Midwave infrared discrete zoom lens

A discrete zoom lens 1 is corrected over a mid-wave infrared (MWIR) or 5.1-3.3μm waveband and includes from a remote object to an image plane a scanning mirror, first 10 and second 20 changeable groups with positive optical power, a fold mirror, a second group of positive optical power 2, an aperture stop and a focal plane array FPA. A ratio between the focal lengths of the first and second changeable groups is 1.41x and a magnification of the second group is about 1x. The first or second changeable group may receive light from the remote object, direct converged light onto the second group and include three optical elements 11,12,13/201,202,203 with in order from the remote object to the image plane, positive, negative and positive optical powers. Both the first and second changeable group may create an intermediate image before the second group and the fold mirror may bend an optical axis between the first or second changeable group and the second group. The second group may include eight elements 21,22,23,24,25,26,27,28 having in order from the remote object to the image plane positive, positive, negative, positive, negative, positive, positive, positive optical powers. The aperture stop may be a cold shield.