Fluid thermal compensation system

A system for adjustably locating a lens (16, 18) as a function of the temperature of the lens, comprising a housing (12) and a lens mount (14) which is movable in an aperture in said housing in an axial direction, the lens being mounted in the lens mount. An axially extending chamber is defined between a radial surface of a radially extending portion (34) of the housing and a radial surface (32) of a radially extending portion (22) of the lens mount. A fluid is accommodated in the chamber, the fluid having variable volume as a function of temperature, such that the distance between the two radial surfaces, and thereby the location of the lens, is variable as a function of the temperature of the fluid.