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lens, diaphragm, focus, stop, depth and focal

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DIAPHANOSCOPE —A dark box for exhibiting transparent pictures with or without a lens. The positive should be placed in the box at a distance from the eye equivalent to the focal length of the lens with which the negative was made.

DIAPHRAGM.—These are metal plates, with a hole in the centre of different diameters, rang ing in geometrical proportion to the focus of the lens to which they belong. Fig. 152 shows the form in which they are usually made. Each plate is made to slip in front of or between the combination of the lens, and obstruct the transmission of the marginal rays, and allow only those to enter which are parallel to the axis of the lens. The larger the aperture, or the longer the focus of the lens, the less depth of focus. By inserting a "diaphragm," or "stop "* as it is some times called, we are able to reduce the aperture of the lens and increase the depth of focus, and the smaller the stop the greater the depth. But in doing this we also reduce the amount of light which it transmits, is directly proportioned to the area of the diaphragm aperture. Some idea of the ef fect of the stop is shown in Fig. 153. Here we have the stop placed close to the lens, which immedi ately reduces it very consider ably. In Figs. 154 and r55 we see the effect of the diaphragm upon the depth of focus of the lens. The rays r r coming from a dis tant point, form, after passing through the lens L, an image at or on the ground glass B. If we move the ground glass to C or to A, the image would immediately spread out and become indistinct. But in the next illustra tion we see the rays r r are caused by the stop to be much less convergent when leaving the lens, in consequence, the ground glass could be moved to A or to C without any appreciable al teration taking place in the image.

If we now move the stop to a proper distance from the lens it becomes a diaphragm. In single lenses it is usually placed from one-fourth to one-half of the focal length in front of the lens. In this positin it limits the dia meter of the pencils of light, causing them to cross the axis at the aperture of the diaphragm before refraction. In symmetrical doublet lenses the position

-of the diaphragm should be in the centre of- the two combinations. In combinations which are not sym metrical the position is proportionate to the foci of the combinations.

Besides giving greater depth of focus the effect of the stop is also to correct spherical and other aberrations in a lens (see Aberration.) The principal points to be remembered in using a diaphragm are these : The larger the aper ture the bolder the picture and the more rapid the exposure required. In focusing always remove the dia phragm. The smaller the diaphragm, the longer the exposure, the greater the depth of focus, and the flatter the image.

Every diaphragm possesses a focal value which is the relation of its diameter to the equivalent focal length of the lens to which it belongs, and it should be num bered accordingly. To find this number first ascertain the equivalent focus of the lens, and then divide this by the diameter of the central aperture of the diaphragm. For example: Focal length of the lens is 6 inches, and the diameter of the diaphragm inch 6 + 12. The number of the diaphragm is therefore, 12, or, as it is expressed, f/12. This system is a very convenient one, as it assists in the calculation of the exposure. If all other conditions be equal the exposures required arc proportional to the squares of the denomina tors of these fractions. The Photographic Society of Great Britain have adopted a different system, taking p4 as the standard, which is termed No. 1. This system is called the Uniform System, and the numbers the U. S. No. By its means the comparative exposures are at once seen. To find out the U. S. No. of any diaphragm marked upon f/x system, the following is the rule: Double the focal length of the lens by the diameter of the diaphragm to f/x, square the result, and divide by sixteen, which will give the U. S. No. Example: Find the U. S. No. of diaphragm marked f/2o. 20 X 20 = 400; 400 4- 16 = 25, the U. S. No.

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