IMAGE FORMING OPTICAL SYSTEM AND ELECTRONIC IMAGING DEVICE HAVING THE SAME

摘要

An image forming optical system includes a positive lens group, a negative lens group, and a diaphragm. The negative lens group is arranged at the object side than the diaphragm. The negative lens group has a cemented lens obtained by cementing a plurality of lenses. In an orthogonal coordinate system having a horizontal axis Nd and a vertical axis ?d, when a straight line expressed by Nd = a × ?d + ß (wherein a = -0.017) is set, the Nd and ?d of at least one of the lenses constituting the cemented lens are contained in both of the regions: one region defined by a straight line as the lower limit and a straight line as the upper limit of the range of the condition expression [1a] given below; and the other region defined by the condition expressions [2a] and [3a] given below. 1.45 < ß < 2.15 ... [1a] 1.30 < Nd < 2.20 ... [2a] 3 < ?d < 12 ... [3a] wherein Nd represents the refractivity and ?d represents the Abbe number.