Optical imaging system having six lenses of +−+−+− refractive powers

1. An optical imaging system comprising:

a first lens, a second lens, a third lens, a fourth lens, a fifth lens, and a sixth lens sequentially arranged from an object side of the optical imaging system,

wherein each of the second lens to the fourth lens has a refractive index greater than 1.61 and at least one lens of the second lens to the fourth lens has a refractive index greater than 1.66,

wherein the fifth lens has a convex object-side surface in a paraxial region;

wherein the sixth lens has a concave object-side surface in a paraxial region;

wherein 0.35<R1/f<0.40;

wherein TTL/f<1.25;

N2+N3+N4>4.8; and

TTL/(2*IMG HT)<0.70,

where R1 is a radius of curvature of an object-side surface of the first lens, f is an overall focal length of the optical imaging system including the first to sixth lenses, N2 is a refractive index of the second lens, N3 is a refractive index of the third lens, N4 is a refractive index of the fourth lens, TTL is a distance on an optical axis from the object-side surface of the first lens to an image capturing surface of an image sensor, and IMG HT is half of a diagonal length of the image capturing surface of the image sensor.

2. The optical imaging system of claim 1 , wherein

V1−V2>30;

−10<V2−V4<10; and

−10<V1−V5<10,

where V1 is an Abbe number of the first lens, V2 is an Abbe number of the second lens, V4 is an Abbe number of the fourth lens, and V5 is an Abbe number of the fifth lens.

3. The optical imaging system of claim 1 , wherein f1/f<1.2, where f1 is a focal length of the first lens.

4. The optical imaging system of claim 3 , wherein −2.5<f2/f<−2.0, where f2 is a focal length of the second lens.

5. The optical imaging system of claim 4 , wherein −0.5<f1/f2<0.

6. The optical imaging system of claim 4 , wherein f3/f>6, where f3 is a focal length of the third lens.

7. The optical imaging system of claim 6 , wherein −0.5<f2/f3<0.

8. The optical imaging system of claim 1 , wherein 0.15<BFL/f<0.25, where BFL is a distance on the optical axis from an image-side surface of the sixth lens to the image capturing surface of the image sensor.

9. The optical imaging system of claim 1 , where 0.015<D1/f<0.025, where D1 is a distance on the optical axis between an image-side surface of the first lens and an object-side surface of the second lens.

10. The optical imaging system of claim 1 , wherein Fno<2.1, where Fno is an F number of the optical imaging system.

11. An optical imaging system comprising:

a first lens, a second lens, a third lens, a fourth lens, a fifth lens, and a sixth lens sequentially arranged from an object side of the optical imaging system,

wherein the third lens has a focal length with a largest absolute value among the first to sixth lenses;

wherein the fifth lens has a convex object-side surface in a paraxial region;

wherein the sixth lens has a concave object-side surface in a paraxial region; wherein among the second lens to the fourth lens, at least one lens has a refractive index greater than 1.66, and

wherein N2+N3+N4>4.8;

wherein TTL/f<1.25;

TTL/(2*IMG HT)<0.70; and

Fno<1.85,

where f is an overall focal length of the optical imaging system, N2 is a refractive index of the second lens, N3 is a refractive index of the third lens, N4 is a refractive index of the fourth lens, TTL is a distance on an optical axis from the object-side surface of the first lens to an image capturing surface of an image sensor, IMG HT is half of a diagonal length of the image capturing surface of the image sensor, and Fno is an F number of the optical imaging system including the first lens to the sixth lens.

12. The optical imaging system of claim 11 , wherein

f1/f<1.2;

−2.5<f2/f<−2.0; and

−0.5<f1/f2<0,

where f1 is a focal length of the first lens, and f2 is a focal length of the second lens.

13. The optical imaging system of claim 12 , wherein 0.35<R1/f<0.40, where R1 is a radius of curvature of the object-side surface of the first lens.

14. The optical imaging system of claim 11 , wherein the first lens has positive refractive power, the second lens has negative refractive power, the third lens has positive refractive power, the fourth lens has negative refractive power, the fifth lens has positive refractive power, and the sixth lens has negative refractive power.