Fast-response liquid lens enables accurate relative depth estimation imaging with cascaded extended depth of field

Conventional mechanical zoom systems still face key challenges including, slow focusing, limited depth of field (DOF), and lack of depth information in two-dimensional sensor. To overcome these bottlenecks, we describe a cascaded relative depth estimation imaging system enabled by extended DOF using elastic membrane liquid lenses. The optical system uses four elastic membrane liquid lenses to achieve continuous focal length tuning from 30 to 120 mm. Different from conventional mechanical systems, the proposed system achieves an approximately 20 ms settling time, improving the response speed by approximately two orders of magnitude. This ms-level response enables rapid scanning across multiple focal planes, which is well aligned with the temporal requirement of extended DOF fusion and generates clearer all-in-focus imaging, thereby improving the accuracy of relative depth estimation. Compared with a conventional system without DOF fusion, the proposed system demonstrates competitive advantages over the depth difference, which improves from 0.80 to 0.88, indicating clearer foreground-background separation. This strategy provides another approach for all-in-focus imaging of near and distant targets and shows potential for compact intelligent surveillance, unmanned aerial vehicle observation, and mobile robotic inspection.