We present a detailed analysis of logarithmic active pixel sensors (log-APS) to be used in complementary metal-oxide-semiconductor (CMOS) imagers for real-time on-chip motion detection. Based on an equivalent circuit model for CMOS-compatible photodiodes. an HSPICE simulation has been used to characterize different configurations of these sensors under various conditions of light intensities and switching speeds. These investigations are supported by the experimental results obtained from the chip fabricated with standard 0.5 mu m CMOS technology. It is concluded that more robust on-chip motion detection in CMOS imagers can be achieved with careful design of its log-APS photocircuits that considers the issues discussed here.