The invention relates to an imaging device which includes a matrix IR detector, a multifocal lens with a focal length ranging from a short focal length to a long focal length, including a front group of lenses, a variator, a compensator and means for positioning the variator and the compensator, the positions of the variator and of the compensator being respectively adapted to the focal length. This device includes a calibration system which comprises a control for the means for positioning the variator in the vicinity of its short focal length position and the compensator upstream of its long focal length position so as to conjugate the plane of the detector with a real object plane located upstream of the front group of lenses, this positioning being called calibration positioning.