Zoom Lens

1. Field of the Invention

The present invention relates to a zoom lens which is suitable for a 35 mm single-lens reflex camera and, more particularly, to a zoom lens suitable for an autofocus camera in which aberrations are satisfactorily corrected although the zoom lens has a focal length at the wide angle end of about 70 mm, a zoom ratio of about 3:1, and a relatively large aperture of an F number of about 2.8.

2. Description of the Related Art

In recent years, in an autofocus camera which is increasingly the subject of development, not only is the performance of accurate focusing important, but also the attainment of rapid focus speed. The focus speed is determined mainly by power supplied to a focus motor, the efficiency of the power focus motor, the load of a lens barrel, the weight and extension of the focus lens, the computation processing speed of the autofocus, and the like. Of these factors, the factors which influence focus speed the most are the weight of the focus lens and the extension thereof. It is therefore important to achieve an optical design in which the influence of the weight and extension factors is limited to be small.

A zoom lens, having a focal length at the wide angle end of about 70 mm, a zoom ratio of about 3:1, and an F number of about 2.8 (constant during zooming), is generally formed of a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power. Such a lens is well known and afocal zooming is performed by moving the second and third groups during zooming while the first and fourth groups are fixed and focusing is performed by the first lens group (front lens focusing) or a part of the first lens group (front lens inner focusing). An example of front lens focusing is proposed in, for example, Japanese Patent Laid-Open Nos. 62-108218, 64-39542, and 63-58324. Although the front lens has a relatively small aberration, the weight of the front lens is fairly heavy when it has a large aperture, for example, when the F number is F2.8. When the closest focusing distance is set at a relatively short distance, for example, about 1.5 m, the extension amount is increased relatively. Therefore, it cannot be said that the zoom lens is suitable for an autofocus camera. When the power of the front lens is increased in order to decrease the extension amount, the spherical aberration, in particular, at a near distance on the long focal point side, will become great and be undercorrected, which is problematic.

An example of front lens inner focusing is disclosed in, for example, Japanese Examined Patent No. 61-053696. Since focusing is performed by a lens group disposed relatively on the image side of the front lens, it is possible to decrease the lens diameter to some extent, and as a result, lessen the weight of the focus lens to some extent. There are advantages to this type of system even if the refracting power is increased to decrease the extension amount. For example, little aberration will occur due to the floating effect with the lens group of the front lens on the object side. Despite the advantages, such a system is nonetheless not suitable for an autofocus camera in terms of its weight.

Further examples in which the weight of the focus lens is reduced are disclosed in, for example, Japanese Examined Patent Nos. 1-25044 and 1-25043. These examples are designed to focus using a part of the fixed fourth lens group (relay); however, there are problems associated with this system, for example, aberrations, such as a spherical aberration or an astigmatism, will be large though the weight of the focus lens is fairly small.

It is an object of the present invention to provide a high-performance zoom lens which is suitable for an autofocus camera, in which the weight of the focus lens and the extension amount thereof are reduced while utilizing the advantages of the aberration characteristics of front lens inner focusing in order to eliminate the above-described problems of the prior art.

To achieve the above-described object, according to the present invention, there is provided a zoom lens comprising: at least a first lens group having a positive refracting power, a second lens group having a positive refracting power, and a third lens group having a negative refracting power in this order from the object side, which performs zooming by varying the distance between respective lens groups, wherein zooming from the wide angle end to the telephoto end is performed by moving the second and third lens groups toward the image side at different speeds and focusing is performed by the second lens group, and the condition described below is satisfied:

where f.sub.T is the focal distance at the telephoto end of the entire system, and m.sub.2 is the amount of zoom movement of the second lens group from the wide angle end to the telephoto end.

The above and further objects and novel features of the invention will more fully appear from the following detailed description when read in connection with the accompanying drawings.

FIGS. 1(a) and 1(b) show the paraxial refracting power arrangement of a zoom lens in accordance with the present invention;

FIG. 2 is a sectional view of lenses in accordance with a first numerical embodiment of the zoom lens in accordance with the present invention;

FIG. 3 is a sectional view of lenses in accordance with a second numerical embodiment of the zoom lens in accordance with the present invention;

FIG. 4 is a sectional view of lenses in accordance with a third numerical embodiment of the zoom lens in accordance with the present invention;

FIG. 5 is a sectional view of lenses in accordance with a fourth numerical embodiment of the zoom lens in accordance with the present invention;

FIG. 6 is a sectional view of lenses in accordance with a fifth numerical embodiment of the zoom lens in accordance with the present invention;

FIG. 7 is a sectional view of lenses in accordance with a sixth numerical embodiment of the zoom lens in accordance with the present invention;

FIGS. 8(A) and 8(B) show various aberrations at the wide angle end in accordance with the first numerical embodiment of the zoom lens of the present invention;

FIGS. 9(A) and 9(B) show various aberrations at the intermediate region in accordance with the first numerical embodiment of the zoom lens of the present invention;

FIGS. 10(A) and 10(B) show various aberrations at the telephoto end in accordance with the first numerical embodiment of the zoom lens of the present invention;

FIGS. 11(A) and 11(B) show various aberrations at the wide angle end in accordance with the second numerical embodiment of the zoom lens of the present invention;

FIGS. 12(A) and 12(B) show various aberrations at the intermediate region in accordance with the second numerical embodiment of the zoom lens of the present invention;

FIGS. 13(A) and 13(B) show various aberrations at the telephoto end in accordance with the second numerical embodiment of the zoom lens of the present invention;

FIGS. 14(A) and 14(B) show various aberrations at the wide angle end in accordance with the third numerical embodiment of the zoom lens of the present invention;

FIGS. 15(A) and 15(B) show various aberrations at the intermediate region in accordance with the third numerical embodiment of the zoom lens of the present invention;

FIGS. 16(A) and 16(B) show various aberrations at the telephoto end in accordance with the third numerical embodiment of the zoom lens of the present invention;

FIGS. 17(A) and 17(B) show various aberrations at the wide angle end in accordance with the fourth numerical embodiment of the zoom lens of the present invention;

FIGS. 18(A) and 18(B) show various aberrations at the intermediate region in accordance with the fourth numerical embodiment of the zoom lens of the present invention;

FIGS. 19(A) and 19(B) show various aberrations at the telephoto end in accordance with the fourth numerical embodiment of the zoom lens of the present invention;

FIGS. 20(A) and 20(B) show various aberrations at the wide angle end in accordance with the fifth numerical embodiment of the zoom lens of the present invention;

FIGS. 21(A) and 21(B) show various aberrations at the intermediate region in accordance with the fifth numerical embodiment of the zoom lens of the present invention;

FIGS. 22(A) and 22(B) show various aberrations at the telephoto end in accordance with the fifth numerical embodiment of the zoom lens of the present invention;

FIGS. 23(A) and 23(B) show various aberrations at the wide angle end in accordance with the sixth numerical embodiment of the zoom lens of the present invention;

FIGS. 24(A) and 24(B) show various aberrations at the intermediate region in accordance with the sixth numerical embodiment of the zoom lens of the present invention; and

FIGS. 25(A) and 25(B) show various aberrations at the telephoto end in accordance with the sixth numerical embodiment of the zoom lens of the present invention.

Preferred embodiments of the present invention will be described below.

FIGS. 1(a) and 1(b) show the paraxial refracting power arrangement of a zoom lens in accordance with the present invention. In particular, FIG. 1(a) shows the state in which the axial light beam is passed at the short focal point end; FIG. 1(b) shows the state in which the paraxial light beam is passed at the long focal point end. Reference characters I to V denote the first to fifth lens groups, respectively, from the object side to the image side. Zooming is increased by moving the third lens group having a negative refracting power toward the image side, while the first lens group having a positive refracting power and the fifth lens group having a positive refracting power are fixed. The variation of the image point caused by zooming is corrected primarily by moving the fourth lens group having a positive refracting power. During this operation, the second lens group is moved in such a manner as to move away from the first lens group, i.e., toward the image plane, and to the long focal length side of the range in which the conditional equation (1) described above is satisfied.

When a change of the paraxial light beam to the first lens group is seen during zooming, the change is low on the short focal-point side and conversely high on the long focal-point side, and the paraxial light beam converges as a result of the positive refracting power of the first lens group. Thus, the second lens group is moved as described above in order to decrease the outer diameter of the lenses of the second lens group determined by the incidence height of the paraxial light beam on the long focal point side.

FIGS. 2 to 7 are sectional views of lenses shown in numerical embodiments which will be described later. Reference characters I to V denote first to fifth lens groups, respectively.

Conditional equation (1) is concerned with the amount of movement of zooming at the long focal point end with the short focal point side end of the second lens group being a reference, and defines a condition for reducing the weight of the lenses while satisfactorily suppressing the aberration. If the amount of movement is increased beyond the upper limit of conditional equation (1), the lens weight is reduced because the outer diameter of the lenses is decreased; however, a reduction effect is increased during zooming because of the positive refracting power of the second lens group. Thus, both the negative refracting power of the third lens group and the positive refracting power of the fourth lens group for the purpose of obtaining a desired zoom ratio are increased, and therefore, even if the number of lenses of the third and fourth lens groups is increased, variations due to zooming, such as spherical aberration or coma aberration, cannot be removed. In addition, at the long focal-point side, the distortion aberration will greatly vary, and pin-cushion distortion will become conspicuous, which is undesirable.

If, conversely, the amount of movement is decreased beyond the lower limit of conditional equation (1), the refracting powers of the third and fourth lens groups are decreased. This is advantageous as regards aberration correction; however, as the outer diameter of the lenses is increased, reducing the weight of the lenses becomes difficult.

Although the object of the present invention can be sufficiently achieved without changing the numerical range of conditional equation (1), more preferably, the range should be the following numerical range:

Still more preferably, if the focal lengths of the first and second lens groups are denoted as f.sub.1 and f.sub.2, respectively, and the following condition is satisfied:

it is easy to give optical performance for practical use.

Conditional equation (2) is concerned with the ratio of the refracting power of the first lens group to that of the second lens group. Conditional equation (2) satisfies conditional equation (1), and also regulates the amount of focus extension of the second lens group while correcting aberration satisfactorily.

If the refracting power of the first lens group is increased and if the refracting power of the second lens group is decreased beyond the lower limit of conditional equation (2), various aberrations which occur in the first lens group increase, thus it is difficult to cancel the aberrations by the second and subsequent lens groups. Further, not only is the amount of extension of the second lens group during focusing increased, but also the lens diameter is increased as the amount of extension is increased, and the weight of the lenses is increased, which are undesirable.

If, conversely, mainly the refracting power of the second lens group is increased beyond the upper limit of conditional equation (2), the amount of extension of the lenses is decreased; however, the spherical aberration and the chromatic aberration (both of the axial color and the chromatic aberration of magnification) will vary greatly, in particular, during focusing, and it is difficult to correct these aberrations. Further, the refracting powers of the third and fourth lens groups for obtaining a desired zoom ratio are increased due to the influence of the positive refracting power of the second lens group, and it is difficult to correct the aberration variations caused by zooming, as described in conditional equation (1) described above, which is undesirable.

Although the object of the present invention can be achieved sufficiently without changing the numerical range of conditional equation (2), more preferably, the numerical range should be the following range:

Still more preferably, the following condition should be satisfied:

It is more preferable that the value of the ratio of f.sub.2 to f.sub.T be in the numerical range below as a condition for optimizing the aberration correction and the amount of extension:

Although it is possible to sufficiently balance the aberration correction and the amount of extension in the numerical range of equation (3), more preferably, the range should be the following range:

The zoom arrangement is desirably formed in such a way that a fixed fifth lens group having a positive refracting power is provided on the image plane side of the fourth lens group, and an afocal zooming system is formed by the first to fourth lens groups, as in the embodiment of the present invention.

The first lens group is formed of a meniscus negative lens having a greatly concaved surface on the image side, a double convex lens and a meniscus positive lens having a greatly convexed surface on the object side in this order from the object side. The second lens group is preferably formed of a meniscus negative lens having a concave surface on the image side and a positive lens whose curvature of the lens surface on the object side is greater than that of the lens surface on the image side. The third lens group is formed of a negative lens having a greatly concaved surface on the image side and a cemented lens comprising a negative and a positive lens, and a meniscus negative lens having a concave surface on the object side. The fourth lens group is formed of at least one positive lens and a meniscus negative lens having a concave surface on the object side. The fifth lens group is formed of a positive lens, a positive lens having a greatly convexed surface on the object side, a negative lens having a greatly concave surface on the image side, a positive lens, a meniscus negative lens having a concave surface on the object side, and a positive lens. Thus, excellent optical performance is maintained.

Use of a glass member with an anomalous dispersion ratio having a refractive index of about 1.5 and an abbe number of about 80 in at least one of the two positive lenses of the first lens group makes it possible to satisfactorily correct chromatic aberration. A glass member having a relatively small specific gravity and a refractive index of 1.65 or less may preferably be used as the positive lens of the second lens group, and a glass member having an abbe number of 40 or more may preferably be used by taking the correction of chromatic aberration into consideration.

Following conditional equation (3) described earlier, it is more preferable that any of conditional equations (4), (5) and (6) described below be satisfied. If the focal length of the third lens group is denoted as f.sub.3, and if the value of the ratio of f.sub.3 to f.sub.T is set within the range:

then, various conditions, such as a zoom ratio, a total length of the lenses, or a back focus, are satisfied, and it is easy to correct aberrations satisfactorily.

If the focal length of the fourth lens group is denoted as f.sub.4, and if the value of the ratio of f.sub.4 to f.sub.T is set within the following range:

then, various conditions, such as a zoom ratio, a total length of the lenses, or a back focus, are satisfied and it is easy to correct aberrations satisfactorily.

If the focal length of the fifth lens group is denoted as f.sub.5, and if the value of the ratio of f.sub.5 to f.sub.T is set within the following range:

then, various conditions, such as a zoom ratio, a total length of the lenses, or a back focus, are satisfied, and it is easy to correct aberrations satisfactorily.

Next, numerical embodiments of the present invention will be described. In the numerical embodiments, reference character Ri denotes the curvature radius of the i-th lens surface sequentially from the first conjugate point; reference character Di denotes the thickness of the i-th lens and an air gap from the first conjugate point; reference characters Ni and vi denote a refractive index of glass of the i-th lens from the conjugate point and its abbe number, respectively, sequentially from the first conjugate point.

The relation between each of the above-described conditional equations and the numerical values in the numerical embodiments is shown in Table 1.

In the aberration figures, (A) shows a figure with respect to an object at infinity, and (B) shows a figure when the zoom lens is focused on an object at a distance of 1.5 m.

As has been described up to this point, by defining the movement of the second lens group so as to satisfy conditional equation (1) described earlier, the diameter of the second lens group can be decreased and focusing can be made speedier. In addition, by defining the focal lengths of the first and second lens groups so as to satisfy conditional equation (2), it is possible to provide an excellent zoom lens free from aberration.

Many different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the claims. The following claims are to be accorded the broadest interpretation so as to encompass all such modifications, equivalent structures and functions.