Recent efforts in CMOS image sensor design have focused on reducing pixel size to increase resolution given a fixed package size. This scaling comes at a cost, as less light is incident on each pixel, potentially leading to poor image quality caused by photon shot noise. One solution to this problem is to allow the imaging or objective lens to capture more light by decreasing its f-number. The larger cone of accepted light resulting from a lower f-number, however, can lead to decreased optical efficiency and increased spatial optical crosstalk at the pixel level when the microlens is not able to properly focus the incident light. In this work, we investigate the effects of imaging lens f-number on sub-2µm CMOS image sensor pixel performance. The pixel is considered as an optical system with an f-number, defined as the ratio of the pixel height to width, and we predict the performance of a realistic pixel structure subject to illumination from an objective lens. For our predictions, we use finite-difference time-domain (FDTD) simulation with continuous-wave, diffraction-limited illumination characterized by the f-number of the imaging lens. The imaging lens f-numbers are chosen to maintain resolution and incident optical power as pixel size scales, while the pixel f-number is varied by modifying the height of the pixel structure. As long as pixel f-number is scaled to match the imaging f-number when pixel size is scaled, optical efficiency and crosstalk for on-axis illumination will not be significantly affected down to the 1.2 &mgr;m pixel node. We find the same trend for system MTF, which does not seem to suffer from diffraction effects.