Abstract:

This paper analyses the problems existing in the measurement of the foci of thin lens，and the method of parallax elimination is used to measure the focal length of thin lens. The first method uses self-collimation and eyes instead of eyepiece system，which can change the adjustment of the image clarity into the adjustment of the lateral object and image parallax，and can greatly reduce the error caused by the focal depth. The second method uses the conjugate method to measure the focal length by the symmetry of object and image，and at the same time skillfully uses the method of eliminating parallax. The measurement results show that the measurement results obtained by the method of parallax elimination are more accurate，reaching 0.4%.

Key words: self-collimating, conjugate, parallax