只有在近轴光线条件下，平行光通过凸透镜才可近似聚焦于一点。
The parallel light rays of the prnicipal axis that approach a convex lens can converge to the focal point.
同时通过近似计算给出了由于加速间隙电压波动引起束聚焦焦斑模糊范围增大的上限值。
The upper limit values of blurred area coverage of focal spot caused by the fluctuation of accelerating gap voltage are also calculated using a approximate method.
文中还对锐聚焦系统给出了最佳归一化离焦量的表达式和相应的ETF近似公式。
For sharp-focusing system, the expressions for the position of optimum image plane and its corresponding ETF are also given.
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