Projection zoom lens

1. A projection zoom lens comprising, from an image side to an object side, a first lens group having a negative refractive power, consisting of a number of lens elements and being movable along an optical axis of the projection zoom lens; a second lens group having a positive refractive power, consisting of a number of lens elements and being movable along the optical axis of the projection zoom lens; and an aperture stop arranged between two lens elements of the second lens group and being axially movable therewith; the projection zoom lens satisfying the following conditions:

−0.90<P2/P1<−0.78

−1.99<P1/PF<−1.67

1.50<P2/PF<1.55

0.58<PF/BF<0.68

5.37<TT/PF<5.82

3.40<TT/BF<3.80

−1.34<EX/BF<−1.13

Where P1 represents the refractive power of the first lens group, P1 represents the refractive power of the second lens group, PF represents the focal length of the projection zoom lens as a whole at the wide-angle end, BF represents the back focal length of the projection zoom lens as a whole at the wide-angle end, TT represents the total optical track of the projection zoom lens as a whole at the wide-angle end, and EX represents the exit pupil position of the projection zoom lens as a whole at the wide-angle end.

2. The projection zoom lens as claimed in claim 1 , wherein lens elements of the first and second lens groups are made of glass.

3. The projection zoom lens as claimed in claim 1 further comprising a cover glass arranged between the second lens group and the object side.

4. The projection zoom lens as claimed in claim 1 , wherein the object side is a surface of an image-forming device, and the image side is a screen side onto which an image is projected.

5. The projection zoom lens as claimed in claim 1 , wherein the first lens group is movable toward the object side, and the second lens group is movable toward the image side, and the distance between the first and the second lens groups is shortened when zooming the projection zoom lens from the wide-angle end to the telephoto end.

6. The projection zoom lens as claimed in claim 5 , wherein the focal length varies between 21.333 mm (W), 24.4907 mm (M) and 27.6359 mm (T), and the effective aperture varies between 2.40 (W), 2.57 (M) and 2.73 (T).

7. The projection zoom lens as claimed in claim 5 , wherein the focal length varies between 20.439 mm (W), 21.852 mm (M) and 23.545 mm (T), and the effective aperture varies between 2.61 (W), 2.69 (M) and 2.78 (T).

8. The projection zoom lens as claimed in claim 5 , wherein the focal length varies between 23.700 mm (W), 26.006 mm (M) and 28.523 mm (T), and the effective aperture varies between 2.48 (W), 2.59 (M) and 2.71 (T).

9. A projection optical system comprising an image-forming device for forming an image, and a projection zoom lens for projecting the image, wherein said projection zoom lens comprising, from an image side to an object side, a first lens group having a negative refractive power, consisting of a number of lens elements and being movable along an optical axis of the projection zoom lens; a second lens group having a positive refractive power, consisting of a number of lens elements and being movable along the optical axis of the projection zoom lens; and an aperture stop arranged between two lens elements of the second lens group and being axially movable therewith; the projection zoom lens satisfying the following conditions:

-

0.90

<

P2

P1

<

-

0.78

⁢

-

1.99

<

P1

PF

<

-

1.67

1.50

<

P2

PF

<

1.55

0.58

<

PF

BF

<

0.68

5.37

<

TT

PF

<

5.82

3.40

<

TT

BF

<

3.80

⁢

-

1.34

<

EX

BF

<

-

1.13

where P1 represents the refractive power of the first lens group, P2 represents the refractive power of the second lens group, PF represents the focal length of the projection zoom lens as a whole at the wide-angle end, BF represents the back focal length of the projection zoom lens as a whole at the wide-angle end, TT represents the total optical track of the projection zoom lens as a whole at the wide-angle end, and EX represents the exit pupil position of the projection zoom lens as a whole at the wide-angle end.

10. The projection optical system as claimed in claim 9 , wherein the image-forming device comprises a reflective modulator.

11. The projection optical system as claimed in claim 10 , wherein the image-forming device comprises a DMD modulator.

12. The projection optical system as claimed in claim 10 , wherein the projection zoom lens has a long back focal length.

13. An image-taking optical system comprising at least a projection zoom lens, which comprising, from an image side to an object side, a first lens group having a negative refractive power, consisting of a number of lens elements and being movable along an optical axis of the projection zoom lens; a second lens group having a positive refractive power, consisting of a number of lens elements and being movable along the optical axis of the projection zoom lens; and an aperture stop arranged between two lens elements of the second lens group and being axially movable therewith; the projection zoom lens satisfying the following conditions:

-

0.90

<

P2

P1

<

-

0.78

⁢

-

1.99

<

P1

PF

<

-

1.67

1.50

<

P2

PF

<

1.55

0.58

<

PF

BF

<

0.68

5.37

<

TT

PF

<

5.82

3.40

<

TT

BF

<

3.80

⁢

-

1.34

<

EX

BF

<

-

1.13

where P1 represents the refractive power of the first lens group, P2 represents the refractive power of the second lens group, PF represents the focal length of the projection zoom lens as a whole at the wide-angle end, BF represents the back focal length of the projection zoom lens as a whole at the wide-angle end, TT represents the total optical track of the projection zoom lens as a whole at the wide-angle end, and EX represents the exit pupil position of the projection zoom lens as a whole at the wide-angle end.