JP2005309061A - Telecentric telephoto zoom lens with long back focus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a projection lens for a telephoto type projector which is bright, made relatively small in size, and long in back focus, and has a sufficiently long exit pupil and no variation. <P>SOLUTION: A nearly telecentric telephoto zoom lens which enlarges and projects an image of a display surface on a screen has a 1st lens group L1 with positive refracting power which is fixed during zooming and only adjusts a distance, a 2nd lens group L2 with negative refracting power which is movable during zooming, a 3rd lens group L3 with negative refracting power which is movable during zooming, a 4th lens group L4 with positive refracting power which is movable during zooming, and a 5th lens group L5 with positive refracting power which is fixed during zooming in order from a screen side. Then when the focal length is varied from the focal length at the wide-angle end to the focal length at the telephoto end, the gap between the 2nd lens group L2 and 3rd lens group L3 is shorter at the telephoto end than at the wide-angle end. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a zoom lens used in a projection apparatus that magnifies and projects an image onto a screen at a fixed finite distance, and in particular, uses a plurality of liquid crystals or the like for each color light as a display body and performs color synthesis to project a single image. The present invention relates to a telecentric telephoto zoom lens that projects a high-definition image via a lens.

  In particular, the present invention relates to a zoom lens for a projector that projects at an appropriate size in the distance.

  Conventionally, a negative lead type zoom lens preceded by a lens unit having a negative refractive power is easy to widen the angle of view with a relatively long back focus, especially for a telecentric wide angle lens that requires a long back focus. Many proposals have been made. However, the negative lead type preceded by negative refracting power has a drawback of increasing the size of the telephoto objective of the present invention.

  On the other hand, as a positive lead type zoom lens having a positive refractive power, it has been proposed in Patent Documents 1 to 5, for example.

  In each of these publications, the zoom lens has a lens group having positive, negative, negative, positive, and positive refractive power in order from the enlargement side, and is composed of five lens groups as a whole. It is moved and zoomed.

  In Patent Document 1, a zoom lens used for image formation on a small image sensor such as a video, a configuration in which the first, third, and fifth groups are fixed and the second and fourth groups are movable during zooming is described. This is a so-called rear focus type zoom lens that performs distance adjustment with four lens groups, and has a drawback that it is a lens that is considerably larger than the image size.

  Patent Documents 2 and 5 are zoom lenses used for image formation on a small image sensor such as a video, etc., and the first and fourth groups are fixed and the second, third and fifth groups are movable during zooming. It has the disadvantage of becoming a considerably large lens with respect to size. In addition, since the aperture stop is disposed in the vicinity of the fourth lens group, and the movable group (fifth lens group) is arranged on the image plane side, there is not only a change in exit pupil position during zooming, but also a telephoto zoom. It wasn't too bad.

Both Patent Documents 3 and 4 are similar in structure to the present invention, but are lenses having a wide-angle side, a short pupil, and a structure in which the lens group on the image plane side from the aperture stop is movable during zooming. The exit pupil position fluctuation is also large. Also, the back focus was short.
Japanese Patent Laid-Open No. 9-15495 JP-A-11-64733 JP-A-11-160621 JP-A-11-305128 JP 2000-214386 A

  These conventional examples are not suitable as a telephoto zoom for a projector that projects to an appropriate size from a long distance in a wide space, and in these configurations, there are many things that have a short exit pupil and are not suitable for a projection lens. In addition, many of them are larger than the image size, which is insufficient for use as a projector.

When enlarging and projecting a display image on a screen at a very long distance as in the present invention, particularly when using a liquid crystal display divided into a plurality of colors and combining each color light and projecting with one projection lens, the following It is necessary to satisfy the conditions.
1) A so-called telecentric optical system in which the exit pupil is at a distance, taking into consideration the light distribution characteristics of the liquid crystal and the projection directivity of the illumination system. Further, when color synthesis is performed using a member having a dichroic film such as a cross dichroic prism, it is preferably telecentric to reduce color unevenness.
2) A long back focus or a space between lenses is required in order to secure a space for a color composition element for combining a plurality of display bodies.
3) When projecting from a long distance to an appropriate size, the focal length of the projection lens must be long enough.

  As described above, in order to increase the back focus with a telecentric optical system, a negative lead zoom type may be used. However, it is easy to become a wide-angle lens, and it is difficult to satisfy the requirement 3).

  In the prior art, there are many cases where the exit pupil position is finite, the variation of the exit pupil is large, the back focus is not sufficiently long, and the lens is not sufficiently telephoto. Moreover, it was easy to become a big thing also with respect to image size.

  An object of the present invention is to provide a projection lens for a telephoto projector that is bright and relatively small in size, has a sufficiently long back focus, and has a sufficiently long exit pupil and no fluctuation.

  In order to achieve the above object, the present invention is characterized by the following configuration.

  A first lens group with positive refractive power that is fixed during zooming and is only distance-adjusted in order from the screen side (magnification side), in order from the screen side (magnification side), in a substantially telecentric telephoto zoom lens that magnifies and projects the image on the screen. A second lens group having a negative refractive power, a third lens group having a negative refractive power that is movable during zooming, a fourth lens group having a positive refractive power that is movable during zooming, and a fifth lens having a positive refractive power that is fixed during zooming. By having a lens group and shifting from the focal length at the wide-angle end to the focal length at the telephoto end, the distance between the second lens group and the third lens group is configured so that the telephoto end is shorter than the wide-angle end. is there.

  At this time, the third lens group contributes most to the multiplication, and the second lens group corrects the image plane position together with the fourth lens group while reducing the magnification.

  In particular, in this case, the second lens group and the third lens group are on the panel side (reduction side) on the telephoto side. Thereby, a desired zoom ratio can be obtained effectively without increasing the size of the entire system. In addition, the negative lens group that contributes to zooming is divided into a second lens group and a third lens group and moved separately to correct tilting of the image plane at the intermediate zoom position.

  In particular, the third lens group may move along a locus convex to the screen side (enlargement side) upon displacement from the wide-angle end to the telephoto end.

  In order to increase the back focus, it is preferable to have at least one negative lens having a strong concave surface on the panel side (reduction side) in the fixed fifth lens group. By using this lens to increase the back focus of the entire system and further disposing the aperture stop in the vicinity of the fifth lens group, it is possible to eliminate the zoom variation of the exit pupil and maintain a long pupil position.

  In particular, it is preferable for the long exit pupil to arrange the aperture stop on the most screen side of the fifth lens group. Further, by disposing a stop in the fifth lens group that is fixed during zooming, there is no pupil variation during zooming and focusing, which is suitable as a liquid crystal projection lens.

  Further, it is preferable that the fifth lens group has the largest number of lenses. As a result, the occurrence of spherical aberration can be suppressed even when the lens is sufficiently bright.

  When the lens is displaced from the focal length at the wide-angle end to the focal length at the telephoto end, the fourth lens group is preferably located on the screen side (enlargement side) from the wide-angle end at the telephoto end. As a result, the combined focal length from the first lens group to the fourth lens group is almost infinite, and the light beam incident on the fifth lens group is almost afocal (considering the projection system, the fifth lens group to the fourth lens group). The fifth lens group is easy to make a telecentric structure, the pupil position can be easily changed, and the occurrence of aberration can be reduced.

  At this time, the fifth lens group may have a convex locus on the screen side (enlargement side) as it is displaced from the wide-angle end to the telephoto end.

In order to achieve a telecentric lens system by increasing the back focus while making the telephoto system, it is preferable to satisfy the following expression.
fw <f5 <ft (1)
Here, fw and ft are focal lengths at the wide-angle end and the telephoto end of the entire system. f5 is the focal length of the fifth lens group.

Here, when the movement amount from the wide-angle end to the telephoto end of the i-th lens group is Mi and the movement toward the panel side (reduction side) is positive (+), 2 <M2 / M3 <11 (2)
Is preferably satisfied.

  If the upper limit value is exceeded, the entire system becomes larger, and if the lower limit value is exceeded, the tilt of the image plane at the zoom intermediate portion becomes large, and it becomes difficult to obtain a desired zoom ratio.

  Further, if the first lens group is fixed during zooming, it is effective for downsizing the entire system, and functions can be shared by using only for distance adjustment, which is effective in terms of performance and mechanism.

  The fifth lens group is closest to the display body and provides a relatively strong positive refractive power to realize a telecentric system.

  The final lens may act like a field lens, and a color synthesis prism may be introduced on the screen side (enlargement side). At that time, a polarizing plate or a color filter may be attached to the final lens. In that case, the final lens is closest to the display surface (panel), especially the panel side (reduction side) surface is flat or close to a flat surface, and a positive lens with a strong convex surface facing the screen side (enlargement side) is arranged. Is preferred.

  By moving and zooming the second lens group, the third lens group, and the fourth lens group, the amount of movement of each group is reduced, and the performance sensitivity due to the positional deviation, tilting, etc. of each group is reduced. By dispersing, it is possible to achieve a zoom lens with a high zoom ratio, and it is possible to shorten the overall length.

  The fifth lens group has a positive refractive power and ensures a long back focus because of the space of the color synthesizing element. In particular, in order to increase the back focus, it is preferable to dispose a negative lens having a strong concave surface on the panel side (reduction side) in the fifth lens group.

  Furthermore, by arranging the refractive power of each group appropriately and fixing the first lens group during zooming, it is possible to achieve a lens system with a simplified overall configuration and a constant overall length.

  In particular, the distance between the first lens group and the second lens group increases and the distance between the second lens group and the third lens group decreases at the focal length at the telephoto end, compared with the focal length at the wide-angle end, It is preferable to increase the distance between the fourth lens group and the fifth lens group in order to appropriately disperse the variable magnification share of each group and to obtain a desired variable magnification ratio.

  At this time, it is preferable that the third lens group is multiplied upon zooming.

For the third lens group which is the main variable power group, it is preferable to satisfy the following expression. If the change βit / βiw in the i-th lens group is Zi and the change in focal length ft / fw in the entire system is Z, then 1.5 <Z3 / Z <20 (3)
This expression (3) appropriately defines the ratio of zooming of the third lens group that is the zooming group. Although the third lens group is magnified during zooming, it is appropriate in view of the variable magnification sharing of the entire system to be within this range in consideration of the balance of zooming with other lens groups.

Air-converted back focus (air-converted interval from the fifth lens group to the panel surface) without a prism color matching block is BF, the image circle (effective image circle) diameter by the lens is D, and from the display panel (reduction side) When the distance from the conjugate position to the exit pupil is tk (the exit pupil here refers to the pupil on the reduction side), it is preferable to satisfy the following formula: 1 <BF / D <1.5 ( 4)
2 <fw / BF <5 (5)
5 <| tk / D | <20 (6)
Equations (4), (5), and (6) are conditions for increasing the back focus and the pupil while having the zoom range shifted to the telephoto side. Regardless of which formula is deviated, a telephoto lens with a long back focus and sufficiently telecentricity cannot be achieved.

The focal length of the first lens group is f1, the focal length of the second lens group is f2, the focal length of the third lens group is f3, the focal length of the fourth lens group is f4, and the focal length of the fifth lens group is f5. , It is preferable that the following formula is satisfied.
0.6 <| f1 / f2 | <1.5 (7)
2.0 <| f1 / f3 | <5.0 (8)
1.5 <f2 / f3 <5.0 (9)
1.3 <f1 / fw <3.0 (10)
The above formulas (7) and (8) appropriately define the relationship between the first lens group and the second lens group involved in the front lens diameter and distortion. When the lower limit is exceeded, the front lens diameter increases, and when the upper limit is exceeded, distortion on the wide-angle side increases, which is not appropriate.

  Expression (9) mainly limits the configuration of the lens group that contributes to zooming. If the value deviates from the upper limit, the amount of movement for obtaining a desired zoom ratio increases, the front lens diameter determined by the first lens group increases, and distortion at the wide-angle end increases, which is not appropriate. If the value deviates from the lower limit, the desired telephoto angle of view (especially the wide angle side) becomes difficult, and the entire system becomes large and is not suitable.

  Expression (10) is for making the power of the first lens group appropriate. When the upper limit is exceeded, the entire system becomes larger and the front lens diameter also becomes larger. If the lower limit is exceeded, negative distortion increases on the wide-angle side, which is not appropriate.

The reason for setting the exit side (reduction side) of the lens to be substantially telecentric is to eliminate the influence of the light distribution characteristics of the liquid crystal or the angle dependency of the color synthesis dichroic mirror when combining multiple color lights as described above. is there. Specifically, in order to eliminate the angle dependency, it is preferable if the following conditions are satisfied.
| Tk | / fw> 1.5 (11)
Here, tk is the distance from the display panel at the wide-angle end to the exit pupil (from the reduction side conjugate position). (The exit pupil here refers to the pupil on the reduction side.) In the present invention, the fifth lens group has an aperture stop, so the exit pupil position is constant throughout the zoom and focus.

It is preferable to satisfy the following conditions in order to set the distortion small while making the telecentric system optimal.
4 <f1 / BF <8 (12)
If the lower limit is exceeded, optimum telecentricity cannot be satisfied, and distortion also increases. If it exceeds the upper limit, it will be unsuitable due to an increase in size.

Furthermore, it is preferable to satisfy the following expression in order to reduce the size by making the movement amount of each group appropriate while making the power arrangement of each group appropriate.
1.0 <f1 / √ (fw × ft) <2.0 (13)
1.0 <| f2 | / √ (fw × ft) <3.0 (14)
0.4 <| f3 | / √ (fw × ft) <0.7 (15)
0.7 <f4 / √ (fw × ft) <1.2 (16)
0.7 <f5 / √ (fw × ft) <1.5 (17)
In particular, it is preferable for miniaturization to satisfy the following conditions. 1.0 <E / D <1.5 (18) where D is the image circle of the lens and E is the effective diameter of the lens immediately before the screen side (enlargement side) of the color synthesis prism.
If the value deviates from the upper limit at this time, the size of the color synthesizing prism increases, which is not appropriate. Further, if the value deviates from the lower limit value, a sufficiently long back focus cannot be secured, which is not appropriate.

  By constructing as described above, the present invention achieves a projection lens for a telephoto projector that is bright and relatively small in size, has a sufficiently long back focus, and has a sufficiently long exit pupil and no fluctuation. We were able to.

  1 to 4 are sectional views of first to fourth embodiments of the zoom lens according to the present invention. 5 to 8 are graphs showing various aberrations of the zoom lenses according to the first to fourth embodiments.

  In FIG. 1, L1 is a first lens unit having a positive refractive power that does not move for zooming and moves during distance adjustment (focusing), L2 is a second lens unit having a negative refractive power that is movable during zooming, and L3 is a zooming unit. A third lens group having a negative refractive power, which is movable in the middle, L4 is a fourth lens group having a positive refractive power, which is movable during the zooming, and a fifth lens group which is fixed during the zooming and has a positive refractive power. P is a color composition prism, and LCD is a display element for displaying an original image such as a liquid crystal panel. Originally, three elements are prepared for each color light of RGB, but are omitted in FIG. The zoom lens of the present embodiment is a projection optical system for a projection apparatus that projects an original image displayed on the display element LCD onto a screen (not shown). The color synthesis prism P plays a role of synthesizing light from the original image corresponding to each color light and guiding it to the projection optical system.

  P in the figure indicates a glass block such as a color synthesis prism.

  The aberration diagrams shown in FIGS. 5 to 8 show spherical aberration, astigmatism (field curvature), and distortion (%) lateral chromatic aberration, respectively. State is written. Spherical aberrations are those at 550 nm and 470 nm. The lateral chromatic aberration shows a value of 470 nm on the basis of 550 nm. In astigmatism, a solid line indicates a sagittal section and a chain line indicates a meridional section.

  In the first and second embodiments, each of the second, third, and fourth lens groups is monotonically moving from the wide-angle end to the telephoto end in the reduction side, the reduction side, and the enlargement side. In this example, the first lens group is composed of two lenses, and one positive lens is arranged on the most reduction side of the fifth lens group. The wide-angle Fno is set to F2.2 in the first embodiment and F2.3 in the second embodiment.

  The third example is an example in which the third lens unit moves from the wide-angle end to the telephoto end to the enlargement side and to the reduction side with an inflection point. At the telephoto end, the third lens group moves to the reduction side from the wide angle end. The second lens group and the fourth lens group move monotonously on the reduction side and the enlargement side, respectively. The first lens group is composed of three lenses.

  The fourth example is an example in which the third lens group and the fourth lens group move from the wide-angle end to the telephoto end, respectively, toward the reduction side and the enlargement side, but each moves with an inflection point. . The third lens group has an inflection point on the wide-angle side, and the fourth lens group has an inflection point on the telephoto side. At the telephoto end, the third lens group is configured on the reduction side of the wide-angle end, the fourth lens group is configured on the magnification side, and the first lens group is configured of three lenses.

  Although it has been described that the focusing is performed by the first lens group, the first lens group and the second lens group are performed at the same time, or in the sixth group, or in a plurality of groups, especially at a finite distance, for each group. The distance may be adjusted by moving the display panel.

  Next, numerical data of numerical examples 1 to 4 corresponding to the first to fourth examples will be described. In the table, f is a focal length, fno is an F number, and ω is a half angle of view. Also, ri is the radius of curvature of the i-th surface from the object side, di is the distance between the i-th surface and the (i + 1) -th surface, and ni and νi are the i-th optical member in order from the object side. The refractive index and Abbe number with respect to the d-line.

  Table 1 shows values corresponding to the above-described conditional expressions of the numerical examples.

  (Numerical example 1)

（数値実施例２）

(Numerical example 2)

（数値実施例３）

(Numerical Example 3)

（数値実施例４）

(Numerical example 4)

It is sectional drawing of 1st Example of the zoom lens of this invention. It is sectional drawing of 2nd Example of the zoom lens of this invention. It is sectional drawing of 3rd Example of the zoom lens of this invention. It is sectional drawing of 4th Example of the zoom lens of this invention. FIG. 6 is a diagram illustrating all aberrations of the zoom lens according to the first example. FIG. 6 is various aberration diagrams of the zoom lens of Example 2. FIG. 12 is a diagram illustrating all aberrations of the zoom lens according to the third example. FIG. 12 is a diagram illustrating all aberrations of the zoom lens according to the fourth example.

Claims (5)

In a substantially telecentric zoom lens that enlarges and projects an image on the display surface onto the screen,
From the screen side (enlarged side),
A first lens unit having a positive refractive power that is fixed during zooming and only performs distance adjustment;
A second lens group movable during zooming and having a negative refractive power,
A third lens group movable during zooming and having a negative refractive power,
A fourth lens group movable during zooming and having a positive refractive power,
A fifth lens unit that is fixed during zooming and has a positive refractive power;
A zoom lens, wherein the distance between the second lens group and the third lens group is shorter at the telephoto end than at the wide-angle end when the focal length at the wide-angle end is displaced to the focal length at the telephoto end.   2. The zoom lens according to claim 1, wherein at least one negative lens having a strong concave surface facing the panel side (reduction side) is provided in the fifth lens group. When shifting from the focal length at the wide-angle end to the focal length at the telephoto end,
The zoom lens according to claim 1, wherein the second lens group and the third lens group are on the panel side (reduction side) on the telephoto side. When shifting from the focal length at the wide-angle end to the focal length at the telephoto end,
The zoom lens according to claim 1, wherein the fourth lens group is located on the screen side (enlargement side) from the wide angle end at the telephoto end.   2. The zoom lens according to claim 1, wherein an aperture stop is disposed in the vicinity of the fifth lens group.

Images