Definition: (1) Noun: act of setting or driving lens in such a way as to make image look sharp. * Achieved either (a) indirectly e.g. by observing image on focusing screen or by using optical range finder e.g. swing wedge type or (b) directly by driving focusing mechanism as in autofocus systems or by direct observation of image on focal plane itself as in enlarging or slide projection. * In rare instances, focus may be achieved by moving the focal plane. (2) Adjective: ~ device, ~ component: designed to achieve or help achieve focus e.g. focusing aid is any device ranging in character from (a) optical e.g. split-image rangefinder or microprism screen to (b) mechanical e.g. focusing handles on a lens that are designed to improve manual focusing.

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