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Angle

degrees, lens, lenses and wide-angle

ANGLE, WIDE." Wide angle" is a term applied to certain forms of lenses so constructed that they will embrace a larger amount of view than the ordinary or narrow angle lens by having their focus very short in comparison to the base line of the plate. Figs. 31 and 32 will serve to show the meaning of this more clearly.

Wide-angle lenses are most useful in photographing buildings or other objects when we require to include a large amount in the picture, and which with an ordinary lens it is impossible, owing to the inability to place the camera at a sufficient distance.

The angle of view of the eye does not exceed 5o degrees. A lens with an angle of 40 degrees gives the most artistically truthful picture, and except for special purposes already men tioned, no lens should be used with a greater angle than 5o degrees. The result of using wide angle lenses is pictures with painfully exaggerated perspective, the objects in the foreground appearing large and out of proportion, while the more distant ones are dwarfed and appear at an exaggerated distance.* This power of making exaggerated photographs has been largely made use of for advertising purposes. Photographs taken by wide-angle lenses can be seen every day advertising houses for sale, illustrating the saloon accommodation of steamboats, etc., in every case deceiving the eye of the observer as to the actual size of the object represented.

Wide-angle lenses, however, possess many drawbacks. They require a small diaphragm, in order to obtain sharp definition, by the use of which the light is considerably reduced and the exposure necessarily increased in proportion, so that they are at best but slow workers.

They also zive unequal definition. If we take two equally bright and equidistant objects, the image of one falling on the margin, and that of the other upon the center of the plate, the area occupied by the first image will be greater than that occupied by the second, and consequently the marginal illumi nation will be less ; the diminution of light from both these causes increases with the angle of view beyond 4o degrees. At this angle the extreme margins receive 8o per cent. of the light falling upon the center at 5o degrees but 7o per cent., at 6o degrees, only 55 per cent., and at 7o degrees no more than 45 per cent. Therefore, it will be seen that with the increase of angle we get an in crease of this defect. Dallmeyer makes a lens with an angle of ioo degrees. A lens of 'this description, however, would only be useful for certain purposes. Wide-angle lenses give dis tortion, particularly noticeable in photographing buildings. The swing back of the camera can be used to remedy it, but with it a small stop must also be added. See Lens.