Imagers designed for different system applications may comply with different requirements. Performance can be optimized only with an appropriate metric. A CMOS imager 'figure-of-merit (FOM)' for such a purpose is presented in this paper. It evaluates a CMOS imager from three performance categories: spatial resolution, sensing speed, and dynamic range. The sensing speed performance index (SP) is obtained by analyzing the imager signal-to-noise ratio versus exposure level. A modulation transfer function normalized to a 35 mm film standard assuming an identical field-of-view is introduced as the spatial resolution performance index (MTF<SUB>35e_Avg</SUB>), regardless of image sensor size. The dynamic range performance index, DR, is defined as ratio of the maximum signal level the imager is able to capture to its noise level. The tradeoffs among SP, MTF<SUB>35e_Avg</SUB>, and DR of a CMOS imager are quantitatively analyzed, and its FOM is found to be FOM equals DR X SP X (MTF<SUB>35e_Avg</SUB>)<SUP>2</SUP>. This paper also gives examples of optimized imager design using the proposed FOM.