In this paper, we present a 3D imaging system providing a semi-dense depth map, using a passive, low-power, compact, static, monocular camera. The demonstrated depth estimation system reconstructs 32 depth-levels in real-time at 25FPS drawing less than 1.9W of power. This is achieved by performing computation on an analog focal-plane processor that analyses frames captured through a vibrating liquid focus-tunable lens. The optical system provides shallow depth of focus images and fast sweeps of optical power, while the use of pixel-level processing removes the sensor-processor bandwidth limitations of depth-from-focus systems built using conventional imaging and processor technologies. All-in-focus images are also obtained.