A micro lens, from an object side to an image side, sequentially includes a stop, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power. The third and fourth lenses respectively comprise at least one aspherical surface. The negative refractive power gradually decreases from the center to the periphery of the fourth lens, and the refractive power of the periphery of the fourth lens is turned to positive. The relationship of the first lens and the focal length of the micro lens of the micro lens and the relationship of the third lens and the first lens satisfy the desired requirements, thus, to reduce the total length of the micro lens, tolerance sensitivity and cost.
1. A micro lens, from an object side to an image side, sequentially comprising a stop, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power, wherein the third and fourth lenses respectively comprise at least one aspherical surface, and the negative refractive power of the fourth lens gradually decreases from the center of the fourth lens toward the periphery of the fourth lens, wherein the micro lens satisfies the following condition:
1.0< F/f 1<2.5
wherein “F” represents a focal length of the micro lens, and f 1 represents a focal length of the first lens.
2. The micro lens as claimed in claim 1 , wherein the micro lens satisfies the following condition:
0.6< f 3/ f 1<2.0
wherein f 3 represents a focal length of the third lens, and f 1 represents a focal length of the first lens.
3. The micro lens as claimed in claim 2 , wherein refractive indexes of the first, second, third and fourth lenses at d-lines are less than 1.6.
4. The micro lens as claimed in claim 3 , wherein the first, second, third and fourth lenses are plastic lens.
5. The micro lens as claimed in claim 4 , wherein both lens surfaces of the first, second, third and fourth lenses are aspherical.
6. The micro lens as claimed in claim 4 , wherein both lens surfaces of the second, third and fourth lenses are aspherical.
7. The micro lens as claimed in claim 1 , wherein the peripheral region of the fourth lens has positive refractive power.
8. The micro lens as claimed in claim 5 , wherein the peripheral region of the fourth lens has positive refractive power.
9. The micro lens as claimed in claim 8 , wherein the second lens is a biconcave lens.
10. The micro lens as claimed in claim 1 , wherein the first lens is a biconvex lens.
11. A micro lens, from an object side to an image side, sequentially comprising a stop, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power, wherein the third and fourth lenses respectively comprise at least one aspherical surface, and the negative refractive power gradually decreases from the center to the periphery of the fourth lens, wherein refractive indexes of the first, second, third and fourth lenses at d-lines less than 1.6.
12. The micro lens as claimed in claim 11 , wherein the micro lens satisfies the following condition:
1.0< F/f 1<2.5
wherein “F” represents a focal length of the micro lens, and f 1 represents a focal length of the first lens.
13. The micro lens as claimed in claim 12 , wherein the micro lens satisfies the following condition:
0.6< f 3/ f 1<2.0
wherein f 3 represents a focal length of the third lens, and f 1 represents a focal length of the first lens.
14. The micro lens as claimed in claim 13 , wherein the first, second, third and fourth lenses are plastic lens.
15. The micro lens as claimed in claim 13 , wherein the first, second, third and fourth lenses are aspherical.
16. The micro lens as claimed in claim 13 , wherein the second, third and fourth lenses are aspherical.
17. The micro lens as claimed in claim 15 , wherein the fourth lens has a periphery with positive refractive power.
18. The micro lens as claimed in claim 17 , wherein the second lens is a biconcave lens.
19. The micro lens as claimed in claim 18 , wherein the first lens is a biconvex lens.