The continued development of focal plane arrays of increasing size provides new challenges in the design of the associated imaging optics. The main aim must be to achieve an acceptable balance amongst a range of conflicting requirements, principally, optical performance, mass, space envelope and low cost. In this case the development of a high relative aperture zoom optic, operating in the 3 to 5 µm waveband, is described. The basic configuration is that of a mechanically compensated zoom objective with additional pupil relay optics. The relatively long focal length and high aperture of the optical system combined with the need for compactness presents particular problems regarding chromatic correction over this waveband. Whilst the conventional silicon/germanium combination is employed, this alone proved to be inadequate, hence the inclusion of diffractive surfaces, also the use of an additional high dispersion material. In order to maintain useful optical performance over the temperature range, a multi-sensor athermalization algorithm is derived to re-position the moving zoom groups. The design process is described, through to component testing.