Because of focus changing when TFT inspection device performing auto focus, the resolution of device will inevitably changes. So there is a need to offer a fast estimation method for device’s actual resolution. Since total resolution is the product of camera resolution and lens resolution, at first, camera contrast limit is set up based on all kinds of noise model. Then, LCD white and black bar fringe are used whose frequency changes gradually to estimate lens MTF degeneration. Finally, through relationship between resolution and contrast, the spatial cut through frequency can be calculated by differentiating imaging system’s light dark blur smallest differences at highest camera contrast distinguish condition. Experimental results indicates that when using optic inspection device to examine cell phone background light panel's defects, article's method estimates the actual resolution less than 17 μm , in contrast test, the calibrated sample defects exceeds 17 μm can be photographed clearly using device. The results prove the effectiveness of the proposed method.