An infrared imaging inverse telephoto optical system (50, 150) for focusing infrared radiation on an infrared detector, wherein the optical system operates in the mid-wavelength infrared spectral band, the optical system comprising: a front lens group (51, 151) having a negative optical power, wherein the front lens group (51, 151) receives an infrared radiation and comprises a first front lens (51a, 151a) with at least one aspherical surface (21, 121) and a second front lens (51b, 151b) with at least one aspherical surface (19, 119); an intermediate lens group (52, 152) that receives the infrared radiation from the front lens group (51, 151), wherein the intermediate lens group comprises a first intermediate lens (52a, 152a) with at least one aspherical surface (18, 118), a second intermediate lens (52b, 152b) with at least one aspherical surface (15, 115), and a third intermediate lens (52c, 152c) with at least one aspherical surface (14, 114); and a rear lens group (53, 153) having positive optical power, wherein the rear lens group (53, 153) receives the infrared radiation from the intermediate lens group (52, 152), and wherein the rear lens group (53, 153) comprises a first rear lens (53a, 153a) with at least one aspherical surface (12, 112), a second rear lens (53b, 153b) with at least one aspherical surface (10, 110), and a third rear lens (53c, 153c); wherein the imaging optical system (50, 150) has a stop (2, 102) between the rear lens group (53, 153) and a focal plane (1, 101) located at said infrared detector, and wherein the ratio of the physical length to the effective focal length is less than about 3.0, wherein the imaging optical system (50, 150) further comprises a corrector plate (54, 154) between the third rear lens (53c, 153c) and the stop (2, 102). a front lens group (51, 151) having a negative optical power, wherein the front lens group (51, 151) receives an infrared radiation and comprises a first front lens (51a, 151a) with at least one aspherical surface (21, 121) and a second front lens (51b, 151b) with at least one aspherical surface (19, 119); an intermediate lens group (52, 152) that receives the infrared radiation from the front lens group (51, 151), wherein the intermediate lens group comprises a first intermediate lens (52a, 152a) with at least one aspherical surface (18, 118), a second intermediate lens (52b, 152b) with at least one aspherical surface (15, 115), and a third intermediate lens (52c, 152c) with at least one aspherical surface (14, 114); and a rear lens group (53, 153) having positive optical power, wherein the rear lens group (53, 153) receives the infrared radiation from the intermediate lens group (52, 152), and wherein the rear lens group (53, 153) comprises a first rear lens (53a, 153a) with at least one aspherical surface (12, 112), a second rear lens (53b, 153b) with at least one aspherical surface (10, 110), and a third rear lens (53c, 153c); wherein the imaging optical system (50, 150) has a stop (2, 102) between the rear lens group (53, 153) and a focal plane (1, 101) located at said infrared detector, and wherein the ratio of the physical length to the effective focal length is less than about 3.0, wherein the imaging optical system (50, 150) further comprises a corrector plate (54, 154) between the third rear lens (53c, 153c) and the stop (2, 102).