An optical system used for an image pickup apparatus including an image pickup element is provided. The optical system includes a negative lens disposed furthest away on the enlargement conjugation side and satisfies the following condition: 80 degrees<2.omega., where 2.omega. (degrees) denotes an angle of view of the optical system. In addition, the following parameters of the optical system are optimally determined: a lens open angle .theta. of a surface of the negative lens on the reduction conjugation side, an index of refraction Ndn and an Abbe number .nu.dn of a material of the negative lens for a d-line, and a relative partial dispersion .theta.gFn of the material of the negative lens for a g-line and an F-line.