First, based on the theory of Fresnel diffraction, the diffraction patterns are analyzed when the point source at the front focus or it's nearby of the lens.

首先，利用菲涅耳衍射理论，分析了无衍射成像系统中点光源在透镜焦点及焦点前后所产生的无衍射光斑形式。

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youdao

Learn to focus on what's in front of you, and find peace in whatever you do.

学会专注于你眼前的事，并在做任何事的时候都心态平和。

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But in front of the focus, and behind it, the energy density is lower, and little or no damage is done.

ECONOMIST: Thermal treatments of cancer are remarkably effective

On the corporate front, the focus was on Vodafone, as Mannesmann's board convened to discuss Vodafone's all-share terms and investors assessed its merits and chances of success.

BBC: Friday close

Dedicated lens rings allow for fingertip control of focus and aperture, while a DSLR-style Focus mode dial on the front of the camera enables easy switching between focus modes.

ENGADGET: Sony's Cyber-shot RX1 compact camera packs a full-frame sensor, 'fits in your palm' (hands-on)