Abstract: To enlarge the measurement range of a potential vision meter and improve its measurement accuracy for patients with high error of refraction, the imaging characteristics of its optical system were investigated. The visual magnification equation of the system was derived. The influence of the focal length variation of the collimating lens and the imaging lens on the magnification was analyzed respectively. The focus of the collimating lens has effects on the slope of the visual magnification curve while the focus of the imaging lens affects visual magnification of the whole moving range in the same way. With ideal parameters, the visual magnification can remain constant independent of the position of the vision pad in the moving range, and the system error can be removed. The optimum parameters for the meter were found by numerical analysis and the measurement scope of the system was expanded from -7D～＋12D to -11D～＋15D.