Dual f/number cooled IR optics allow future systems (i.e. 3rd Gen FLIR) to be optimized for each operational mode, while maintaining the same size packages. This permits improved signal-to-noise ratios for search applications while maintaining long ranges for targeting applications. In addition, it enables the optimization of dual band sensor optics (MWIR and LWIR) based on the change of the diffraction limit with wavelength and f/number. In essence, dual f/number optics allow for improvements in either sensitivity or resolution for a given operational mode of a sensor, depending on which is more critical. To make the transition from 2nd Generation systems to 3rd Generation dual f/number systems, both the optics and cold stop require changes. This paper details the concept of dual f/number optics, their design requirements, and the optics related issues that are inherent in their design.