The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Diaphanoscope                                                 Diaphragms
Diaphanoscope. A device for looking at a photograph some-what similar to the Alethoscope, Pantoscope, or Lanternoscope.
Diaphragmsa partition) are either loose plates
of metal, or rotating metal screens, both having apertures of certain diameters; or another form is composed of tongues of metal actuated by an external pin or ring, which can be closed and opened out to any desired size. The Iris diaphragm consists of thin flat tongues of metal fastened to a ring in the lens "mount, by means of which the aperture of the diaphragm may be enlarged or diminished by turning the ring backwards or forwards, causing the tongues to contract or enlarge the opening, the use of which obviates all chance of losing or mis-placing the diaphragms. The diaphragms of the ordinary or Waterhouse pattern can be pinned together by a brass rivet just by the tongue, on which the numbers are stamped, thus lessening the chance of losing them. The influence of the diaphragm on the picture is great and of considerable practical importance, not only on the character of the picture, but also on the duration of exposure, as we shall see when treating of that subject. The influence of the diaphragm on the character of the image transmitted by the lens is seen first in a reduction of the amount of light admitted by the lens; secondly, by increase of the marginal definition; and, thirdly, by increase of the depth of Focus. For single lenses the diaphragm is usually placed from of the focal length in front of the lens, in which position it limits the djameter of the pencils of light, and causes them to cross the axis at the aperture of the diaphragm, before refraction. (See Distortion.) The distance of the diaphragm is in many instances, when placed in front of the lens, the cause of Flare (q.v.). This can be obviated by altering the position, one-eighth of an inch either way being generally sufficient to obliterate it. In all symmetrical doublet lenses, the proper position of the diaphragm is equidistant between the two combinations; in unsymmetrical combinations, the position is proportionate to the foci of the combinations. For general use the following maxims should be remembered : - A large diaphragm gives a bolder picture than a small one ; focus with the largest aperture, then insert the smaller diaphragms till sharpness is obtained over the whole screen. The S7naller the stop the longer the ex-
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