Imaging optical system, image capturing unit and electronic device

1. An imaging optical system comprising six lens elements, the six lens elements being, in order from an object side to an image side, a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element and a sixth lens element, and each of the six lens elements having an object-side surface facing toward the object side and an image-side surface facing toward the image side;

wherein the second lens element has positive refractive power, the object-side surface of the second lens element is concave in a paraxial region thereof, the image-side surface of the second lens element is convex in a paraxial region thereof, the fifth lens element has positive refractive power, the image-side surface of the sixth lens element is concave in a paraxial region thereof, and at least one lens element of the imaging optical system has at least one aspheric surface having at least one inflection point;

wherein a central thickness of the third lens element is CT3, an axial distance between the third lens element and the fourth lens element is T34, an axial distance between the fourth lens element and the fifth lens element is T45, an axial distance between the fifth lens element and the sixth lens element is T56, a curvature radius of the object-side surface of the second lens element is R3, a focal length of the imaging optical system is f, a focal length of the first lens element is f1, a focal length of the second lens element is f2, and the following conditions are satisfied:

0<CT3/ T 34<1.80;

2.00< T 45/ T 56<100;

| R 3|/ f< 6.40; and

| f 2/ f 1|<2.30.

2. The imaging optical system of claim 1 , wherein the focal length of the first lens element is f1, the focal length of the second lens element is f2, and the following condition is satisfied:

| f 2/ f 1|<0.90.

3. The imaging optical system of claim 1 , wherein the axial distance between the fourth lens element and the fifth lens element is T45, the axial distance between the fifth lens element and the sixth lens element is T56, the curvature radius of the object-side surface of the second lens element is R3, the focal length of the imaging optical system is f, and the following conditions are satisfied:

3.00< T 45/ T 56<70.0; and

| R 3|/ f< 3.50.

4. The imaging optical system of claim 1 , wherein an Abbe number of the third lens element is V3, an Abbe number of the fourth lens element is V4, an axial distance between the object-side surface of the first lens element and an image surface is TL, the focal length of the imaging optical system is f, a maximum image height of the imaging optical system is ImgH, and the following conditions are satisfied:

20.0< V 3+ V 4<60.0;

1.5 [mm]< TL< 8.0 [mm];

1.0< TL/f< 2.0; and

1.0< TL /Img H <1.8.

5. The imaging optical system of claim 1 , wherein an Abbe number of the sixth lens element is V6, a curvature radius of the object-side surface of the sixth lens element is R11, a curvature radius of the image-side surface of the sixth lens element is R12, and the following conditions are satisfied:

10.0< V 6<32.0; and

1.45<( R 11+ R 12)/( R 11− R 12)<3.45.

6. The imaging optical system of claim 1 , wherein a central thickness of the fifth lens element is CT5, a central thickness of the sixth lens element is CT6, and the following condition is satisfied:

1.50<CT5/CT6<4.50.

7. The imaging optical system of claim 1 , wherein the focal length of the imaging optical system is f, a curvature radius of the object-side surface of the fourth lens element is R7, and the following condition is satisfied:

f/|R 7|<1.40.

8. The imaging optical system of claim 1 , wherein the focal length of the imaging optical system is f, a curvature radius of the object-side surface of the fifth lens element is R9, and the following condition is satisfied:

f/|R 9|<1.35.

9. The imaging optical system of claim 1 , wherein the focal length of the second lens element is f2, a focal length of the fifth lens element is f5, and the following condition is satisfied:

1.60<| f 2/ f 5|<4.00.

10. The imaging optical system of claim 1 , wherein the focal length of the first lens element is f1, a focal length of the third lens element is f3, and the following condition is satisfied:

| f 3/ f 1|<1.35.

11. The imaging optical system of claim 1 , wherein the first lens element has positive refractive power, the object-side surface of the first lens element is convex in a paraxial region thereof, the image-side surface of the first lens element is concave in a paraxial region thereof, the third lens element has negative refractive power, and the sixth lens element has negative refractive power.

12. The imaging optical system of claim 1 , wherein the object-side surface of the fifth lens element is concave in a paraxial region thereof, the image-side surface of the fifth lens element is convex in a paraxial region thereof, the object-side surface of the sixth lens element is convex in a paraxial region thereof, the object-side surface of the sixth lens element is aspheric and has at least one inflection point, the image-side surface of the sixth lens element is aspheric and has at least one inflection point, the image-side surface of the sixth lens element has at least one critical point in an off-axis region thereof, a vertical distance between the critical point on the image-side surface of the sixth lens element and an optical axis is Yc62, a maximum effective radius of the image-side surface of the sixth lens element is Y62, and the following condition is satisfied:

0.60< Yc 62/ Y 62<1.0.

13. An image capturing unit, comprising:

the imaging optical system of claim 1 ; and

an image sensor disposed on an image surface of the imaging optical system.

14. An electronic device, comprising:

the image capturing unit of claim 13 .

15. An imaging optical system comprising six lens elements, the six lens elements being, in order from an object side to an image side, a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element and a sixth lens element, and each of the six lens elements having an object-side surface facing toward the object side and an image-side surface facing toward the image side;

wherein the second lens element has positive refractive power, the object-side surface of the second lens element is concave in a paraxial region thereof, the image-side surface of the second lens element is convex in a paraxial region thereof, the sixth lens element has negative refractive power, the image-side surface of the sixth lens element is concave in a paraxial region thereof, and at least one lens element of the imaging optical system has at least one aspheric surface having at least one inflection point;

wherein a central thickness of the third lens element is CT3, an axial distance between the third lens element and the fourth lens element is T34, an axial distance between the fourth lens element and the fifth lens element is T45, an axial distance between the fifth lens element and the sixth lens element is T56, a focal length of the imaging optical system is f, a focal length of the first lens element is f1, a focal length of the second lens element is f2, a curvature radius of the image-side surface of the second lens element is R4, and the following conditions are satisfied:

0<CT3/ T 34<2.50;

2.00< T 45/ T 56<100;

1.58< f/|R 4|; and

| f 2/ f 1|<2.30.

16. The imaging optical system of claim 15 , wherein the central thickness of the third lens element is CT3, the axial distance between the third lens element and the fourth lens element is T34, an axial distance between the object-side surface of the first lens element and an image surface is TL, a maximum image height of the imaging optical system is ImgH, half of a maximum field of view of the imaging optical system is HFOV, and the following conditions are satisfied:

0<CT3/ T 34<1.80; and

1.00< TL /Img H +cot(HFOV)<2.50.

17. The imaging optical system of claim 15 , wherein the axial distance between the fourth lens element and the fifth lens element is T45, the axial distance between the fifth lens element and the sixth lens element is T56, and the following condition is satisfied:

3.00< T 45/ T 56<70.0.

18. The imaging optical system of claim 15 , wherein the focal length of the first lens element is f1, the focal length of the second lens element is f2, and the following condition is satisfied:

| f 2/ f 1|<0.90.

19. The imaging optical system of claim 15 , wherein the first lens element has positive refractive power, the object-side surface of the first lens element is convex in a paraxial region thereof, the image-side surface of the first lens element is concave in a paraxial region thereof, the third lens element has negative refractive power, the image-side surface of the third lens element is concave in a paraxial region thereof, at least one lens surface of the third lens element has at least one inflection point, and the at least one lens surface of the third lens element has at least one critical point in an off-axis region thereof.

20. The imaging optical system of claim 15 , wherein the fifth lens element has positive refractive power, the image-side surface of the fifth lens element is convex in a paraxial region thereof, the object-side surface of the sixth lens element is convex in a paraxial region thereof, at least one lens element of the imaging optical system has at least one lens surface having at least one inflection point, and the at least one lens surface of the at least one lens element has at least one critical point in an off-axis region thereof.

21. The imaging optical system of claim 15 , further comprising an aperture stop disposed between the first lens element and the second lens element, wherein an axial distance between the aperture stop and an image surface is SL, an axial distance between the object-side surface of the first lens element and the image surface is TL, and the following condition is satisfied:

0.85< SL/TL< 0.95.

22. An imaging optical system comprising six lens elements, the six lens elements being, in order from an object side to an image side, a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element and a sixth lens element, and each of the six lens elements having an object-side surface facing toward the object side and an image-side surface facing toward the image side;

wherein the object-side surface of the second lens element is concave in a paraxial region thereof, the image-side surface of the second lens element is convex in a paraxial region thereof, the third lens element has negative refractive power, the sixth lens element has negative refractive power, the image-side surface of the sixth lens element is concave in a paraxial region thereof, and at least one lens element of the imaging optical system has at least one aspheric surface having at least one inflection point;

wherein a central thickness of the first lens element is CT1, a central thickness of the third lens element is CT3, an axial distance between the first lens element and the second lens element is T12, an axial distance between the third lens element and the fourth lens element is T34, an axial distance between the fourth lens element and the fifth lens element is T45, an axial distance between the fifth lens element and the sixth lens element is T56, and the following conditions are satisfied:

0<CT3/ T 34<2.50;

2.00< T 45/ T 56<100; and

0<CT1/ T 12<1.80.

23. The imaging optical system of claim 22 , wherein the central thickness of the third lens element is CT3, the axial distance between the third lens element and the fourth lens element is T34, and the following condition is satisfied:

0<CT3/ T 34<1.80.

24. The imaging optical system of claim 22 , wherein the axial distance between the fourth lens element and the fifth lens element is T45, the axial distance between the fifth lens element and the sixth lens element is T56, a focal length of the third lens element is f3, a focal length of the fourth lens element is f4, and the following conditions are satisfied:

3.00< T 45/ T 56<70.0; and

0< f 3/ f 4<1.40.

25. The imaging optical system of claim 22 , wherein the central thickness of the first lens element is CT1, the axial distance between the first lens element and the second lens element is T12, a curvature radius of the object-side surface of the second lens element is R3, a focal length of the imaging optical system is f, and the following conditions are satisfied:

0.50<CT1/ T 12<1.60; and

| R 3|/ f< 20.0.

26. The imaging optical system of claim 22 , wherein a sum of central thicknesses of all lens elements of the imaging optical system is ΣCT, a sum of axial distances between each of all adjacent lens elements of the imaging optical system is ΣAT, a focal length of the second lens element is f2, a focal length of the sixth lens element is f6, and the following conditions are satisfied:

1.60<ΣCT/Σ AT <4.50; and

1.00<| f 2/ f 6|<3.40.

27. The imaging optical system of claim 22 , wherein an f-number of the imaging optical system is Fno, half of a maximum field of view of the imaging optical system is HFOV, a maximum effective radius of the object-side surface of the first lens element is Y11, a maximum effective radius of the image-side surface of the sixth lens element is Y62, and the following conditions are satisfied:

1.0< Fno< 2.6;

45.0 [deg.]<HFOV<60.0 [deg.]; and

2.0< Y 62/ Y 11<5.0.

28. The imaging optical system of claim 22 , wherein the second lens element has positive refractive power, the image-side surface of the fourth lens element is concave in a paraxial region thereof, the image-side surface of the fourth lens element is aspheric, the image-side surface of the fourth lens element has at least one inflection point, the image-side surface of the fourth lens element has at least one critical point in an off-axis region thereof, the fifth lens element has positive refractive power, and the image-side surface of the fifth lens element is convex in a paraxial region thereof.