Abstract: An infra-red imaging system has two variable focal length optical components (6, 7) whose positions are fixed relative to one another. The system also comprises an image detector (8). Control means (13) is provided such that the focal length of one of the variable focal length optical components (7) can be varied in relation to the focal length of the other variable focal length optical component (6). The system can be used to produce an image of variable magnification while maintaining an in-focus image at the detector (8). The control means (13) may comprise a mechanical linkage, an electronic circuit, or a computer program. In an alternative embodiment the optical system is a beam expander. Controlling a focusing mirror can achieve a dither effect. Controlling a focusing mirror can also de-focus the image, giving a mean scene temperature evaluation.