Patent ID: 8477230

Claim:
An image forming optical system comprising three groups having, in order from an object side to an image side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power, or four groups with one more lens group added to the image side, the first lens group comprises one lens component facing its concave surface toward the object side, the second lens group comprises a single lens having a positive refractive power and a cemented lens having a negative refractive power as a whole, and the third lens group comprises a lens component having a positive refractive power, wherein upon zooming from a wide-angle end toward a telephoto side, the amount of movement of the first lens group along an optical axis direction in an area, where a focal length of the entire image pickup optical system is 3.4 times or more with respect to at the wide-angle end, satisfies the following conditional expression (1): −1≦( G 1( W )− G 1( H )/ fw≦ 0.5 (1) where G 1 (W) is distance from an image plane to a surface of the first lens group on the most image plane side at the wide-angle end of the image forming optical system, and G 1 (H) is distance from the image plane to the surface of the first lens group on the most image plane side in the area, where the focal length of the image forming optical system is 3.4 times or more with respect to at the wide-angle end.