System for imaging field rays (7, 8) from any point of an observed scene, comprising: - a detector; - a diaphragm positioned upstream from the detector and located at a distance d 0 from the any point of the observed scene; and - a device for focusing the field rays on the detector, the focusing device comprising an active dioptre comprising a set of discontinuities of refractive index or of thickness, the effect whereof is, for the design length λ 0, that of producing a diffraction efficiency near 100% at a diffraction order p, the phases of the field rays transmitted at the level of these discontinuities being offset by a whole number of times p2n, the focusing device being of design wavelength λ 0 comprised in the spectral band of the observed scene, the active dioptre of the focusing device being positioned between the diaphragm and the detector: o at a distance d I from the diaphragm minimising the astigmatism aberration at the level of the detector; o at a distance d d from the detector, the distance d d, dependent on the distance d 0, being the focal distance of said active dioptre; the imaging system being characterised in that: ▪ the focusing device comprises a blade comprising at least one Fresnel dioptre which is the active dioptre of the focusing device and which carries most of the optical power of said focusing device, this optical power being supplied by the phase continuity zones of said active dioptre, said optical power being suitable for focusing the field rays on the detector, ▪ said active dioptre is designed at the order p for the design wavelength, the order p being greater than or equal to three; ▪ the aperture L d of the diaphragm, positioned upstream from the active dioptre, is less than L dmax = L 4.d 0 / (d 0 + d I), where L 4 is the least distance between four consecutive discontinuities of the Fresnel lens, in order to, on one hand, limit to three the maximum number of phase discontinuity zones illuminated by said field rays of a given angle of incidence and, on the other, enable a local correction of aberrations, adapted to the angles of incidence of said field rays; ▪ said active dioptre is designed to perform said local aberration correction. - a detector; - a diaphragm positioned upstream from the detector and located at a distance d 0 from the any point of the observed scene; and - a device for focusing the field rays on the detector, the focusing device comprising an active dioptre comprising a set of discontinuities of refractive index or of thickness, the effect whereof is, for the design length λ 0, that of producing a diffraction efficiency near 100% at a diffraction order p, the phases of the field rays transmitted at the level of these discontinuities being offset by a whole number of times p2n, the focusing device being of design wavelength λ 0 comprised in the spectral band of the observed scene, the active dioptre of the focusing device being positioned between the diaphragm and the detector: o at a distance d I from the diaphragm minimising the astigmatism aberration at the level of the detector; o at a distance d d from the detector, the distance d d, dependent on the distance d 0, being the focal distance of said active dioptre; o at a distance d I from the diaphragm minimising the astigmatism aberration at the level of the detector; o at a distance d d from the detector, the distance d d, dependent on the distance d 0, being the focal distance of said active dioptre; ▪ the focusing device comprises a blade comprising at least one Fresnel dioptre which is the active dioptre of the focusing device and which carries most of the optical power of said focusing device, this optical power being supplied by the phase continuity zones of said active dioptre, said optical power being suitable for focusing the field rays on the detector, ▪ said active dioptre is designed at the order p for the design wavelength, the order p being greater than or equal to three; ▪ the aperture L d of the diaphragm, positioned upstream from the active dioptre, is less than L dmax = L 4.d 0 / (d 0 + d I), where L 4 is the least distance between four consecutive discontinuities of the Fresnel lens, in order to, on one hand, limit to three the maximum number of phase discontinuity zones illuminated by said field rays of a given angle of incidence and, on the other, enable a local correction of aberrations, adapted to the angles of incidence of said field rays; ▪ said active dioptre is designed to perform said local aberration correction.