Focus: plane of focus; focal length; and depth of field.
焦距：焦点平面，焦距和景深。
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 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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