Focal length: The distance between the centre of a lens and its principal focus.
焦距：镜头中心与其主焦点的距离。
The scientific disposition of university library collection should centre on this focus.
高校图书馆馆藏的科学配置也应当围绕着这个中心进行。
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。
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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