Definition: Verb: (1) to make image look sharp, usually in order to bring film or projection plane into coincidence with focal plane of optical system. * To adjust an optical system so that an observer sees a clear image. Noun: (2) Rear principal focal point. (3) The point or area on which e.g. attention or compositional elements converge.
Focus on focusing
There are many methods of adjusting a system to obtain a clear image: in close-up work e.g. microscopy or macrophotography it is often easiest to move the whole assembly. With long distances, you can adjust the focus by moving lens elements e.g. as in camera lenses or binoculars. But for very long distances it may be easiest to adjust just the eyepiece optics e.g. astronomical telescopes.

Related Terms: focal point, focusing

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