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We demonstrate a new method to measure 3D position of a light source by tracking the ring images transformed through a single hemispherical lens. The ring images are formed by the large spherical aberration of the lens and always clear regardless of the distance and angle of the light source without any focusing mechanism. Dynamic change of diameter and center position of the ring image enables 3D position measurement of the light source with accuracy and high resolution. Experimental results verified that this new principle solves the 3D optical measurement problems of complex structures, narrow-angle views and slow measuring speed due to focusing mechanism, indicating the possibility for wide applications.