A mid infrared spectral band continuous zoom system (100, 200) for thermally imaging an object, comprising: a positive element (102, 202) that focuses incoming light (150, 250) from the object; a negative focal length group (104, 204) configured to move for zooming, and that intercepts the light being focused by the positive element and creates a virtual image; a positive focal length group (106, 206) configured to move for zooming, that relays the virtual image created by the negative focal length moving group to an intermediate image (156, 256); a relay group (110, 208) that relays the intermediate image to a final image plane (158, 258); and a cooled infrared camera (114, 210) that thermally images the object at the final image plane, characterised in that the negative focal length group and the positive focal length group are moved relative to one another, and the relay group is configured to be fixed during zooming, while providing a continuous zoom range with a focal length change greater than 25X; the positive element is made from zinc selenide; the negative focal length moving group includes a first element (104a, 204a) made from Ge 33 As 12 Se 55 glass, and a second element (104b, 204b) made from germanium; the positive focal length moving group includes a first element (106a, 206a) made from silicon, a second element (106b, 206b) made from zinc selenide, and a third element (106c, 206c) made from germanium; and the relay group includes a first element (110a, 208a) made from germanium, a second element (110b, 208b) made from silicon, a third element (110c, 208c) made from germanium, a fourth element (110d, 208d) made from zinc sulfide, and a fifth element made from chalcogenide. a positive element (102, 202) that focuses incoming light (150, 250) from the object; a negative focal length group (104, 204) configured to move for zooming, and that intercepts the light being focused by the positive element and creates a virtual image; a positive focal length group (106, 206) configured to move for zooming, that relays the virtual image created by the negative focal length moving group to an intermediate image (156, 256); a relay group (110, 208) that relays the intermediate image to a final image plane (158, 258); and a cooled infrared camera (114, 210) that thermally images the object at the final image plane, characterised in that the negative focal length group and the positive focal length group are moved relative to one another, and the relay group is configured to be fixed during zooming, while providing a continuous zoom range with a focal length change greater than 25X; the positive element is made from zinc selenide; the negative focal length moving group includes a first element (104a, 204a) made from Ge 33 As 12 Se 55 glass, and a second element (104b, 204b) made from germanium; the positive focal length moving group includes a first element (106a, 206a) made from silicon, a second element (106b, 206b) made from zinc selenide, and a third element (106c, 206c) made from germanium; and the relay group includes a first element (110a, 208a) made from germanium, a second element (110b, 208b) made from silicon, a third element (110c, 208c) made from germanium, a fourth element (110d, 208d) made from zinc sulfide, and a fifth element made from chalcogenide.