Image-forming optical system

1. An optical system, having an object side and an image side, comprising, in order from an object side:

an iris diaphragm;

a first lens having a positive refractive power and at least one aspheric surface; and

a second lens which is a meniscus lens having a convex image side, a negative refractive power and at least one aspheric surface,

the optical system satisfying the following conditional expression:

1.25

<

L

T

f

<

1.58

wherein L T denotes the total length of the optical system from the object side to an image side; and f denotes the total focal length of the optical system.

2. The optical system as claimed in claim 1 , wherein the optical system further satisfies the following conditional expression:

-

1.5

<

W

p

-

W

W

p

<

-

0.1

wherein W p is defined by

Tan

-

1

⁡

(

y

f

)

;

W denotes the half angle of view for a maximum effective image circle; and y is the height of a maximum effective image.

3. The optical system as claimed in claim 1 , wherein the optical system further satisfies the following conditional expression:

25 <Vd 1 −Vd 2 <30

where Vd 1 denotes the Abbe's value of the d-line ray of the first lens; and Vd 2 denotes the Abbe's value for the d-line ray of the second lens.

4. The optical system as claimed in claim 1 , wherein the first and second lenses respectively have an aspheric surface on both sides thereof.

5. The optical system as claimed in claim 1 , wherein the first lens is a lens having a convex image side.

6. An optical system, having an object side and an image side, comprising, in order from an object side:

an iris diaphragm;

a first lens having a positive refractive power and at least one aspheric surface; and

a second lens having a negative refractive power and at least one aspheric surface, the optical system satisfying the following conditional expression:

-

1.5

<

W

p

-

W

W

p

<

-

0.1

wherein W p is defined by

Tan

-

1

⁡

(

y

f

)

;

W denotes the half angle of view for a maximum effective image circle; and y is the height of a maximum effective image.

7. The optical system as claimed in claim 6 , wherein the optical system further satisfies the following conditional expression:

25 <Vd 1 −Vd 2 <30

where Vd 1 denotes the Abbe's value of the d-line ray of the first lens; and Vd 2 denotes the Abbe's value for the d-line ray of the second lens.

8. The optical system as claimed in claim 6 , wherein the first and second lenses respectively have an aspheric surface on both sides thereof.

9. The optical system as claimed in claim 6 , wherein the second lens is a meniscus lens having a convex image side.

10. The optical system as claimed in claim 6 , wherein the first lens is a lens having a convex image side.

11. An optical system, having an object side and an image side, comprising, in order from an object side:

an iris diaphragm;

a first lens having a positive refractive power and at least one aspheric surface; and

a second lens which is a meniscus lens having a convex image side, a negative refractive power and at least one aspheric surface,

the optical system satisfying the following conditional expressions:

0.75

<

f

B

f

<

0.95

and



R

1



f

>

2.1

wherein f denotes the total focal length of the optical system; f B denotes the distance on the optical axis between the image side surface of the second lens and the image planes; and Rl denotes the curvature of the object side of the first lens.

12. The optical system as claimed in claim 11 , wherein the optical system further satisfies the following conditional expression:

L

T

f

B

<

2.4

wherein L T denotes the total length of the optical system from the object side to an image side.

13. The optical system as claimed in claim 11 , wherein the optical system further satisfies the following conditional expression:

0.35

<

t

2

f

<

0.52

wherein t 2 denotes the thickness of the first lens.

14. The optical system as claimed in claim 11 , wherein the optical system further satisfies the following conditional expression:

0.14

<

t

4

f

<

0.23

wherein t 4 denotes the thickness of the second lens.

15. The optical system as claimed in claim 11 , wherein the optical system further satisfies the following conditional expression:

-

1.5

<

W

p

-

W

W

p

<

-

0.02

wherein W p is defined by

Tan

-

1

⁡

(

y

f

)

;

W denotes the half angle of view for a maximum effective image circle; and y is the height of a maximum effective image.

16. The optical system as claimed in claim 11 , wherein the first and second lenses respectively have an aspheric surface on both sides thereof.

17. The optical system as claimed in claim 11 , wherein the first lens is a lens having a convex image side.

18. An optical system, having an object side and an image side, comprising, in order from an object side:

an iris diaphragm;

a first lens having a positive refractive power and at least one aspheric surface; and

a second lens having a negative refractive power and at least one aspheric surface, the optical system satisfying the following conditional expression:

-

1.5

<

W

p

-

W

W

p

<

-

0.02

wherein W p is defined by

Tan

-

1

⁡

(

y

f

)

;

W denotes the half angle of view for a maximum effective image circle; and y is the height of a maximum effective image.

19. The optical system as claimed in claim 18 , wherein the optical system further satisfies the following conditional expression:



R

1



f

>

2.1

wherein R 1 denotes the curvature of the object side of the first lens.

20. The optical system as claimed in claim 18 , wherein the optical system further satisfies the following conditional expression:

L

T

f

B

<

2.4

wherein L T denotes the total length of the optical system from the object side to an image side.

21. The optical system as claimed in claim 18 , wherein the optical system further satisfies the following conditional expressions:

0.35

<

t

2

f

<

0.52

0.14

<

t

4

f

<

0.23

wherein t 2 denotes the thickness of the first lens, and t 4 denotes the thickness of the second lens.

22. The optical system as claimed in claim 18 , wherein the first and second lenses respectively have an aspheric surface on both sides thereof.

23. The optical system as claimed in claim 18 , wherein the second lens is a meniscus lens having a convex image side.

24. The optical system as claimed in claim 18 , wherein the first lens is a lens having a convex image side.