JPH0629908B2 - Telephoto zoom lens - Google Patents

Description

The present invention relates to a telephoto zoom lens having a simple and compact structure and a high zoom ratio.

In order from the object 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. A telephoto zoom lens in which the first lens group is moved on the optical axis during focusing and the second and third lens groups are moved during zooming is known from Japanese Patent Application Laid-Open No. 58-202420. However, in such a lens, in order to increase the zoom ratio, the first
Since it is necessary to move the lens group largely forward, the total length of the lens is increased and the number of constituent elements is increased.

Therefore, the first method for increasing the zoom ratio and making the overall lens length compact and simplifying the configuration is to configure the fourth lens group arranged closest to the image side by a telephoto type having a small telephoto ratio. . That is, the fourth lens group is divided into two groups, a front group and a rear group, and the front group is positive and the rear group is negative. However, in order to reduce the telephoto ratio, strong refractive power is required for both the front and rear groups, so aberration correction becomes extremely difficult. Therefore, conventionally, each of the front and rear groups is usually composed of a combination of a positive lens and a negative lens.
It is composed of sheets. The second method is to increase the negative combined refractive power of the second lens group that is moved during zooming to reduce the amount of movement, but then, the other lens groups must also have strong refractive power. ,
The difficulty in aberration correction increases. A third method is to reduce the number of constituent lenses of each lens group as much as possible, but this increases the difficulty in aberration correction.

Therefore, the present invention overcomes the above-mentioned difficulties and provides a telephoto zoom lens having a high zoom ratio of 3.5 to 4 and a compact and simple structure, with good aberration correction. With the goal.

In order to achieve the above object, the telephoto zoom lens according to the present invention includes, in order from the object side, a first lens group having a positive refractive power.
(I), a second lens group (II) having a negative refractive power, a third lens group (III) having a positive refractive power, and a fourth lens group (IV) having a positive refractive power. Therefore, the first lens group (I) is moved along the optical axis during focusing, and the first lens group (I) and the second lens group during zooming.
(II) and the third lens group (III) are moved on the optical axis, and the fourth lens group (IV) is composed of one or two single lenses having a positive refractive power and has a positive refractive index as a whole. As a whole, it consists of a front lens group (IVa) having a refractive power, a single lens having a positive refractive power in order from the object side, a single lens having a negative refractive power, and a single lens having a positive refractive power. It is characterized by comprising a rear group (IVb) having a negative refractive power and satisfying the following conditions.

(1) 0.05 <Δx / f ₁ <1.0 (2) 0.4 <fa / f ₄ <1.3 where Δx is the first lens group during zooming.
Movement amount of (I), f ₁ is the focal length, fa is the focal length, f ₄ is the fourth lens group of the front group in the fourth lens group (IVa) of the first lens group (I)
It is the focal length of (IV).

Each condition will be described below. First, in the present invention, in order to achieve a zoom ratio of 3.5 to 4 while making the overall lens length compact and simplifying the configuration, the above-mentioned first method is used and the fourth lens group (IV) is set to the telephoto ratio. It consists of a small telephoto type. That is, the fourth lens group (IV) is composed of a front group (IVa) having a positive refractive power and a rear group (IVb) having a negative refractive power. Furthermore, in order to make the configuration simple and compact and to reduce the telephoto ratio of the fourth lens group (IV),
The front group (IVa) of the fourth lens group is composed of only one positive lens or two. In most conventional lenses, the front lens group is composed of a combination of a positive lens and a negative lens, but in the present invention, it is difficult to correct aberrations because it consists of only positive lenses. Therefore, the rear group (IVb) of the fourth lens group is composed of three positive, negative and positive single lenses to maintain good aberration correction.

The condition (1) defines the amount of movement of the first lens group (I) during zooming in order to achieve a zoom ratio of 3.5 to 4 in the above-mentioned configuration, based on its focal length. conditions
If the lower limit of (1) is exceeded, it will be difficult to obtain the desired zoom ratio, while if the upper limit of condition (1) is exceeded, the amount of movement of the first lens group (I) will be too large and compactness will be maintained. Can not.

The condition (2) defines the refractive power of the front group (IVa) of the fourth lens group. When the value goes below the lower limit of the condition (2), the refractive power of the front group (IVa) becomes strong and the lens system becomes compact, but the aberration balance is lost, and it becomes difficult to satisfactorily correct various aberrations. If the upper limit of the condition (2) is exceeded, the refracting power of the front lens group (IVa) will be weakened, and aberration correction will be advantageous, but the lens system itself will become large.

In addition to the above conditions (1) and (2), preferable conditions for aberration correction, compactness, and high zoom ratio are shown below.

(3) 0.15 <| fa / fb | <0.7 (4) 0.7 <f 1 / f 4 <1.4 (5) 0.15 <de / f 4 <0.4 where, where, fb the rear group of the fourth lens group ( The focal length of IVb), de is the axial air distance between the front group (IVa) and the rear group (IVb) of the fourth lens group.

In the present invention, since a high zoom ratio is realized with good aberration correction, it is necessary to maintain the balance of the refractive powers of the front group (IVa) and the rear group (IVb) of the fourth lens group. Condition (3) defines this balance. If the lower limit of condition (3) is exceeded, it is advantageous for compactness of the lens system, but it becomes difficult to correct various aberrations. On the other hand, when the value exceeds the upper limit of the condition (3), the lens system becomes large, which is contrary to the gist of the present invention.

The condition (5) defines the axial air distance between the front lens group (IVa) and the rear lens group (IVb) of the fourth lens group, and in combination with the condition (3), the high zoom ratio is compact. It is a necessary condition to realize. When the value exceeds the upper limit of the condition (5), not only the lens system becomes large, but also it becomes difficult to satisfactorily correct spherical aberration and coma. On the other hand, when the value goes below the lower limit of the condition (5), the lens system becomes compact, but it becomes difficult to satisfactorily correct spherical aberration and astigmatism.

Further, in order to achieve compactness in the present invention, the method of reducing the moving amount of the second lens group (II) is adopted as is conventionally done. For this purpose, the negative refracting power of the second lens group (II) is strengthened, but along with that, both the first, third, and fourth lens groups (I) (III) (IV) are strong. Refractive power is required. However, in order to obtain good aberration correction, the balance of the refracting power given to each group is important. The condition (4) defines this, and the first lens group (I) is defined by the ratio of the focal length to the fourth lens group (IV),
It defines the balance of the refractive power of each group. If the lower limit of condition (4) is exceeded, the power of the zooming system will become too strong, making it difficult to correct high-order spherical aberration, coma, and chromatic aberration over the entire zoom range. Although it is advantageous, the compactness of the entire lens system is lost, which is contrary to the gist of the present invention.

In the present invention, it is desirable that the following conditions are further satisfied in addition to the above-mentioned conditions. That is, when the front group (IVa) of the fourth lens group is composed of one single lens having a positive refractive power, and the refractive index of the lens with respect to the d-line is Na, (6) 1.49 <Na It is desirable to satisfy <1.8. Alternatively, when the front group (IVa) of the fourth lens group is composed of two single lenses each having a positive refractive power, and the refractive indices of the lenses with respect to d line are Na ₁ and Na ₂ , respectively, 7) It is desirable to satisfy.

Conditions (6) and (7) are that the front lens group (IVa) of the fourth lens group is 1
It defines the range of the refractive index of the glass used when it is composed of one single lens and when it is composed of two single lenses. That is, the Petzval sum for compacting the lens system becomes too small, which leads to deterioration of image plane property. In order to keep this favorable, it is necessary to configure the positive lens of the front lens group (IVa) of the fourth lens group by a glass type having a relatively low refractive index. By constructing within the ranges of the conditions (6) and (7), the spherical aberration and the field curvature can be corrected well, and beyond this, it becomes difficult to correct the aberrations in the entire zoom range.

Furthermore, when zooming, it is advantageous to move the fourth lens group (IV) as well, so that a high zoom ratio can be obtained. In particular, when the fourth lens group (IV) is moved integrally with the first lens group (I), the configuration of the lens barrel can be improved. Can be simplified.

As described above, in the present invention, the fourth lens group (IV), which is the main lens system, includes the positive single lens 1 to 2 front group positive, negative, positive 3 rear groups 4 to 5 By using a single lens, a compact zoom lens system having good aberration performance over the entire zoom range is realized.

Next, examples based on the present invention will be shown. ri (i = 1, 2,
3 ...) is the radius of curvature of the lens surface counted from the object side, di
Is the axial upper surface distance counted from the object side, Ni is the refractive index for the d-line of the lens counted from the object side, and νi is its Abbe number.

[Brief description of drawings]

FIGS. 1, 3, 5 and 7 show the first, second and third embodiments of the present invention, respectively.
3 and 4 are schematic views showing a cross section of the longest focal length state and a movement to the shortest focal length state, and FIGS. 2, 4, 6, and 8 are aberration diagrams of the above embodiment in order. I ... First lens group II ... Second lens group III ... Third lens group IV ... Fourth lens group IVa ... Front group of fourth lens group IVb ... Rear group of fourth lens group

Claims (6)

[Claims] 1. A first lens element having a positive refractive power in order from the object side.
The lens unit includes a second lens unit having a negative refracting power, a third lens unit having a positive refracting power, and a fourth lens unit having a positive refracting power. The first lens group, the second lens group, and the third lens group are moved along the optical axis during zooming, and the fourth lens group is a single lens having a positive refractive power. Alternatively, a front group consisting of two lenses having a positive refracting power as a whole, a single lens having a positive refracting power, a single lens having a negative refracting power, and a single lens having a positive refracting power are sequentially arranged from the object side. Telephoto zoom lens composed of a rear group consisting of three lenses and having a negative refracting power as a whole, and satisfying the following conditions; 0.05 <Δx / f ₁ <1.0 0.4 <fa / f ₄ < 1.3 However, here Δx; during zooming Of the first lens group f ₁ ; focal length fa of the first lens group fa; focal length f ₄ of the front group of the fourth lens group f ₄ ; focal length _{4 of} the fourth lens group. 2. A telephoto zoom lens according to claim 1, further satisfying the following condition: 0.15 <| fa / fb | <0.7 0.7 <f ₁ / f ₄ <1.4 0.15 < de / f ₄ <0.4 where fb is the focal length of the rear group of the fourth lens group de is the axial air distance between the front group and the rear group of the fourth lens group. 3. The telephoto zoom lens according to claim 2, wherein the front group of the fourth lens group is composed of one single lens having a positive refractive power, and the following condition is satisfied: 1.49 <Na <1.8 where Na is the refractive index of the single lens constituting the front group of the fourth lens group with respect to the d-line. 4. The front lens unit of the fourth lens unit is composed of two single lenses each having a positive refractive power, and satisfies the following condition. Telephoto zoom lens; 1.47 <(Na ₁ + Na ₂ ) / 2 <1.82 where Na ₁ and Na ₂ are the refractive indices of the two single lenses forming the front group of the fourth lens group with respect to the d-line. Is. 5. The telephoto zoom lens according to claim 1, wherein the fourth lens group is moved along the optical axis during zooming. 6. The fourth lens unit is provided with a first lens unit for zooming.
The telephoto zoom lens according to claim 5, wherein the telephoto zoom lens is movable integrally with the lens group on the optical axis.