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。
Focal point is, the distance between the two is the focus of the focal length.
定点是焦点，两焦点的距离是焦距。
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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