JP2000131610A - Zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized zoom lens of about three-variable power ratio suitable for a small-sized image pickup device such as a video camera and a digital still camera, etc. SOLUTION: The zoom lens 1 is constituted of a 1st lens group GR1 whose refractive power is positive, a 2nd lens group GR2 whose refractive power is negative, a 3rd lens group GR3 whose refractive power is positive and a 4th lens group GR4 whose refractive power is positive in order from an object side to an image field IMG side, and zooming is accomplished by moving the 2nd lens group GR2 and the 4th lens group GR4. In this case, the 1st lens group GR1 is constituted of a 1st single lens L1 whose refractive power is negative, a prism P for bending an optical path and a 2nd single lens L2 whose refractive power is positive in order from the object side.

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 3 which is optimal for a small-sized video camera, digital still camera or the like.

[0002]

2. Description of the Related Art In recent years, there has been a demand for further miniaturization of small-sized imaging devices such as video cameras and digital still cameras.
Zoom lenses are required to be reduced in size by shortening the overall length.

In addition, in the above-mentioned photographing lens, particularly for a digital still camera, demands for a zoom lens including a wide-angle region having an angle of view of about 70 to 80 ° at a wide-angle end have been increased with miniaturization. At the same time, there is a demand for an improvement in lens performance in response to an increase in the number of pixels of an image sensor.

[0004]

As a small zoom lens for a small imaging device, a first lens group having a negative refractive index and a second lens group having a positive refractive power are arranged in order from the object side. There is a retrofocus type zoom lens having a two-group configuration. However, in such a two-unit zoom lens, it is difficult to increase the zoom ratio, and the overall length changes with the zooming operation, which is not suitable for a small-sized imaging device.

Further, 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, and a third lens group (compensator) having a positive refractive power. And a fourth lens group (master) having a positive refractive power. However, such a four-group zoom lens is unsuitable for use in a small-sized imaging device because the overall length is long.

Further, a zoom lens described in JP-A-8-248318, ie, a first lens group having a positive refractive power and a second lens group having a negative refractive power are arranged in order from the object side.
A lens group (variator), a third lens group (compensator) having a positive refractive power, and a fourth lens group having a positive refractive power
Like a four-group zoom lens composed of a lens group (master), a prism is disposed between the lens at the object side position of the first lens group, and the first lens group is sandwiched by the prism. In some cases, an afocal system is configured by dividing the lens into a lens group having a negative refractive power on the object side and a positive refractive power on the image plane side, and the front-back length is shortened by bending the optical path by a prism. However, this type of zoom lens has a problem that the number of lenses is large, the overall length is still long, and the manufacturing cost is high.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a small zoom lens having a zoom ratio of about three times, which is optimal for a small imaging device such as a video camera and a digital still camera.

[0008]

In order to solve the above-mentioned problems, a zoom lens according to the present invention comprises a first lens unit having a positive refractive power and a negative refractive power in order from an object side to an image plane side. A second lens group, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. The zooming is performed by moving the second lens group and the fourth lens group. In a zoom lens that performs
The first lens group includes, in order from the object side, a first lens of a single lens having a negative refractive power, a prism for bending an optical path, and a second lens of a single lens having a positive refractive power.

Therefore, it is possible to reduce the size of a zoom lens having a zoom ratio of about three times, which is optimal for a small-sized imaging device such as a video camera and a digital still camera.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the accompanying drawings. still,
1 to 4 show the first embodiment (numerical example 1), FIGS. 5 to 8 show the second embodiment (numerical example 2), and FIG.
12 to 12 show a third embodiment (Numerical Example 3).

First, common items in each embodiment will be described.

In the following description, "Si" is the i-th surface counted from the object side, "Ri" is the radius of curvature of the surface Si, and "di" is the i-th surface and the (i + 1) -th surface from the object side. “NdLi” is the refractive index of the i-th lens (Li) at d-line (wavelength 587.6 nm), “νdLi”
Is the Abbe number of the i-th lens (Li) at the d-line, “f” is the focal length of the entire lens system, “Fno.” Is the open F value, and “ω”
It indicates a half angle of view. However, those in which P, LP, IR, and CG are added after nd or vd indicate the refractive index or Abbe number of the prism, low-pass filter, infrared cut filter, and cover glass of the image sensor, respectively.

The lenses used in the embodiments include those having a lens surface formed by an aspherical surface.

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

The zoom lenses 1, 2 and 3 in the first to third embodiments are, as shown in FIGS.
In order from the object side to the image plane IMG side, a first lens group GR1 having a positive refractive power, a second lens group GR2 having a negative refractive power, and a third lens group GR3 having a positive refractive power
And a fourth lens group GR4 having a positive refractive power. In the zoom lenses 1 to 3, the first lens group GR1 has two lenses including a first lens L1 and a second lens L2 and a prism disposed therebetween, and the second lens group GR2 has a third lens L3 and a fourth lens. Three lenses including L4 and a fifth lens L5, the third lens group GR3 is a sixth lens L6, and the fourth lens group GR4 is a seventh lens L7,
This is a four-group, nine-element configuration having three lenses each including an eighth lens L8 and a ninth lens L9.

An aperture ID is provided between the second lens group GR2 and the third lens group GR3, and a low-pass filter LP and an infrared light are provided between the fourth lens group GR4 and the image plane IMG in order from the object side. The cut filter IR and the cover glass CG of the CCD are arranged.

The zooming is performed by moving the second lens group GR2 and the fourth lens group GR4. The zooming is performed from a short focal length end (wide angle end) to a long focal length end (telephoto end). During zooming, the second lens silver GR2 is moved from the object side to the image plane side, and the fourth lens group GR4 is moved so as to maintain the image position.

The focus adjustment of the zoom lenses 1 to 3 is performed by moving the fourth lens group GR4.

The first lens group GR1 is arranged in order from the object side.
It comprises a first lens L1 of a meniscus-shaped single lens having a negative refractive power, a prism P for bending the optical path by 90 °, and a second lens L2 of a single lens having a positive refractive power.

The zoom lenses 1 to 3 satisfy the following conditional expression 1.
And conditional expression 2 are satisfied, or the first lens L1
It is preferable that at least one of the surfaces is formed by an aspheric surface. ndL1> 1.75 (conditional expression 1) νdL1 <30 (conditional expression 2) where ndL1 is the refractive index of the first lens L1 at the d-line, and νdL1 is the Abbe number of the first lens L1 at the d-line. is there.

Conditional expression 1 defines the amount of distortion generated by the first lens L1 which is a single lens having a negative refractive power and constitutes the first lens group GR1 having a positive refractive power. . That is, when the value of ndL1 is out of the range defined by the conditional expression 1, the amount of distortion that is generated with respect to the required refractive power of the first lens group GR1 increases, and this is changed to the fourth lens group. The correction cannot be performed due to the aspherical surface of GR4.

Conditional expression 2 defines the amount of chromatic aberration caused by L1 by the first lens which is a single lens having a negative refractive power and constitutes the first lens group GR1 having a positive refractive power. . That is, when the value of νdL1 is out of the range defined by the conditional expression 2, the amount of chromatic aberration generated in the first lens group GR1 having a positive refractive power increases, and correction of the chromatic aberration cannot be performed even in the entire lens system. It will be difficult.

It is preferable that the object-side surface S1 of the first lens L1 of the zoom lenses 1 to 3 be convex toward the object side. This is because, if the surface S1 is concave toward the object side, the negative distortion generated on the concave surface S1 increases, and it becomes difficult to correct this by the entire lens system.

The fourth lens group GR of the zoom lenses 1 to 3
It is desirable that at least one of the surfaces of the lens constituting the lens 4 should be constituted by an aspheric surface, and in particular, at least one surface of the lens located closest to the image plane should be constituted by an aspheric surface. .

As described above, when at least one of the lens surfaces in the fourth lens group GR4 is formed by an aspherical surface, it is possible to correct the negative distortion at the wide-angle end generated by the first lens group GR1. As a result, the power of the single lens (first lens) L1 having a negative refractive power of the first lens group GR1 can be increased, and a wider angle of view can be obtained. Become like

Further, it is preferable that the zoom lenses 1 to 3 are configured to satisfy the following conditional expression 3. 4.5 <f _GR1 / fw <12 (conditional expression 3) where f _GR1 is the focal length of the first lens group GR1,
fw is the focal length of the entire lens system at the wide-angle end.

Conditional expression 3 defines the ratio between the focal length of the first lens group GR1 having a positive refractive power and the focal length of the entire lens system. That is, the value of f _GR1 / fw is 4.
When the value is 5 or less, the positive power of the first lens group GR1 becomes too strong, and the power of the second lens L2, which is a single lens having a positive refractive power, in the first lens group GR1 increases. With L2, spherical aberration cannot be corrected, or the power of the first lens, which is a single lens having a negative refractive power, becomes weak, making it difficult to sufficiently widen the angle of view. Further, when the value of f _GR1 / fw becomes 12 or more, the positive power of the first lens group GR1 becomes too weak, and the total length of the zoom lenses 1 to 3 becomes long, which makes downsizing difficult.

Next, specific items of the zoom lenses 1 to 3 according to the first to third embodiments will be described.

Table 1 shows each numerical value of the zoom lens 1.
The surface with (ASP) added after the numerical value of Ri is constituted by an aspherical surface (see Tables 4 and 7 described later).
The same applies. ).

[0030]

[Table 1]

As shown in Table 1, the distance d between the surfaces of the zoom lens 1 accompanying the zooming and focusing operations is
7, d12, d15 and d19 are variable (variable
e). Therefore, Table 7 shows that d7, d12, d15, and d19 at the wide-angle end (f = 5.3), the telephoto end (f = 15.6), and the intermediate focal position (f = 9.0) between the wide-angle end and the telephoto end. , And FNo. , F and ω.

[0032]

[Table 2]

Also, the sixth lens L of the third lens group GR3
The surface S14 on the object side of No. 6 and the surface S19 on the image surface side of the ninth lens L9 of the fourth lens group GR4 are formed by aspheric surfaces. Table 3 below shows the fourth order of the surfaces S14 and S19,
6th, 8th and 10th order aspherical coefficients C4, C6, C8 and C10 are shown.

[0034]

[Table 3]

It is to be noted that "E" in the above Table 3 means an exponential expression having a base of 10. (The same applies to Tables 7 and 11 described below.)

FIGS. 2 to 4 show the wide-angle end of the zoom lens 1,
FIGS. 3A and 3B respectively show spherical aberration, astigmatism, and distortion at an intermediate focal position between the wide-angle end and the telephoto end and at the telephoto end. In the spherical aberration diagram, the solid line is the e-line (wavelength 54
6.1 nm), a dotted line (dashed line with a shorter pitch) is a C line (wavelength 656.3 nm), a dashed line is a d line, a broken line is an F line (wavelength 486.1 nm), and a two-dot chain line is a g line (wavelength 43 nm).
5.8 nm), and 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.

In the zoom lens 1, the fourth lens group GR4 is constituted by a cemented lens of three lenses L7, L8 and L9.
The inclination of the image plane due to the eccentricity in the inside is reduced, and the manufacturing is facilitated.

Table 4 shows each numerical value of the zoom lens 2.

[0039]

[Table 4]

As shown in Table 4, the distance d between the surfaces of the zoom lens 2 accompanying the zooming and focusing operations of the zoom lens 2 is increased.
7, d12, d15 and d20 are variable (variable
e). Accordingly, Table 7 shows that d7, d12, d15, and d20 at the wide-angle end (f = 5.3), the telephoto end (f = 15.5), and the intermediate focal position (f = 9.0) between the wide-angle end and the telephoto end. , And FNo. , F and ω.

[0041]

[Table 5]

Further, the sixth lens L of the third lens group GR3
The object side surface S14 and the object side surface S19 and the image surface side S20 of the ninth lens L9 of the fourth lens group GR4 are constituted by aspheric surfaces. Table 6 below shows the surface S1
The fourth, sixth, eighth and tenth order aspherical coefficients C4, C6, C8 and C10 of 4, S19 and S20 are shown.

[0043]

[Table 6]

FIGS. 6 to 8 show the wide-angle end of the zoom lens 2,
FIGS. 3A and 3B respectively show spherical aberration, astigmatism, and distortion at an intermediate focal position between the wide-angle end and the telephoto end and at the telephoto end. In the spherical aberration diagram, the solid line is e-line, and the dotted line is C
The dash-dot line indicates the value at the d-line, the dashed line indicates the value at the F-line, and the two-dot chain line indicates the value at the g-line. In the astigmatism diagram, the solid line indicates the value at the sagittal image plane, and the dashed line indicates the value at the meridional image plane. is there.

In the zoom lens 2, a plastic aspherical lens is used for the ninth lens L9 of the fourth lens group GR4 to constitute a zoom lens that can be manufactured at a low cost while achieving miniaturization and high performance. I have.

Table 7 shows the numerical values of the zoom lens 3.

[0047]

[Table 7]

As shown in Table 7, the distance d between the zoom lens 3 and the zooming operation of the zoom lens 3 depends on the zooming and focusing operations.
7, d12, d15 and d19 are variable (variable
e). Therefore, Table 8 shows that d7, d12, d15, and d19 at the wide-angle end (f = 5.3), the telephoto end (f = 15.5), and the intermediate focal position (f = 9.0) between the wide-angle end and the telephoto end. , And FNo. , F and ω.

[0049]

[Table 8]

The image-side surface S2 of the first lens L1 of the first lens unit, the object-side surface S14 of the sixth lens L6 of the third lens unit GR3, and the ninth lens L9 of the fourth lens unit GR4 The surface S19 on the image plane side is formed by an aspheric surface. The following Table 3 shows the four surfaces S2, S14 and S19.
Next, sixth, eighth and tenth order aspherical coefficients C4, C6, C
8 and C10 are shown.

[0051]

[Table 9]

In the zoom lens 3, as described above, the surface S2 on the image plane side of the first lens L1 in the first lens group GR1 is constituted by an aspheric surface, so that the field curvature and the long focal length range are obtained. Is corrected.

FIGS. 10 to 12 show the spherical aberration, astigmatism, and distortion at the wide-angle end, the intermediate focal position between the wide-angle end and the telephoto end, and the telephoto end of the zoom lens 3, respectively. In the spherical aberration diagram, the solid line is e-line, and the dotted line is C
The dash-dot line indicates the value at the d-line, the dashed line indicates the value at the F-line, and the two-dot chain line indicates the value at the g-line. In the astigmatism diagram, the solid line indicates the value at the sagittal image plane, and the dashed line indicates the value at the meridional image plane. is there.

In the zoom lens 3, similarly to the zoom lens 1 in the first embodiment, the fourth lens group G
By configuring R4 as a cemented lens of three lenses L7, L8, and L9, the image plane tilt due to eccentricity in the fourth lens group GR4 is reduced, and manufacturing is facilitated. .

Table 10 below shows numerical values and values of the conditional expressions for obtaining the conditions of the conditional expressions 1 to 3 of the zoom lenses 1 to 3 shown in the first to third embodiments.

[0056]

[Table 10]

As is clear from Table 10, the zoom lenses 1 to 3 satisfy the conditions of the conditional expressions 1 to 3, and as shown in the aberration diagrams, the wide-angle end, the wide-angle end and the telephoto end At the intermediate focal position and the telephoto end, various aberrations are corrected in a well-balanced manner.

As described above, the zoom lenses 1 to 3 have a field angle at the wide-angle end of 74 °, which includes a sufficiently wide-angle region, and various aberrations are well corrected. It is suitable for a digital still camera using many image pickup devices.

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 are not limited thereto. The scope should not be construed as limiting.

[0060]

As is apparent from the above description, the zoom lens according to the present invention is arranged in order from the object side to the image plane 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 fourth lens group having a positive refractive power; In a zoom lens system in which zooming is performed by moving the second lens unit and the fourth lens unit, the first lens unit includes a first lens having a negative refractive power, a first lens having a negative refractive power, and an optical path. Since it is composed of a prism that bends and a second lens that is a single lens having a positive refractive power, it is possible to reduce the size of a zoom lens having a zoom ratio of about three times, which is optimal for a small-sized imaging device such as a video camera and a digital still camera. Can be.

In the zoom lens according to the second aspect, ndL1 is the refractive index of the first lens at the d-line, νdL1
Is the Abbe number at the d-line of the first lens, ndL1>
Since the respective conditions of 1.75 and νdL1 <30 are satisfied, distortion and chromatic aberration generated in the first lens group can be favorably corrected.

In the zoom lens according to the third aspect, since at least one surface of the first lens is formed by an aspherical surface, it is possible to satisfactorily correct curvature of field and spherical aberration in a long focal length range. it can.

In the inventions described in claims 4 to 6, since the surface of the first lens facing the object side is a convex surface, it is difficult to correct the entire lens system with negative distortion. Does not become large.

According to the seventh to twelfth aspects of the present invention, at least one of the surfaces of the lenses constituting the fourth lens group is constituted by an aspherical surface. Since it is possible to effectively correct the generated negative distortion at the wide-angle end, the first
The power of the negative single lens of the lens group can be increased, and a wider angle of view can be obtained.

In the inventions described in claims 13 to 24, if f _GR1 is the focal length of the first lens group and fw is the focal length at the wide end of the entire lens system, 4.5 <
Since the condition of f _{GR 1} / fw <12 was satisfied,
Correction of spherical aberration, sufficient widening of the angle of view, and miniaturization can be achieved.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of a zoom lens according to the present invention together with FIGS. 2 to 4, and FIG. 1 is a schematic diagram showing a lens configuration.

FIG. 2 is a diagram illustrating spherical aberration, astigmatism, and distortion at a wide-angle end.

FIG. 3 is a diagram illustrating spherical aberration, astigmatism, and distortion at an intermediate focal position between a wide-angle end and a telephoto end.

FIG. 4 is a diagram illustrating spherical aberration, astigmatism, and distortion at the telephoto end.

FIG. 5 shows a second embodiment of the zoom lens of the present invention together with FIG. 6 to FIG. 8, and FIG. 5 is a schematic diagram showing a lens configuration.

FIG. 6 is a diagram showing spherical aberration, astigmatism, and distortion at the wide-angle end.

FIG. 7 is a diagram illustrating spherical aberration, astigmatism, and distortion at an intermediate focal position between a wide-angle end and a telephoto end.

FIG. 8 is a diagram showing spherical aberration, astigmatism, and distortion at the telephoto end.

9 shows a third embodiment of the zoom lens according to the present invention together with FIGS. 10 to 12, and FIG. 9 is a schematic diagram showing a lens configuration.

FIG. 10 is a diagram showing spherical aberration, astigmatism, and distortion at the wide-angle end.

FIG. 11 is a diagram illustrating spherical aberration, astigmatism, and distortion at an intermediate focal position between a wide-angle end and a telephoto end.

FIG. 12 is a diagram illustrating spherical aberration, astigmatism, and distortion at the telephoto end.

[Explanation of symbols]

1: zoom lens, 2: zoom lens, 3: zoom lens, GR1: first lens group, GR2: second lens group, G
R3: third lens group, GR4: fourth lens group, L1: first lens, L2: second lens, P: prism, IMG ...
Image plane

Continued on the front page F term (reference) 2H087 KA03 MA15 PA06 PA07 PA19 PB09 QA02 QA06 QA17 QA21 QA26 QA32 QA37 QA42 QA45 RA05 RA12 RA32 RA41 RA43 SA23 SA27 SA29 SA32 SA63 SA65 SB03 SB14 SB22 SB34 TA03

Claims (24)

[Claims] 1. 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, in order from the object side to the image plane side. And a fourth lens group having a positive refractive power, wherein the zooming is performed by moving the second lens group and the fourth lens group. In order from the first lens of a single lens having a negative refractive power, a prism that bends the optical path,
A zoom lens comprising a second single lens having a positive refractive power. 2. The zoom lens according to claim 1, wherein the following condition is satisfied. ndL1> 1.75 νdL1 <30, where ndL1: refractive index of the first lens at the d-line, and νdL1: Abbe number of the first lens at the d-line. 3. The zoom lens according to claim 1, wherein at least one surface of the first lens is formed by an aspheric surface. 4. The zoom lens according to claim 1, wherein a surface of the first lens facing the object side is a convex surface. 5. The zoom lens according to claim 2, wherein a surface of the first lens facing the object side is a convex surface. 6. The zoom lens according to claim 3, wherein a surface of the first lens facing the object side is a convex surface. 7. The zoom lens according to claim 1, wherein at least one of the surfaces of the lens constituting the fourth lens group is constituted by an aspheric surface. 8. The zoom lens according to claim 2, wherein at least one of the surfaces of the lens constituting the fourth lens group is constituted by an aspheric surface. 9. The zoom lens according to claim 3, wherein at least one of the surfaces of the lens constituting the fourth lens group is constituted by an aspheric surface. 10. The zoom lens according to claim 4, wherein at least one of the surfaces of the lens constituting the fourth lens group is constituted by an aspheric surface. 11. The zoom lens according to claim 5, wherein at least one of the surfaces of the lens constituting the fourth lens group is constituted by an aspheric surface. 12. The zoom lens according to claim 6, wherein at least one of the surfaces of the lens constituting the fourth lens group is constituted by an aspheric surface. 13. The zoom lens according to claim 1, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 14. The zoom lens according to claim 2, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 15. The zoom lens according to claim 3, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 16. The zoom lens according to claim 4, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 17. The zoom lens according to claim 5, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 18. The zoom lens according to claim 6, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 19. The zoom lens according to claim 7, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 20. The zoom lens according to claim 8, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 21. The zoom lens according to claim 9, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 22. The zoom lens according to claim 10, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens unit, and fw is the focal length of the entire lens system at the wide end. 23. The zoom lens according to claim 11, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens group, and fw is the focal length of the entire lens system at the wide end. 24. The zoom lens according to claim 12, wherein the following condition is satisfied. 4.5 <f _GR1 / fw <12 where f _{GR1 is} the focal length of the first lens group, and fw is the focal length of the entire lens system at the wide end.