Projection zoom lens and projection display apparatus

1. A projection zoom lens substantially consisting of a first lens group having a negative power, a second lens group having a positive power, a third lens group having a positive power, a fourth lens group having a negative power, a fifth lens group having a positive power, and a sixth lens group having a positive power arranged in this order from the magnification side and is configured telecentric on the reduction side, wherein:

the projection zoom lens is configured such that the first and sixth lens groups are fixed while the second to fifth lens groups are moved along the optical axis at the time of zooming;

the fourth lens group is composed of one negative lens whose magnification side surface has a greater curvature in absolute value than that of the reduction side surface;

the fifth lens group is composed of a negative lens, a positive lens, a negative lens with a concave surface on the magnification side, a positive lens with a convex surface on the reduction side, and a positive lens arranged in this order from the magnification side; and

the zoom lens satisfies a conditional expression (1) given below:

−12.0 <f 4 /fw<− 5.0  (1),

where:

f4 is a focal length of the fourth lens group; and

fw is a focal length of the entire lens system at the wide angle end.

2. The projection zoom lens of claim 1 , wherein the projection zoom lens satisfies a conditional expression (2) given below:

3.0< f 5 /fw< 10  (2),

where f5 is a focal length of the fifth lens.

3. The projection zoom lens of claim 2 , wherein the projection zoom lens satisfies a conditional expression (2A) given below:

4.0< f 5 /fw< 8.0  (2A).

4. The projection zoom lens of claim 2 , wherein the second lens group comprises at least a cemented lens composed of a negative lens with a concave surface on the reduction side and a positive lens with a convex surface on the reduction side.

5. The projection zoom lens of claim 2 , wherein the projection zoom lens satisfies a conditional expression (3) given below:

1.8< Bf/fw< 4.0  (3),

where, Bf is an air equivalent back focus of the entire lens system on the reduction side.

6. The projection zoom lens of claim 2 , wherein the projection zoom lens satisfies a conditional expression (4) and a conditional expression (5) given below:

ft/fw≧ 1.5  (4); and

2ω>65°  (5),

where:

ft is a focal length of the entire lens system at the telephoto end; and

2ω is a total angle of view at the wide angle end.

7. The projection zoom lens of claim 2 , wherein the projection zoom lens includes a variable aperture stop for changing the size of the aperture diameter according to the zooming on the magnification side or reduction side of the fourth lens group, and the variable aperture stop is configured to maintain the F-number of the projection zoom lens constant at the time of zooming.

8. The projection zoom lens of claim 2 , wherein the projection zoom lens is configured such that focusing is performed by moving the first lens group in an optical axis direction.

9. The projection zoom lens of claim 2 , wherein:

the one negative lens constituting the fourth lens group has a meniscus shape with a convex surface on the reduction side; and

the zoom lens satisfies a conditional expression (6) given below:

30<ν d   (6),

where νd is an Abbe number of the negative lens constituting the fourth lens group with respect to d-line.

10. The projection zoom lens of claim 1 , wherein the second lens group comprises at least a cemented lens composed of a negative lens with a concave surface on the reduction side and a positive lens with a convex surface on the reduction side.

11. The projection zoom lens of claim 1 , wherein the projection zoom lens satisfies a conditional expression (3) given below:

1.8< Bf/fw< 4.0  (3),

where, Bf is an air equivalent back focus of the entire lens system on the reduction side.

12. The projection zoom lens of claim 11 , wherein the projection zoom lens satisfies a conditional expression (3A) given below:

2.0< Bf/fw< 3.0  (3A).

13. The projection zoom lens of claim 1 , wherein the projection zoom lens satisfies a conditional expression (4) and a conditional expression (5) given below:

ft/fw≧ 1.5  (4); and

2ω>65°  (5),

where:

ft is a focal length of the entire lens system at the telephoto end; and

2ω is a total angle of view at the wide angle end.

14. The projection zoom lens of claim 1 , wherein the projection zoom lens includes a variable aperture stop for changing the size of the aperture diameter according to the zooming on the magnification side or reduction side of the fourth lens group, and the variable aperture stop is configured to maintain the F-number of the projection zoom lens constant at the time of zooming.

15. The projection zoom lens of claim 1 , wherein the projection zoom lens is configured such that focusing is performed by moving the first lens group in an optical axis direction.

16. The projection zoom lens of claim 1 , wherein:

the one negative lens constituting the fourth lens group has a meniscus shape with a convex surface on the reduction side; and

the zoom lens satisfies a conditional expression (6) given below:

30<ν d   (6),

where νd is an Abbe number of the negative lens constituting the fourth lens group with respect to d-line.

17. The projection zoom lens of claim 1 , wherein the projection zoom lens satisfies a conditional expression (1A) given below:

−10.0 <f 4 /fw<− 5.0  (1A).

18. A projection display apparatus, comprising the projection zoom lens of claim 1 , a light source, a light valve, and an illumination optical unit for guiding a luminous flux from the light source to the light valve, wherein the luminous flux from the light source is optically modulated by the light valve and the optically modulated luminous flux is projected onto a screen through the projection zoom lens.