An optical system for forming a final image of a non-circular light source on a workpiece, wherein the final image has a desired non-circular cross-section and a desired size B, the optical system comprising: a combination of five lenses having first, second and third upstream lenses and fourth and last, fifth downstream lenses sequentially spaced apart along a light path transmitting a beam which is emitted by the light source along the path, wherein said last, fifth downstream lens is an F-theta lens, the spaced three upstream lenses being configured to form a preliminary non-circular image of the source upstream from the fourth downstream lens with a size A different from the desired size B; wherein the beam incident on the first upstream lens is collimated and has a first waist thereof downstream from the first upstream lens , the three upstream lenses being configured to form an initial non-circular image of the source upstream from the first waist , wherein the second upstream lens is spaced downstream from the first upstream lens , and the third upstream lens is located between the second upstream lens and the downstream lenses , the three upstream lenses being configured to form the preliminary image in a focal plane (F3) of the third upstream lens , wherein the second upstream lens is configured to transmit the beam incident thereon so as to form a second waist , the third upstream lens being configured with a back focus (F3) coinciding with the second waist , wherein the F-theta lens is spaced downstream from the fourth downstream lens so that F 4 Fth = A B , with F4 and Fth being focal lengths of respective fourth downstream and F-theta lenses , wherein the F-theta lens converges the beam incident thereon so that the final image and a final waist of the converged beam substantially coincide with one another; and wherein the final image is formed within a Rayleigh range of the converged beam and spaced from the final waist at a distance d1 not exceeding ± 5% of the Rayleigh range. a combination of five lenses having first, second and third upstream lenses and fourth and last, fifth downstream lenses sequentially spaced apart along a light path transmitting a beam which is emitted by the light source along the path, wherein said last, fifth downstream lens is an F-theta lens, the spaced three upstream lenses being configured to form a preliminary non-circular image of the source upstream from the fourth downstream lens with a size A different from the desired size B; wherein the beam incident on the first upstream lens is collimated and has a first waist thereof downstream from the first upstream lens , the three upstream lenses being configured to form an initial non-circular image of the source upstream from the first waist , wherein the second upstream lens is spaced downstream from the first upstream lens , and the third upstream lens is located between the second upstream lens and the downstream lenses , the three upstream lenses being configured to form the preliminary image in a focal plane (F3) of the third upstream lens , wherein the second upstream lens is configured to transmit the beam incident thereon so as to form a second waist , the third upstream lens being configured with a back focus (F3) coinciding with the second waist , wherein the F-theta lens is spaced downstream from the fourth downstream lens so that F 4 Fth = A B , with F4 and Fth being focal lengths of respective fourth downstream and F-theta lenses , wherein the F-theta lens converges the beam incident thereon so that the final image and a final waist of the converged beam substantially coincide with one another; and wherein the final image is formed within a Rayleigh range of the converged beam and spaced from the final waist at a distance d1 not exceeding ± 5% of the Rayleigh range. <EOS>