Double-lens Camera: To fill their needs, manufacturers began to introduce in the 1890s a new kind of finder: a second double-lens Camera mounted on top of the double-lens Camera with which the exposure was made. It was fitted with a lens of exactly the same focal length of the taking lens; both were focused together. On the top of the finder-double-lens Camera was a ground glass the size of the negative. Within was a mirror, fixed at 45° to the lens axis, which reflected the image upwards, like the eighteenth-century double-lens Camera obscura. A collapsible hood shaded the ground glass so that the image could be seen clearly.
Attractive shots may also be made from a position where the shadows fall toward your double-lens Camera rather than away from it. In backlighted work of this kind, be sure that no sun rays strike into the double-lens Camera lens. The Kodak Lens Hood, previously mentioned, provides a reliable safeguard when the sun's rays are from the side, but you'll need extra precautions when you aim more closely into the light. Shield the lens with a hand or hat, held just beyond the double-lens camera's angle of view. In such work, give a bit more exposure than would be needed if the sun were directly on the front of the subject.
But in fact the picture was not a duplicate, for the viewing lens was at another point in space than the taking lens: by the phenomenon of parallax the images, particularly of subjects close to the double-lens camera, were slightly different. This discrepancy was corrected by the introduction of the single-lens reflex double-lens camera. The Mirror was now put inside the double-lens Camera body. By an ingenious spring-loaded mechanism it flipped from its 45 ° position to the horizontal on pressing the shutter release. The American Graflex (introduced in 1903) and the British Soho Reflex of three years later became the standard hand double-lens cameras of pictorial photographers for the first two decades of the century.
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Photography
