Optical imaging system

1. An optical imaging system, comprising:

a first lens having positive refractive power and comprising a convex object-side surface and a concave image-side surface at an optical axis of the optical imaging system;

a second lens having negative refractive power;

a third lens having refractive power, and a focal length equal to or greater than 18 mm;

a fourth lens having refractive power and comprising a convex image-side surface at the optical axis of the optical imaging system;

a fifth lens having positive refractive power, and a focal length equal to or greater than 50 mm;

a sixth lens having positive refractive power and comprising a convex image-side surface; and

a seventh lens having negative refractive power and comprising a concave image-side surface,

wherein the first to seventh lenses are disposed in order from an object side to an imaging side, and

wherein:

1 <f/f 6

V 2<40,

where f is a focal length of the optical imaging system, f6 is a focal length of the sixth lens, and V2 is an Abbe number of the second lens.

2. The optical imaging system of claim 1 , wherein an F No. is 1.8 or less.

3. The optical imaging system of claim 1 , wherein:

0.05 <OAL /HFOV<0.3,

where OAL is a distance, in millimeters, from an object-side surface of the first lens to an imaging plane, and HFOV is a half field of view, in degrees, of the optical imaging system.

4. The optical imaging system of claim 1 , wherein:

0.2< Th 7/ Th 6<0.9,

where Th6 is a thickness of the sixth lens at an optical center, and Th7 is a thickness of the seventh lens at an optical center.

5. The optical imaging system of claim 1 , wherein the fourth lens has negative refractive power.

6. The optical imaging system of claim 1 , further comprising a stop disposed between the first lens and the second lens.

7. The optical imaging system of claim 1 , wherein the fifth lens comprises an inflection point on one or both of an object-side surface and an image-side surface of the fifth lens.

8. The optical imaging system of claim 1 , wherein the sixth lens comprises an inflection point on one or both of an object-side surface and an image-side surface of the sixth lens.

9. The optical imaging system of claim 1 , wherein the seventh lens comprises an inflection point on one or both of an object-side surface and an image-side surface of the seventh lens.

10. The optical imaging system of claim 1 , wherein the fifth lens comprises a convex object-side surface at the optical axis of the optical imaging system.

11. The optical imaging system of claim 10 , wherein the seventh lens comprises a concave object-side surface at the optical axis of the optical imaging system.

12. The optical imaging system of claim 1 , wherein the third lens comprises a convex object-side surface and a concave image-side surface.

13. The optical imaging system of claim 1 , wherein the third lens has a positive refractive power.

14. The optical imaging system of claim 1 , wherein a thickness along an optical axis of the first lens is greater than a distance between the image-side surface of the sixth lens and an object-side surface of the seventh lens along the optical axis.

15. An optical imaging system, comprising:

a first lens comprising a concave image-side surface at an optical axis of the optical imaging system, a second lens, a third lens having a focal length equal to or greater than 18 mm, a fourth lens comprising a convex image-side surface at the optical axis of the optical imaging system, a fifth lens having positive refractive power, and a focal length equal to or greater than 50 mm, a sixth lens, and a seventh lens disposed in order from an object side to an imaging side,

wherein:

F No.<1.8

V 2<40,

where V2 is an Abbe number of the second lens, and

wherein a thickness along an optical axis of the first lens is greater than a distance between the image-side surface of the sixth lens and an object-side surface of the seventh lens along the optical axis.

16. The optical imaging system of claim 15 , wherein the third lens has positive refractive power.

17. The optical imaging system of claim 15 , wherein the fourth lens has negative refractive power.

18. The optical imaging system of claim 15 , wherein the third lens comprises a convex object-side surface.

19. The optical imaging system of claim 15 , wherein the fourth lens comprises a concave object-side surface at an optical axis of the optical imaging system.

20. The optical imaging system of claim 15 , wherein the fifth lens, the sixth lens, and the seventh lens each comprise an inflection point on one or both of a respective object-side surface and a respective image-side surface.

21. An optical imaging system, comprising:

a first lens comprising a concave image-side surface at an optical axis of the optical imaging system;

a second lens comprising a refractive power;

a third lens comprising a refractive power, and a focal length equal to or greater than 18 mm;

a fourth lens comprising a convex image-side surface at the optical axis of the optical imaging system;

a fifth lens comprising a refractive power, and a focal length equal to or greater than 50 mm;

a sixth lens comprising a refractive power; and

a seventh lens comprising a refractive power,

wherein the first to seventh lenses are sequentially disposed in order from an object side to an imaging side,

wherein F No.<1.8 and V2<40, where V2 is an Abbe number of the second lens, and

wherein a radius of curvature of an object-side surface of the fifth lens is greater than a radius of curvature of an object-side surface of the fourth lens.