Optical imaging system

1. An optical imaging system, comprising:

a first lens, a second lens, a third lens, a fourth lens, a fifth lens, a sixth lens, and a seventh lens, sequentially disposed from an object side to an imaging plane,

wherein the fourth lens and the fifth lens have negative refractive power, and the seventh lens has positive refractive power, and

wherein the focal length of the first lens is within a range of 2.2 to 2.8 mm.

2. The optical imaging system of claim 1 , wherein the first lens has a convex image-side surface along an optical axis.

3. The optical imaging system of claim 1 , wherein the second lens has a convex object-side surface along an optical axis.

4. The optical imaging system of claim 1 , wherein the third lens has a convex object-side surface along an optical axis.

5. The optical imaging system of claim 1 , wherein the fourth lens has a meniscus form in which one side surface is concave along an optical axis, and the other side surface is convex along the optical axis.

6. The optical imaging system of claim 1 , wherein the fifth lens has a concave object-side surface along an optical axis.

7. The optical imaging system of claim 1 , wherein the sixth lens has a concave object-side surface and an opposing concave image-side surface along an optical axis.

8. The optical imaging system of claim 1 , wherein the seventh lens has a convex object-side surface and an opposing convex image-side surface along an optical axis.

9. An optical imaging system, comprising:

first to seventh lenses, sequentially disposed from an object side to an imaging plane, wherein a ratio (TL/f) of a distance TL from an object-side surface of the first lens to an imaging plane to an overall focal length f is lower than or equal to 1.0,

wherein the focal length of the first lens is within a range of 2.2 to 2.8 mm.

10. The optical imaging system of claim 9 , wherein the optical imaging system satisfies the following expression:

BFL/f< 0.15

where BFL represents a distance from an image-side surface of the seventh lens to an imaging plane.

11. The optical imaging system of claim 9 , wherein the optical imaging system satisfies the following expression:

0.1 <f /(IMG HT)<2.5

where IMG HT is a half diagonal length of the imaging plane.

12. The optical imaging system of claim 9 , wherein the optical imaging system satisfies the following expression:

1.5< Nd 7<1.7

where Nd7 represents a refractive index of the seventh lens.

13. The optical imaging system of claim 9 , wherein the optical imaging system satisfies the following expression:

−45< f 5/ f< 45

where f5 represents a focal length of a fifth lens.

14. The optical imaging system of claim 9 , wherein the optical imaging system satisfies the following expression:

2.0< f /EPD<2.8

where EPD represents a diameter of an entrance pupil.

15. The optical imaging system of claim 9 , wherein a second lens, a fourth lens, and a sixth lens each have a negative refractive power.

16. The optical imaging system of claim 9 , wherein a sixth lens has a concave object-side surface along an optical axis.

17. An optical imaging system, comprising:

a first lens comprising a positive refractive power, a convex object-side surface along an optical axis, and a convex image-side surface along the optical axis;

a second lens to sixth lens; and

a seventh lens comprising a positive refractive power, a convex object-side surface along the optical axis and a convex image-side surface along the optical axis,

wherein the first lens to seventh lens are sequentially disposed from an object side to an imaging plane, and

wherein the first lens to seventh lens are disposed adjacent to one another.

18. The optical imaging system of claim 17 , wherein the object-side surface of the first lens comprises a most convex point of the optical imaging system.