An optical system comprising at least a light-transmitting member forming one of a plurality of lenses, and an aperture diaphragm, wherein: said light-transmitting member is formed with an optical surface which is concave with respect to said aperture diaphragm; and an anti-reflection coating is provided on said optical surface; wherein said anti-reflection coating comprises multiple layers, the multiple layers comprising a magnesium fluoride layer, formed by a sol-gel method, whose refractive index is equal to or smaller than 1.3, and a plurality of lower layers (1a-1c) made by a vacuum deposition, located between the transmitting member and the magnesium fluoride layer, said light-transmitting member, whose refractive index is 1.52, is provided with said anti-reflection coating that comprises: a first layer, which is formed on said optical surface of said light-transmitting member, said first layer having a refractive index of 1.65 and an optical film thickness of 0.27λ; a second layer, which is formed on said first layer, said second layer having a refractive index of 2.12 and an optical film thickness of 0.07λ; a third layer, which is formed on said second layer, said third layer having a refractive index of 1.65 and an optical film thickness of 0.30λ; and said magnesium fluoride layer, which is formed on said third layer, said magnesium fluoride layer having a refractive index of 1.25 and an optical film thickness of 0.26λ; "λ" being a reference light-ray wavelength of 550 nm. said light-transmitting member is formed with an optical surface which is concave with respect to said aperture diaphragm; and an anti-reflection coating is provided on said optical surface; wherein said anti-reflection coating comprises multiple layers, the multiple layers comprising a magnesium fluoride layer, formed by a sol-gel method, whose refractive index is equal to or smaller than 1.3, and a plurality of lower layers (1a-1c) made by a vacuum deposition, located between the transmitting member and the magnesium fluoride layer, said light-transmitting member, whose refractive index is 1.52, is provided with said anti-reflection coating that comprises: a first layer, which is formed on said optical surface of said light-transmitting member, said first layer having a refractive index of 1.65 and an optical film thickness of 0.27λ; a second layer, which is formed on said first layer, said second layer having a refractive index of 2.12 and an optical film thickness of 0.07λ; a third layer, which is formed on said second layer, said third layer having a refractive index of 1.65 and an optical film thickness of 0.30λ; and said magnesium fluoride layer, which is formed on said third layer, said magnesium fluoride layer having a refractive index of 1.25 and an optical film thickness of 0.26λ; "λ" being a reference light-ray wavelength of 550 nm. a first layer, which is formed on said optical surface of said light-transmitting member, said first layer having a refractive index of 1.65 and an optical film thickness of 0.27λ; a second layer, which is formed on said first layer, said second layer having a refractive index of 2.12 and an optical film thickness of 0.07λ; a third layer, which is formed on said second layer, said third layer having a refractive index of 1.65 and an optical film thickness of 0.30λ; and said magnesium fluoride layer, which is formed on said third layer, said magnesium fluoride layer having a refractive index of 1.25 and an optical film thickness of 0.26λ; "λ" being a reference light-ray wavelength of 550 nm.