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 (58, 60, 62) and fourth and last, fifth downstream lenses (64, 66) 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 (58, 60, 62) 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 (58,60, 62) 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 (64, 66), the three upstream lenses (58, 60, 62) being configured to form the preliminary image in a focal plane 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 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 (64, 66), 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 (58, 60, 62) and fourth and last, fifth downstream lenses (64, 66) 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 (58, 60, 62) 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 (58,60, 62) 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 (64, 66), the three upstream lenses (58, 60, 62) being configured to form the preliminary image in a focal plane 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 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 (64, 66), 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.