This is the distance (usually in mm.) from the lens (or primary mirror) to the point where the telescope is in focus (focal point).

就是从透镜(或者主反射镜)到焦点的距离，通常单位是毫米(mm)。

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The Pentax ME-F, which used focus sensors in the camera body coupled with a motorized lens, became the first autofocus 35 mm SLR in 1981.

宾得我，男，该传感器用于集中在机身加上机动镜头，成为第一个自动对焦35毫米单镜反光相机于1981年。

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