In this experiment, we use two gratings with the same frequency to generate Moire fringes and use them to measure the long focus length of lens.

本实验是利用两块空间频率相同的黑白光栅产生的莫尔条纹来测量长焦距透镜的焦距。

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When zoom lens are used for military sighting, the drift of optical center caused by the change of focus length will reduce the sighting accuracy severely.

长焦变焦镜头用于军事瞄准时，变焦过程中较大的光中心漂移，降低瞄准精度。

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A child that suffers from ADHD may have trouble following instructions includingthose for finishing school work, performing chores or anything else that requires focus for any length of time.

一个患有多动症的小孩可能会有困难跟着教学指令，比如说需要更长的时间来完成家庭作业，做些杂物或者其他的一些事情。

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Tried to make the report, despite its technical focus and length, something that could be read widely.

FORBES: Live Blogging The President's Economic Recovery Advisory Board Meeting On Taxes

Yesterday, we were able to focus at length on the extraordinary economic dynamism of the Asia Pacific region and the U.S. interest in, again, expanding our own presence here in the region.

WHITEHOUSE: Press Briefing with Ben Rhodes and Admiral Robert Willard

If you focus on that, length will usually take care of itself.

FORBES: How long should an Internet video be? -media training