A novel, to the best of our knowledge, depth-sensing technology that enables a shallow depth of field was developed by adding a diffuser to the rear end of a mechanical control lens that can capture 2D images. The sensor in the optical depth-sensing system obtains the function curve between the motor step and the focus distance through calibration and imports the measured values into the control program's database. The optical depth-sensing system scans the visible range of an interval, and the Laplacian equation can be applied to confirm whether the interval was in focus by judging the sharpness of the contour of the objects captured in the interval and to define the outline of the objects. Then, the depth information can be obtained by calculating the focus distance based on the motor step during scanning. Finally, the focus images of individual objects are used to calculate the image contours in the depth direction. The focus images of each object are combined to reconstruct a 2.5D model within the sensing range. The optical depth-sensing system is not affected by sunlight or the material of the measured object. Furthermore, the system can be used to obtain color images by using a modified lens. The optical path is simple and does not require complex calculations. Therefore, the proposed system is not easily affected by the environment and exhibits high resolution and calculation speed.