Image forming optical system and electronic image pickup apparatus using the same

An image forming optical system comprising in order from an object side:

• a first lens group G1 having a positive refractive power, which comprises only one cemented-lens component;
• a second lens group G2 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 G3 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 G4 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 G1 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)
• νd(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.