JP2783667B2 - Zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens, particularly to a zoom lens suitable for projection.

[Conventional technology]

First, FIG. 9 shows a configuration diagram of a projection television for projecting an image formed on a general color liquid crystal screen.
1 emits light collimated by a white light source. 2 (2
a, 2b, 2c) are liquid crystal display elements, 3 is a reflection mirror, 4,
Reference numerals 5 and 6 denote a red reflecting dichroic mirror, a green reflecting dichroic mirror, a blue reflecting dichroic mirror, respectively.
Reference numeral 7 denotes a projection lens. Under such a configuration, at least two mirrors such as a reflection mirror or a dichroic mirror need to be disposed between the final surface of the projection lens and the liquid crystal display element (between the bike focuses). Must be secured.

[Problems to be solved by the invention]

By the way, a so-called two-group zoom lens having a long back focus and a lens having a negative refractive power as a simple zoom lens followed by a lens having a positive refractive power can be considered. Second
There is a problem that the amount of movement of the lens group becomes large and the entire zoom lens system becomes long.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a zoom lens having good optical performance while securing a relatively long back focus and reducing the size.

[Means for solving the problem]

The present invention provides, in order from the largest conjugate side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a third lens group having a positive refractive power. When zooming from the wide-angle side to the telephoto side, 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. The lens group and the fourth lens group each include a configuration that independently moves so as to draw a locus convex toward a small conjugate side.

Further, the second lens group is arranged in order from the largest conjugate side.
The second group includes a negative lens having a large power on a small conjugate side, a negative lens having a large power on a small conjugate side, a positive lens having a large power on a large conjugate side, and a meniscus negative lens convex on a small conjugate side. When the radius of curvature on the large conjugate side of the meniscus negative lens on the smallest conjugate side is R1 and the radius of curvature on the small conjugate side is R2, the following conditional expression is satisfied.

SF = (R1 + R2) / (R1-R2) -5 <SF <-3 (1) On the other hand, the third lens group is a meniscus lens convex to a small conjugate side in order from a large conjugate side, and a convex lens having a large power. , Composed of a meniscus concave lens convex to the small conjugate side.

Apart from this, the focal length of the entire system at the wide-angle end is f _w ,
When the focal length of the first lens group is f ₁ , the focal length of the second lens group is f ₂ , and the principal point interval between the second and third lens groups at the wide angle end is e _2w , the following conditional expression To be satisfied.

0.2 <e _2w / f _w <0.6 (2) 1.0 <− (1 / f ₁ + 1 / f ₂ ) · _fw <2.0 (3) The meaning of the extreme value in the above equation will be described later.

〔Example〕

FIGS. 1 and 2 are lens cross-sectional views corresponding to numerical examples of the present invention described later.

11 or 21 is located on a screen (large conjugate) side (not shown), the first lens group having a positive refractive power, 12 or 22 is the second lens group having a negative refractive power, and 13 or 23 is a positive lens group. The third lens unit 14 or 24 having a refractive power is located on the side of an original image (small conjugate) such as a liquid crystal (not shown) and is a fourth lens unit having a positive refractive power, and zooming from the wide-angle side to the telephoto side. At the time of movement (trajectory when unfolded) as shown by the solid line
Are moving according to. W is the wide-angle end, M is the middle, T
Indicates the telephoto end.

Note that the third and fourth units are synchronously and independently moved to suppress the fluctuation of the image curvature aberration at the intermediate focal length.

The smallest conjugate side of the second group (the right end in the figure) and the third conjugate side
By placing a meniscus lens convex to the small conjugate side on the largest conjugate side (left end in the figure) of the group, the occurrence of field curvature aberration is suppressed to a small value.

The present invention also aims at high performance, and it is necessary to minimize axial chromatic aberration and lateral chromatic aberration.
It is necessary to increase the Abbe number of the positive lens of the group, the third group, and the fourth group. As a result, the refractive index of the positive lens of the first group, the third group, and the fourth group cannot be increased. Therefore, in order to suppress the image curvature aberration, the refractive index of the negative lens must be reduced. However, since the glass having a low refractive index does not have a small Abbé number, it is difficult to sufficiently correct the chromatic aberration. Therefore, glass having a high refractive index and a low Abbé number is used for the negative lens, and the generated field curvature aberration is corrected by the field flattening lenses arranged in the second and third units.

The above-mentioned conditional expression (1) limits the shape of the meniscus negative lens on the smallest conjugate side of the second lens unit. If the upper limit of conditional expression (1) is exceeded, the image plane becomes over, and the conditional expression (1) If the lower limit value is exceeded, the image surface becomes under, which is not preferable.

In the zoom lens of the present embodiment, the third and fourth lens groups located on the original image side and having a positive refractive power have convex trajectories (trajectories when developed and drawn) toward the original image side. It is moved to mainly correct the image position. However, as described above, it is necessary to dispose an optical component such as a dichroic mirror between the final lens surface of the fourth lens group and the original image, so that the distance (hereinafter referred to as the distance) is required. (Referred to as back focus) must be sufficiently secured near the wide-angle end.

When considering the basic structure a thin system, it comes to a Batsukufuokasu S _w at the wide-angle end to the following equation.

_{S w = {1- (e 1w} + e 2w ') / f 1 = (1-e 1w / f 1) e 2w' / f 2} · f w ... (A) where, e _{1 w} first at the wide-angle end The distance between the principal points between the lens group and the second lens group, and e _2w ′ is the distance between the principal points between the second lens group and the third and fourth lens groups. Since the distance between the first lens group and the second lens group is minimum at the wide-angle end, (A)
The value of e ₁ w in the equation cannot be too large to make the whole lens system compact. Therefore, the equation (A) is expressed as S _w = _wK −e _2w ′ · (1 / f ₁ + 1 / f ₂ ) · f (B) (where K = 1−e _1w / f ₁ + e _1w · e _2w ′) / (1 / f ₁ + 1 / f ₂ )) Then, when the value of e _{2w and} the value of − (1 / f ₁ + 1 / f ₂ ) are increased from the formula (B), the back focus SW of the thin-walled system is increased, and the back focus of the thick-walled system also tends to be longer. You can see that.

Therefore, if the lower limit value of the conditional expression (2) is exceeded, e _2w ′ becomes large and it becomes difficult to keep the back focus long.
On the other hand, when the value exceeds the upper limit, the entire lens system becomes large, and it is difficult to reduce the size.

On the other hand, even if the lower limit value of the conditional expression (3) is exceeded, it becomes difficult to keep the back focus long as well.
Since the formula focal length f ₁ is also increased in the first group exceeds the upper limit value of, that to achieve a movement of the first lens group becomes large compact lens system during zooming becomes difficult.

Next, when the value goes below the lower limit of conditional expression (4), the focal length | f ₂ | of the second lens unit becomes small, and the image plane is excessively generated. This aberration can be corrected by another lens. It will be difficult. Also, barrel-shaped distortion occurs on a small conjugate plane (liquid crystal display element), and it becomes difficult to correct aberrations, especially at the wide-angle end.

On the other hand, if the value exceeds the upper limit of the expression (4), that is, if the focal length | f ₂ | of the second group becomes too large, the moving amount of the second lens group in zooming becomes large and a predetermined zoom ratio is obtained. Becomes difficult.

Next, numerical examples of the present invention will be described. In numerical examples
R ₁ is the radius of curvature of the first lens surface in order from the screen side, D ₁ is the first lens thickness and air gap in order from the screen side, and N ₁ and ν ₁ are the first lenses in order from the screen side. Are the refractive index and the Atsube number of the glass.

In the first and second embodiments, the focusing is performed by moving the first lens groups 11 and 21 along the optical axis. Further, the variable spacing D ₈ shows the interval at the screen distance ∞.

The values of each numerical example corresponding to each numerical conditional expression are shown in the following table.

[Brief description of the drawings]

1 and 2 are lens cross-sectional views of Numerical Examples 1 and 2 of the present invention, respectively. FIG. 3, FIG. 4, and FIG. 5 are various aberration diagrams on a small conjugate plane at a widening end, a middle position, and a telephoto end in a distance of 1/20 in Numerical Embodiment 1 of the present invention. 6, 7, and 8 are various aberration diagrams on a small conjugate plane at a distance of 1/20 at a wide-angle end, a middle position, and a telephoto end in numerical example 2 of the present invention in order. FIG. 9 is a configuration diagram of a video projector. W indicates a wide-angle end, M indicates a middle end, T indicates a telephoto end, 1 indicates a light source, 2 indicates a liquid crystal display element, and 3 indicates a mirror.

Claims (3)

(57) [Claims] A first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a positive refractive power. And a fourth lens group. When zooming from the wide-angle side to the telephoto side, the distance between the first lens group and the second lens group increases, the distance between the second lens group and the third lens group decreases, and 3
The lens group and the fourth lens group are zoom lenses that move independently to draw a locus convex toward the small conjugate side, and the second lens group has a large power on the small conjugate side in order from the large conjugate side. A negative lens having a large power on the small conjugate side, a positive lens having a large power on the large conjugate side, and a meniscus negative lens convex on the small conjugate side. A zoom lens characterized by satisfying the following conditional expression when the radius of curvature on the large conjugate side of the meniscus negative lens is R1, and the radius of curvature on the small conjugate side is R2. SF = (R1 + R2) / (R1-R2) -5 <SF <-3 (1) 2. The third lens group comprises, in order from a large conjugate side, a meniscus lens convex to a small conjugate side, a positive lens having a large power, and a negative meniscus lens convex to a small conjugate side. The zoom lens according to claim 1, wherein 3. A first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a positive refractive power, in order from the largest conjugate side. And a fourth lens group. When zooming from the wide-angle side to the telephoto side, the distance between the first lens group and the second lens group increases, the distance between the second lens group and the third lens group decreases, and 3
Each of the lens group and the fourth lens group is a zoom lens that moves independently so as to draw a locus of convexity toward the small conjugate side. The focal length of the entire system at the wide-angle end is f _w , f ₁ , the focal length of the second lens group is f ₂ , and the principal point distance between the second and third lens groups at the wide-angle end is e.
A zoom lens characterized by satisfying the following conditional expression when _2w is satisfied. 0.2 <e _2w / f _w <0.6 (2) 1.0 <− (1 / f ₁ + 1 / f ₂ ) · _fw <2.0 (3) 0.3 <−f ₂ / f _w <0.5 (4)