Patent ID: 8472124

Claim:
An image forming optical system comprising in order from an object side: a first lens group G 1 having a positive refractive power, which comprises only one cemented-lens component; a second lens group G 2 having a negative refractive power, which comprises two lens components namely, a negative lens component and a lens component of a cemented lens of a positive lens LA having a meniscus shape and a negative lens LB in order; a third lens group G 3 having a positive refractive power, which comprises two lens components namely, a positive lens component and a negative lens component; and a fourth lens group G 4 which comprises one lens component, wherein the image forming optical system comprises maximum of five lens groups, and at the time of zooming from a wide angle end to a telephoto end, when a focal length of the overall image forming optical system is in a range of 1.2 fw to 1.8 fw, the first lens group G 1 moves to be positioned more toward the object side than a position at the wide angle end, and in a rectangular coordinate system in which, a horizontal axis is let to be νd (LA) and a vertical axis is let to be nd(LA), when a straight line expressed by nd ( LA )= a×νd ( LA )+ b ( LA )(provided that a=− 0.0267) is set, nd and νd of the positive lens LA are included in an area which is determined by a straight line when it is a lower limit value of a range of the following conditional expression (1) and a straight line when it is an upper limit value of the range of the following conditional expression (1), and an area determined by the following conditional expression (2) 2.0 <b ( LA )<2.4(provided that nd ( LA )>1.3) (1) ν d ( LA )<30 (2) where, fw denotes a focal length of the image forming optical system at wide angle end, νd(LA) denotes Abbe's number (nd(LA)−1)/(nF(LA)−nC(LA)) for the positive lens LA, and nd(LA), nC(LA), nF(LA) denote refractive indices of the positive lens LA for a d-line, a C-line, and an F-line respectively.