平行于主轴的光线，通过镜头后相交于一点，这一相交点称为焦点。通过薄透镜两个球面球心的直线，叫做主光轴，也称主轴。主光轴在相机等光学元件中尤为重要。
从主光轴外伸端向泵看，为顺时针扭转。
Overhang from the spindle to the pump to see, for clockwise rotation.
针孔成像模型下非主光轴上的球体成像为椭圆，椭圆中心偏离球心投影点。
The image of sphericity which does not located the principal optic axis is an ellipse based on pinhole model, and the image center offsets from the projection of the sphere center.
当主光轴旋转时，因为失衡品质离心惯性力的效用，主光轴将萌生屈曲变型。
When the axis of rotation, because of the imbalance in the quality of centrifugal force between the utility, will the initiation of buckling deformation.
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