PURPOSE: To apply the system to a photographic optical system using two and more solid-state image pickup elements, to lengthen a back focal length, to reduce the angle of off-axis principal ray, to make the system high performance and to decrease a number of lenses by properly prescribing an air gap between a fourth lens and a fifth lens and the refractive power of the fourth lens.
CONSTITUTION: In order from an object side, this system is composed of a first positive lens, a second positive lens having more intensive curvature on the object side than the image side, a third negative lens having more intensive curvature on the image side than the object side, a fourth negative lens having more intensive curvature on the object side than the image side, a fifth positive lens having more intensive curvature on the image side than the object side and a sixth positive lens. It comprises an air gap between the fourth lens and the fifth lens. The conditions: 0.25<dg4A/Dg4< 0.33 and -3.4<Ψg4/ΨT<-2.4 are satisfied, where dg4A: the air gap between the fourth lens and the fifth lens, Dg4: larger one of effective diameters of the fourth lens, Ψg4: the refractive power of the fourth lens and ΨT: the refractive power of the whole system.