A method for precision-adjustable optical paths is proposed to measure the central thickness of a lens based on low-coherence interference with transmitted illumination. A pair of wedge prisms are used to translate the movement that is nearly perpendicular to the optical axis with low accuracy into movement along the optical axis with high accuracy, and thus the optical path along the optical axis can be precisely adjusted. The accuracy for adjusting the optical-path difference (OPD) is related to the wedge angle. The smaller the wedge angle, the more accurate the adjustable OPD. A pair of prisms with a wedge angle of 5°30' and a linear stage with 5 μm accuracy are used to realize the movement along the optical axis with an accuracy below 0.5 μm, and the central thickness accuracy of the tested lens is less than 0.9 μm. The proposed method is employed to improve the contrast of interference fringes. Moreover, the central thicknesses of lenses with different shapes can be measured by the proposed method and the thickness-measuring scope can be extended easily.