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Focus Adjuster

lens, camera, image and object

FOCUS ADJUSTER.- .An arrangement fitted to a lens, and by means of which the focus can be adjusted to different lengths for various purposes. Traill Taylor in his work on photo graphic optics there describes a convenient form devised by himself. It consists in a sliding piece of brass made hollow in order to secure lightness. This contains four apertures, into each of which is fitted a thin achromatized lens of a negative power. This piece slides through the lens mount by means of an aperture. The combination to which this system is attached is a doublet composed of two slightly meniscus lenses, which, when used alone, do not give a flat field. By inserting the slide the influence of either of the four concave lenses contained in it is to flatten the field and lengthen the focus—the marginal pencils being well corrected with a moderately large aperture. With No. i lens the focus was 7 inches, with No. 2 it was length ened to 9, with No. 3 to 12, and with No. 4 it was increased to 15 inches.

FOCUSING.—The act of bringing the image into focus. When the camera is set up it should be racked out to its equivalent focus, which, for convenience sake should be marked on the baseboard of the camera. The cloth is then covered over the head, and an object about midway in the middle distance of the view is sharply focused. The image of nearer objects will

be indistinct, however. The stops are then inserted in order, commencing with the largest until the whole of the image becomes as clearly defined as desired. It is not always advisable, from an artistic point of view, to stop the lens down so completely that every object near and far is in focus. Many pictures require the principal object in the foreground to be sharp, the objects in the background being thrown out somewhat. For obtaining very sharp images the ground glass screen system is often very imperfect. The telescopic method devised by Taylor will be found far more perfect, and, further, permits of a moving object being focused while the plate is in the camera ready for exposure. A small telescope is obtained of exactly the same focus as the lens. The front part is attached to a pin fitted to the front part of the camera, and the eyepiece end is attached to the frame of the camera which contains the plate-holder. Care must be taken in fitting this part that the telescope and the camera are exactly in focus together. The image on the screen must be carefully examined with a magnifier. The quality of the ground glass will be found very important in focusing. See Focusing Screen.