同样的锐度下降问题也出现在镜头的长焦段（包括中央的锐度也下降得厉害）。
The same applies to the sharpness in general (including the image center) at the lens' tele setting.
手持拍摄时，最低安全快门速度为所用镜头焦距的倒数，低于这一速度，相机震动可能导致照片的锐度下降。
Camera shake rule: The slowest shutter speed at which you can safely handhold a camera is one over the focal length of the lens in use.
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