Optical centre: The visual centre o fa page.

视觉中点：页面视觉上的中心点。

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Focal length is the distance between optical centre and principal focus.

焦距就是镜片中心和焦点之间的距离。

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Widely applied for repair centre (circuit board and component) jewelry shop (silver) optical shop (glasses) watch shop (metal bracelet) beauty shop (scissors clip) and home user (denture) etc.

适用于维修中心(电路板及电子组件)，首饰店(银器)，钟表店(金属表带)，眼镜店(眼镜)，美容店(剪刀及夹子)，家用(假牙)等等。

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On 17 March 2005, Andrzej Udalski, professor of astronomy at Warsaw University, Poland, and leader of the Optical Gravitational Lensing Experiment (Ogle), noticed that just such an event was about to occur near the centre of our galaxy.

BBC: Artist's concept of a distant world (BBC)