Projection optical system, projection apparatus, and projection system

1. A projection optical system to enlarge and project an image displayed on an image display element onto a projection surface, the projection optical system comprising:

a reflective optical system on a magnification side along an optical path of the projection optical system, the reflective optical system including at least one reflective optical element having a power; and

a refractive optical system on a reduction side along the optical path, the refractive optical system including:

a front group on the magnification side; and

a rear group on the reduction side having a positive refractive power,

the front group including in order from the magnification side toward the reduction side:

a first lens group with a positive or negative refractive power;

a second lens group with a negative refractive power; and

a third lens group with a positive refractive power,

wherein the first lens group moves to the magnification side, and the second lens group and the third lens group move to the reduction side in a change in focus from a long distance to a short distance, and

wherein the rear group of the refractive optical system is fixed during the change in focus.

2. The projection optical system according to claim 1 ,

wherein conditional formula (1) is satisfied:

−0.20< F 2/ F 3<−0.5,  (1)

where,

F 2 is a focal length of the second lens group in the front group of the refractive optical system, and

F 3 is a focal length of the third lens group in the front group of the refractive optical system.

3. The projection optical system according to claim 1 ,

wherein each of the first lens group and the third lens group in the front group of the refractive optical system consists of a single lens.

4. The projection optical system according to claim 1 ,

wherein the rear group of the refractive optical system includes a plurality of lenses,

wherein the plurality of lenses are divided into two lens groups at a point with a greatest difference in outer diameter between adjacent lenses, and

wherein one lens of the adjacent lenses disposed on the magnification side has a greater outer diameter than another lens of the adjacent lenses.

5. The projection optical system according to claim 4 ,

wherein the two lens groups are a fourth lens group on the magnification side and a fifth lens group on the reduction side.

6. The projection optical system according to claim 1 ,

wherein the at least one reflective optical element having the power in the reflective optical system is a concave mirror with a free-form surface.

7. The projection optical system according to claim 1 ,

wherein conditional formula (2) is satisfied:

10< F 1/ FL< 80,  (2)

where,

FL is a focal length of the refractive optical system, and

F 1 is a focal length of the first lens group when the image enlarged and projected onto the projection surface is largest.

8. A projection apparatus comprising:

an image display device including the image display element;

the projection optical system according to claim 1 ; and

a lens holding device to hold the refractive optical system of the projection optical system to perform focusing.

9. The projection apparatus according to claim 8 ,

wherein the rear group of the refractive optical system includes a plurality of lenses divided into a fourth lens group and a fifth lens group at a point with a greatest difference in outer diameter between adjacent lenses,

wherein the fourth lens group disposed on the magnification side has a greater outer diameter than the fifth lens group,

wherein the lens holding device includes a plurality of lens-group holders to hold the front group and the rear group of the refractive optical system, and

wherein among the lens-group holders, a second-lens-group holder to hold the second lens group of the front group and a fourth-lens-group holder to hold the fourth lens group are in contact with and held by a common member of the lens holding device.

10. The projection apparatus according to claim 9 ,

wherein the common member is a rotary barrel.

11. The projection apparatus according to claim 9 ,

wherein the common member restricts rotation of the first lens group.

12. The projection apparatus according to claim 11 ,

wherein the common member is a straight barrel.

13. The projection apparatus according to claim 12 ,

wherein the common member is part of the fourth-lens-group holder.

14. The projection apparatus according to claim 11 ,

wherein the common member is a pin.

15. The projection apparatus according to claim 9 ,

wherein the lens holding device holds the fifth lens group to move the fifth lens group along an optical axis.

16. The projection apparatus according to claim 15 ,

wherein the plurality of lens-group holders includes a fifth-lens-group holder to hold the fifth lens group, and

wherein the lens holding device includes a helicoid on both the fourth-lens-group holder and the fifth-lens-group holder.

17. The projection apparatus according to claim 9 ,

wherein the lens holding device holds the fifth lens group to rotate the fifth lens group around an optical axis.

18. A projection system comprising:

the projection apparatus according to claim 8 ; and

a screen to display an image enlarged and projected by the projection apparatus,

wherein conditional formula (3) is satisfied:

TR< 0.30,  (3)

where,

TR is a ratio of a distance from a point of at least one refractive optical element having the power intersecting with an optical axis of the refractive optical system to the screen in a direction perpendicular to the screen, with respect to a lateral width of the screen.