To obtain a single focus lens small in size, reduced in the number of lenses, low in cost and excellent in performance while securing sufficient back focus by using a diffraction optical element.
This single focus lens is constituted of a front group I provided with a lens whose concave surface faces to an image surface side and having negative refractive power, a diaphragm S, and a rear group II having positive refractive power in order from an object side. The front group I is constituted of a meniscus lens L1 whose concave surface faces to the image surface side and which has negative refractive power and a positive lens L2 whose convex surface faces to the image surface side in order from the object side, and further the rear group II is constituted of a negative meniscus lens L3 whose concave surface faces to the image surface, and a biconvex lens L4 in order from the object side. An aspherical surface and a diffraction optical surface are applied to the image surface side lens surface of the lend L4 which is the surface of the rear group II on the nearest side to the image surface.
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