography, June 22, 1906.