Optical imaging system

An optical imaging system comprising:

• a first lens comprising positive refractive power, a convex object-side surface and a concave image-side surface;
• a second lens comprising negative refractive power, a convex object-side surface and a concave image-side surface;
• a third lens comprising positive refractive power;
• a fourth lens comprising negative refractive power;
• a fifth lens comprising refractive power;
• and
• a sixth lens comprising negative refractive power,
• wherein the first to sixth lenses are sequentially arranged from an object side to an image side,
• wherein the first to sixth lenses are formed of plastic material,
• wherein the optical imaging system has a total of six lenses,
• wherein f/(CT3+CT4+CT5)<4.0 is satisfied, where f is an overall focal length of the optical imaging system, CT3 is a thickness of the third lens in a paraxial region, CT4 is a thickness of the fourth lens in the paraxial region, and CT5 is a thickness of the fifth lens in the paraxial region, and
• wherein the second lens has the greatest refractive index among the first to sixth lenses and the second lens has a refractive index greater than 1.66.