Visual input can be used to recover the 3-D structure of a scene by estimating distances (depth) to the observer. Depth estimation is performed in various applications, such as robotics, autonomous driving, or surveillance. We present a low-power, compact, passive, and static imaging system that computes a semi-dense depth map in real time for a wide range of depths. This is achieved by using a focus-tunable liquid lens to sweep the optical power of the system at a high frequency, computing depth from focus on a mixed-signal programmable focal-plane processor. The use of local and highly parallel processing directly on the focal plane removes the sensor-processor bandwidth limitations typical in conventional imaging and processor technologies and allows real-time performance to be achieved.