A passive IR imaging system comprising, on its optical axis (z): - a matrix-array detector placed in a cryostat comprising a cold diaphragm; - an optical device for forming images on the detector, having a focal length that is continuously variable from a Large Field configuration of focal length F GC to a Small Field configuration of focal length F PC, with, in the range of focal lengths, a constant numerical aperture and an aperture diaphragm located at the cold diaphragm of the cryostat, said optical device comprising: ∘ a head group with a fixed position and a constant focal length comprising at least one lens mounted in a mechanical holding means, with a diameter that is determined by the Small Field configuration; ∘ a first and a second movable group able to be positioned in order to ensure the change of focal length between F GC and F PC as well as to ensure that the image is focussed on the detector; ∘ an image-transport group with a fixed position and with constant magnification, able to image the aperture diaphragm in order to limit the diameter of the envelope of the Small Field useful beams on the lenses of the head group Gf1; characterised in that the optical device comprises a Very Small Field configuration of a predetermined focal length F TPC, with the first and second movable groups (Gm1, Gm2) being positioned in order to obtain the focal length F TPC; and an aperture diaphragm for the Very Small Field configuration implemented in the mechanical holding means of the head group, such that the image in the detector space of this aperture diaphragm is located in the vicinity of the cold diaphragm of the cryostat. - a matrix-array detector placed in a cryostat comprising a cold diaphragm; - an optical device for forming images on the detector, having a focal length that is continuously variable from a Large Field configuration of focal length F GC to a Small Field configuration of focal length F PC, with, in the range of focal lengths, a constant numerical aperture and an aperture diaphragm located at the cold diaphragm of the cryostat, said optical device comprising: ∘ a head group with a fixed position and a constant focal length comprising at least one lens mounted in a mechanical holding means, with a diameter that is determined by the Small Field configuration; ∘ a first and a second movable group able to be positioned in order to ensure the change of focal length between F GC and F PC as well as to ensure that the image is focussed on the detector; ∘ an image-transport group with a fixed position and with constant magnification, able to image the aperture diaphragm in order to limit the diameter of the envelope of the Small Field useful beams on the lenses of the head group Gf1; characterised in that the optical device comprises a Very Small Field configuration of a predetermined focal length F TPC, with the first and second movable groups (Gm1, Gm2) being positioned in order to obtain the focal length F TPC; and an aperture diaphragm for the Very Small Field configuration implemented in the mechanical holding means of the head group, such that the image in the detector space of this aperture diaphragm is located in the vicinity of the cold diaphragm of the cryostat. ∘ a head group with a fixed position and a constant focal length comprising at least one lens mounted in a mechanical holding means, with a diameter that is determined by the Small Field configuration; ∘ a first and a second movable group able to be positioned in order to ensure the change of focal length between F GC and F PC as well as to ensure that the image is focussed on the detector; ∘ an image-transport group with a fixed position and with constant magnification, able to image the aperture diaphragm in order to limit the diameter of the envelope of the Small Field useful beams on the lenses of the head group Gf1;