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LENSES
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circle of least confusion has a greater diameter than
T±j>". In this case the distance in feet beyond which all
objects may be regarded as in focus is given by the formula
8*3 / /2, in which /= intensity ratio and / is the focal
length in inches of the lens used. Thus, suppose the
intensity / is equal to J, and the focal length of lens 6",
8*3 x62 then the distance will be equal to — =37.5 feet.
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From this it is apparent that the greater the intensity, the greater the distance which the camera must be placed from the objects to get uniform clearness, in other words, the smaller the stop used, the shorter will be the distance beyond which all things may be regarded as in focus.
The Rapid Rectilinear Lens. — The Rapid Rectilinear (R. R.) lens is the most useful for general purposes.
From the diagram this can be seen to consist of two compound lenses, one at each end of the tube, the stops being placed between these lenses.
Such lenses are the best to employ for all kinds of outdoor work, whether landscapes, architecture, or instantaneous effects.
All the best classes of these lenses are so made that the front or back lens may be used by itself, and in some cases, when these lenses have different focal lengths, it is possible to obtain from the same standpoint three pictures graded in size.
Another rapid rectilinear lens which works well at a large aperture (//5*6) is that known as the " Ross Homocentric ." The name implies that ideal definition can be produced by this lens, each point of the object being represented by a corresponding point in the image, and