JP2000227552A - Zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a zoom lens whose variable power ratio is about 10 times compact without deteriorating optical performance. SOLUTION: This zoom lens is constituted of a 1st lens group GR-I having positive refractive power, a 2nd lens group GRII having negative refractive power, a 3rd lens group GRIII having positive refractive power and a 4th lens group GR-IV having positive refractive power in turn from an object side. By moving the groups GR-II and GR-IV, a zooming action is executed. Then, the group GR-II is constituted of at least one spherical-surface lens L7 provided with positive refractive power and one aspherical-surface lens L8 whose concave aspherical surface faces an image surface side and which is provided with negative refractive power. Besides, the spherical-surface lens of the group GRIII having the positive refractive power and the aspherical-surface lens thereof having the negative refractive power are joined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens having a zoom ratio of about 10 times, which is used for a small video camera, a digital still camera or the like.

[0002]

2. Description of the Related Art In recent years, video cameras, digital still cameras, and the like have been required to be further miniaturized, and accordingly, a reduction in the overall length of a photographic lens, particularly, a zoom lens has been required.

As a conventional zoom lens, for example, as described in Japanese Patent Application Laid-Open No. Hei 5-60974,
In order from the object side, a first lens group having a positive refractive power, a second lens group (variator) having a negative refractive power, a third lens group (compensator) having a positive refractive power, and a second lens group having a positive refractive power. In a four-group lens system including four lens groups (master), a third lens group includes an aspheric lens having a strong positive refractive power and a meniscus lens having a negative refractive power with a concave surface facing the image surface side. And the overall length is shortened.

However, in order to reduce the total length of the zoom lens, it is necessary to increase the curvature of the object-side surface of the convex lens in the third lens group and the curvature of the image-side surface of the concave lens.
There is a problem that the optical performance is greatly deteriorated due to the eccentricity of the lens in the lens group.

[0005]

SUMMARY OF THE INVENTION In view of the above problems, the present invention does not deteriorate optical performance. It is an object to reduce the size of a zoom lens having a zoom ratio of about 10 times.

[0006]

In order to solve the above-mentioned problems, a zoom lens according to the present invention comprises, in order from the object side, a first lens group having a positive refractive power and a second lens group having a negative refractive power. And a third lens group having a positive refractive power and a fourth lens group having a positive refractive power, wherein zooming is performed by moving the second lens group and the fourth lens group. In the lens, the third lens group includes at least one spherical lens having a positive refractive power;
The third lens group includes a spherical lens having a positive refractive power and an aspherical surface having a negative refractive power. It is the one that is joined with a lens.

Therefore, it is possible to reduce the size of a zoom lens having a zoom ratio of about 10 without deteriorating the optical performance.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the zoom lens according to the present invention will be described.

First, the outline of the zoom lens of the present invention will be described.

That is, the zoom lens of the present invention comprises, in order from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. And a fourth lens group having a positive refractive power,
In a zoom lens that performs zooming by moving the second lens group and the fourth lens group,
The third lens group includes at least one spherical lens having a positive refractive power and one aspherical lens having a negative refractive power with a concave aspheric surface facing the image plane side. The present invention is characterized in that a spherical lens having a positive refractive power of a lens group and an aspherical lens having a negative refractive power are cemented.

It is desirable that the zoom lens of the present invention satisfies the following conditions. 1.2 <Di / Do <1.45 (Conditional expression 1) where Di: diameter of light beam incident on the third lens unit at the short focal length end (wide angle end), Do: third light at the short focal length end Let the diameter of the light beam emitted from the lens group be:

Furthermore, it is desirable that the zoom lens of the present invention also satisfies the following conditions. 3 < _{fGRIII /} fw <5.5, where _fGRIII : focal length of the third lens group, fw: focal length at the short focal length end of the entire lens system.

Furthermore, in the zoom lens of the present invention, it is desirable that the surface on the object side of the aspherical lens included in the third lens group is a flat surface.

Next, each feature of the present invention will be described.

In the zoom lens according to the present invention, in order from the object side,
A first lens group GRI having a positive refractive power, a second lens group GRII having a negative refractive power, a third lens group GRIII having a positive refractive power, and a fourth lens group GRIV having a positive refractive power. The zooming is performed by moving the second lens group GRII and the fourth lens group GRIII.

When zooming from the short focal length end (wide angle end) to the long focal length end (telephoto end), the second lens group GR2 moves from the object side to the image plane side, and the fourth lens group GR2 moves. The GRIV is appropriately moved so as to maintain the image position. The focus adjustment is performed by moving the fourth lens group GRIV.

The third lens group GRIII includes at least one spherical lens having a positive refractive power and one aspherical lens having a negative refractive power with a concave aspheric surface facing the image plane side. The spherical lens having a positive refractive power and the aspherical lens having a negative refractive power of the third lens group GRIII are cemented to form a so-called cemented lens. This is because, by using the two lenses as cemented lenses, the allowable amount of eccentricity without the third lens group GRIII is increased, and manufacturing is facilitated.

Conditional expression 1 defines the magnification for reducing the diameter of the light beam by the third lens group GRIII having a positive refractive power. Therefore, as defined in conditional expression 1, Di / D
If the value of o exceeds the lower limit, the overall length of the entire lens system becomes longer. On the other hand, if the value of o exceeds the upper limit, a sufficient back focus cannot be secured.

Conditional expression 2 defines the ratio between the third lens group GRIII having a positive refractive power and the focal length of the entire lens system. Therefore, as defined in conditional expression 2, f
_If the value of _{GRII I} / fw exceeds the lower limit, the third lens group G
The positive power of RIII becomes too strong, that is, the power of the positive spherical lens in the third lens group GRIII becomes too large to correct spherical aberration with an aspheric lens having negative refractive power, The flatness of the surface cannot be maintained, and the optical performance deteriorates. Conversely, f _{GR III} /
If the value of fw exceeds the upper limit, the positive power of the third lens group GRIII becomes too weak, and the overall length of the entire lens system becomes longer, making it difficult to reduce the size of the zoom lens.

Further, by making the object-side surface of the aspherical lens having a negative refractive power in the third lens group GRIII flat, the manufacture of the aspherical lens having a negative refractive power is facilitated, and the manufacturing cost is increased. Can be reduced.

Hereinafter, numerical examples 1 and 2 in the embodiment of the zoom lens according to the present invention will be described with reference to the accompanying drawings.

In the following description, "FNo." Is the F-number, "f" is the focal length, "ω" is the half angle of view, "r"
“i” is the radius of curvature of the i-th lens surface from the object side, and “d”
“i” is the surface distance between the i-th lens surface and the (i + 1) -th lens surface from the object side, “ndi” is the refractive index at the d-line of the i-th lens, and “νdi” is the d-line of the i-th lens. Abbe number of each is shown.

Of the surfaces of each lens, those constituted by an aspherical surface are indicated by "ASP" in the table.

The aspheric shape is defined by the following equation. ^{x = (y 2 / r)} / {1+ (1-κ · y 2 / r 2)} 1/2 +
C4 · y ⁴ + C6 · y ⁶ + C8 · y ⁸ + C10 · y ¹⁰ where “x” is the distance in the optical axis direction from the vertex of the lens surface,
“Y” is the radius of curvature at the lens vertex, “κ” is the conic constant,
“C4”, “C6”, “C8”, and “C10” are fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients, respectively.

FIG. 1 schematically shows the structure of a zoom lens 1 according to Numerical Example 1.
Are the first lens L1 to the third lens L in order from the object side.
3, a first lens group GRI having a positive refractive power as a whole, and a second lens group GRI having a negative refractive power as a whole, including a fourth lens L4 to a sixth lens L6.
A third lens group GRII having a positive refracting power as a whole, comprising I, a stop S, a seventh lens L7 and an eighth lens L8.
I and a ninth lens L9 and a tenth lens L10,
The lens system includes a fourth lens group GRIV having a positive refractive power as a whole and a four-group ten-lens lens system.

Note that a low-pass filter, an infrared cut filter, and a cover glass FL of a CCD are provided between the fourth lens group GRIV and the image plane IMG.

Particularly, the third lens group GRIII is a plano-convex lens having a positive refractive power with the convex surface facing the object side.
A lens L7 and an eighth lens L8, which is an aspheric plano-concave lens having a negative refractive power with a concave aspheric surface facing the image plane side, and the seventh lens L7 and the eighth lens L8 are a cemented lens. Has become.

The zoom lens 1 performs zooming by moving the second lens group GR2 and the fourth lens group GRIV. When zooming from the short focal length end to the long focal length end, the second lens group GR2 Moving from the object side to the image plane side, the fourth lens group GR4 is appropriately moved to the object side or the image plane side so as to maintain the image position.

Table 1 shows each numerical value of the zoom lens 1.

[0030]

[Table 1]

Further, as shown in Table 1 above, in the third lens group GRIII and the fourth lens group GRIV, the surfaces (r) on the image plane side of the eighth lens L8 and the tenth lens L10, respectively.
14 and r17) are constituted by aspherical surfaces. Table 2 below shows aspheric coefficients of r14 and r17.

[0032]

[Table 2]

In Table III, "e" indicates an exponential expression with a base of 10. (The same applies to Table 4 below.)

As shown in Table 1, the surface distances d5 and d1
0, d14 and d17 are variable. Therefore, Table 3 shows the above-mentioned surface distances and FNo. , F, and ω.

[0035]

[Table 3]

FIGS. 2 to 4 show the zoom lens 1 respectively.
Shows the spherical aberration, astigmatism, and distortion at the short focal length end, intermediate focal length, and long focal length end. In the spherical aberration diagram, the solid line is the e-line (wavelength 546.1 nm), the dotted line is the C line (wavelength 656.3 nm), the broken line is the F line (wavelength 486.1 nm), the one-dot chain line is the d line (wavelength 587.6 nm) and The chain line is the g-line (wavelength 435.8n
m). In the astigmatism diagram, the solid line shows the value on the sagittal image plane, and the broken line shows the value on the meridional image plane (the same applies to FIGS. 6 to 8).

FIG. 5 schematically shows the structure of a zoom lens 1A according to Numerical Example 2, similar to the zoom lens 1 in Numerical Example 1 described above.
Are the first lens L1 to the third lens L in order from the object side.
3, a first lens group GRI having a positive refractive power as a whole, and a second lens group GRI having a negative refractive power as a whole, including a fourth lens L4 to a sixth lens L6.
A third lens group GRII having a positive refracting power as a whole, comprising: I, an aperture S, a seventh lens L7 to a ninth lens L9.
A fourth lens group GRIV having a positive refracting power as a whole, comprising an I and a tenth lens L10 and an eleventh lens L11;
And a lens system composed of 11 elements in 4 groups.

Note that a low-pass filter, an infrared cut filter, and a cover glass FL of a CCD are disposed between the fourth lens group GRIV and the image plane IMG.

In particular, the third lens group GRIII is a seventh lens L7 which is a meniscus lens having a positive refractive power with the convex surface facing the object side, and a plano-convex lens having a positive refractive power with the convex surface facing the object side. An eighth lens L8 is constituted by a certain eighth lens L8 and a ninth lens L9, which is an aspheric plano-concave lens having a negative refractive power with the concave aspheric surface facing the image plane side.
The eighth and ninth lenses L9 are cemented lenses.

The zoom lens 1A includes a second lens group GR2
And the fourth lens group GRIV are moved to perform zooming. When zooming from the short focal length end to the long focal length end, the second lens group GR2 moves from the object side to the image plane side, and The lens group GR4 is appropriately moved to the object side or the image plane side so as to maintain the image position.

Table 4 shows each numerical value of the zoom lens 1A.

[0042]

[Table 4]

Further, as shown in Table 4, in the third lens group GRIII and the fourth lens group GRIV, the image-side surface (r16) of the ninth lens L9 and the object-side surface (r16) of the tenth lens L10. r17) is composed of an aspherical surface. Table 5 below shows the aspheric coefficients of r16 and r17.

[0044]

[Table 5]

As shown in Table 4 above, the surface distances d5 and d1
0, d16 and d19 are variable. Therefore, Table 6 shows the above-mentioned surface intervals and FNo. , F, and ω.

[0046]

[Table 6]

FIGS. 6 to 8 show the zoom lens 1 respectively.
A shows spherical aberration, astigmatism, and distortion at the short focal length end, the intermediate focal length, and the long focal length end of A.

Table 7 shows numerical values in the conditional expressions 1 and 2 of the above-mentioned Numerical Examples 1 and 2.

[0049]

[Table 7]

It should be noted that the specific shapes and structures of the respective parts shown in the above-described embodiments are merely examples for embodying the present invention, and the technical features of the present invention will be described below. The scope should not be construed as limiting.

[0051]

As is apparent from the above description, the zoom lens according to the present invention comprises, in order from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, In a zoom lens which includes a third lens group having a positive refractive power and a fourth lens group having a positive refractive power, and performs zooming by moving the second lens group and the fourth lens group. , The third lens group includes at least one spherical lens having a positive refractive power and one aspheric lens having a negative refractive power with a concave aspheric surface facing the image plane side. Since the spherical lens having the positive refractive power and the aspherical lens having the negative refractive power of the three lens groups are cemented, it is possible to increase the allowable eccentricity of the lens in the third lens group. That the optical performance is degraded It is possible to facilitate the workability in manufacturing is possible stop.

According to the second aspect of the present invention, Di is the diameter of a light beam incident on the third lens unit at the short focal length extremity, and Do is emission from the third lens unit at the short focal length extremity. Assuming that the beam diameter is 1.2 <Di / Do <1.45, it is possible to reduce the size of the zoom lens having a zoom ratio of about 10 times.

Further, according to the third and fourth _aspects of the _present invention, f _GRIII is a focal length of the third lens _unit , f _GR
If w is the focal length at the short focal length extremity of the entire lens system,
Since the condition of 3 <f _GRIII /fw<5.5 is satisfied, the optical performance can be prevented from deteriorating.
A zoom lens having a zoom ratio of about 10 can be reduced in size.

[Brief description of the drawings]

FIG. 1 shows Numerical Example 1 of an embodiment of a zoom lens according to the present invention, along with FIGS. 2 to 4, and FIG. 1 does not schematically show a lens configuration.

FIG. 2 is a diagram illustrating spherical aberration, astigmatism, and distortion at a short focal length extremity.

FIG. 3 is a diagram illustrating spherical aberration, astigmatism, and distortion at an intermediate focal length.

FIG. 4 is a diagram illustrating spherical aberration, astigmatism, and distortion at a long focal length extremity.

FIG. 5 shows Numerical Example 2 of the embodiment of the zoom lens of the present invention together with FIGS. 6 to 8, and this drawing does not schematically show the lens configuration.

FIG. 6 is a diagram illustrating spherical aberration, astigmatism, and distortion at a short focal length extremity.

FIG. 7 is a diagram showing spherical aberration, astigmatism, and distortion at an intermediate focal length.

FIG. 8 is a diagram illustrating spherical aberration, astigmatism, and distortion at a long focal length extremity.

[Explanation of symbols]

1: zoom lens, 1A: zoom lens, GRI: first
Lens group, GRII: second lens group, GRIII: third lens group, GRIV: fourth lens group

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H087 KA03 PA06 PA07 PA16 PB10 PB11 QA02 QA07 QA17 QA21 QA25 QA34 QA37 QA41 QA42 QA46 RA05 RA12 RA13 RA32 RA42 RA43 SA23 SA27 SA29 SA32 SA63 SA65 SA72 SA74 SB04 SB33 SB24 SB23

Claims (4)

[Claims] 1. 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 object side. And a fourth lens group having the following formula: wherein the zooming is performed by moving the second lens group and the fourth lens group, wherein the third lens group has at least one positive lens. A spherical lens having a refractive power and a single aspherical lens having a negative refractive power with a concave aspheric surface facing the image surface side, and the spherical lens having a positive refractive power of the third lens group. A zoom lens, wherein an aspheric lens having a negative refractive power is cemented. 2. The zoom lens according to claim 1, wherein the following condition is satisfied. 1.2 <Di / Do <1.45, where Di: the diameter of the light beam incident on the third lens group at the short focal length extremity, Do: the light flux diameter from the third lens group at the short focal length extremity, and I do. 3. The zoom lens according to claim 1, wherein the following condition is satisfied. 3 < _{fGRIII /} fw <5.5, where _fGRIII : focal length of the third lens group, fw: focal length at the short focal length end of the entire lens system. 4. The zoom lens according to claim 2, wherein the following condition is satisfied. 3 < _{fGRIII /} fw <5.5, where _fGRIII : focal length of the third lens group, fw: focal length at the short focal length end of the entire lens system.