US Patent 11340431 Optical imaging system

An optical imaging system includes: a first lens having a concave image-side surface; a second lens having refractive power; a third lens having a concave image-side surface; a fourth lens having a concave object-side surface or a convex image-side surface; a fifth lens having refractive power; a sixth lens having a concave image-side surface; and a seventh lens having negative refractive power. The first to seventh lenses are sequentially disposed from an object side and 1.62<(N2+N5+N6)/3, where N2 is a refractive index of the second lens, N5 is a refractive index of the fifth lens, and N6 is a refractive index of the sixth lens.