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View Finder

lens, camera, ground and image

VIEW FINDER (Pr., Viseur ; Ger., Bild sucher, Sucher) An accessory showing the amount of subject included by the lens of a camera. The com monest form A resembles a miniature camera obscura. A small convex lens I, throws upon a mirror M the rays proceeding from objects in front of the camera. ia is inclined at an angle of 45, and reflects an image upon a horizontal ground-glass screen s. 'Usually a hood shields the ground glass from extraneous light. The frame, or mask, surrounding the ground glass should be of such a size that the image contains the same amount of subject as is shown by the camera lens on the focusing screen or plate. A new finder should be tested, and if it includes too much, pencil lines should be drawn round the margins of the ground glass to exclude the surplus subject, the outer space being then blocked out with black paint. If it shows too little, a mental allowance will have to be made when exposing. With this type of finder the image is dull, due to loss of light caused by the ground glass.

In the brilliant finder B the ground glass is re placed by a second convex lens, which receives the rays from the mirror, forming a very bright image. The first finder of this kind was con structed by A. L. Adams and H. Hill in 1894, and there have been many modifications. Some

patterns have a reflecting prism 'instead of a mirror, with one side ground to a curve to form a lens. An unfortunate peculiarity of the bril liant finder in its simplest form is that the amount of image included varies slightly accord ing to the position of the observer's eye. This defect was overcome, in 1898, by Beck, who placed a rectangular opening or mask Ai between the two lenses at the focal distance of the first lens. (See diagram C.) There is a growing tendency to use a finder in which the object is directly viewed. This may consist either of a small concave lens or an open wire frame with a sight. (See" Direct Pinder.") A few cameras for special purposes are fitted with field-glass or telescopic finders.

Among more recent improvements in finders must be mentioned the Adams Identoscope (195), illustrated at D, the front lens of which is made to move in unison with the camera lens, so that the exact effect of using the rising or falling front is immediately shown.

The reflex and twin-lens cameras have full size finders of the camera obscura type, the camera lens itself, in the case of the reflex, serv ing also as the finder lens.