Single aperture multispectral systems are becoming prevalent thanks to advances in multispectral detectors, new optical materials, and new methods for selecting materials that minimize chromatic and thermal focal shift. This design study focuses on design of a three field-of-view, multispectral lens operating across the MWIR and LWIR spectral regions. The lens in question will have an f-number of f/3 with a 3X zoom ratio. The narrow full field-ofview of the lens is 3.33° with a wide full field-of view of 9.99°, the length of the system is 163 mm. The performance goal for the lens is diffraction limited over the thermal region. The study will provide an overview of material selection using an updated γv-v diagram, to provide achromatic and athermal characteristics. The study will then step through first order layout, optimization with key constraints, and tolerancing for manufacturability. Finally, the study will provide detailed analysis of system performance including as-built MTF over temperature, aberration analysis, and NETD contributions from narcissus.