This paper presents a 4× compressive CMOS imager for always-on operation that achieves an energy efficiency of 51pJ/pixel, while maintaining high image quality of PSNR>32dB and SSIM>0.84. This is enabled by an energy-efficient compressive-sensing (CS) encoder, which replaces a densely populated CS encoding method with a highly sparse pseudo-diagonal one. Since the proposed CS encoder can be implemented with an energy-efficient switched-capacitor matrix multiplier at pixel outputs, data compression is achieved prior to pixel digitization, thereby greatly reducing ADC power, data size, and I/O power. The energy efficiency of the imager is further improved by incorporating it into dynamic single-slope ADCs. A prototype VGA imager consumes only 0.7mW at 45 fps. The corresponding energy per pixel (51pJ/pixel) amounts to a 20× improvement over the previous low-energy benchmark on CS imagers.