Rear attachment lens and image-forming optical system using the same

A rear attachment lens detachably attached to the image side of a main lens system to change a focal length of an entire system so as to become longer than a focal length of the main lens system, the rear attachment lens comprising:

• one or more positive lenses GMp,
• wherein, an Abbe number and a partial dispersion ratio difference of a material of the positive lens GMp are respectively νd—P and ΔθgF—P, a focal length of the positive lens GMp is f—P, a focal length of the rear attachment lens is f, and (ΔθgF—P×f)/(f—P×νd—P) is a ratio P,
• wherein, one among the one or more positive lenses GMp having a maximum ratio P is a positive lens P1, the Abbe number, a partial dispersion ratio, and a partial dispersion ratio difference of a material of the positive lens P1 are respectively νd—P1, θgF—P1 and ΔθgF—P1, a focal length of the positive lens P1 is f—P1, and (ΔθgF—P1×f)/(f—P1×νd—P1) is a ratio P—P1, such that the following conditions are satisfied:
• −1.665×10−7×νd—P13+5.213×10−5×νd—P12−5.656×10−3×νd—P1+0.7006<θgF—P1<−1.61783×10−3×νd—P1+0.64046
• 20<νd—P1<50
• 1×10−4<P—P1<1×10−3.