JP2901144B2 - Zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens, and more particularly to a high-magnification and large-aperture ratio rear focus suitable for a still camera, a video camera, a broadcast TV camera and the like. The present invention relates to a zoom lens employing a system. 2. Description of the Related Art There is a so-called four-unit zoom lens as a zoom lens having a relatively high zoom ratio and a large aperture ratio, which has been conventionally used for a still camera, a video camera and the like. This 4
The group zoom lens includes, in order from the object side, a first lens group for focusing, a second lens group for zooming, a third lens group for correcting an image plane variation due to zooming, and a focal length of the entire system. It comprises a fourth lens unit for balancing aberration correction.
The four-unit zoom lens employs a configuration in which a total of three lens units including two lens units for zooming and one lens unit for focusing are moved. For this reason, the lens barrel tends to be relatively complicated. Also, when focusing on a short-distance object, the diameter of the front lens tends to increase if an attempt is made to secure a sufficient off-axis light beam because the first lens group is extended toward the object side. For this reason, conventionally, various zoom lenses using a so-called rear focus system in which a lens group other than the first lens group is moved to perform focusing have been proposed. For example, in US Pat. No. 4,364,642,
Focusing is performed by moving the third lens group in the four-unit zoom lens. In U.S. Pat. No. 4,460,251, focusing is performed by moving the second lens unit and the third lens unit together in a four-unit zoom lens. Further, Japanese Patent Application Laid-Open No. 58-136012 proposes a zoom lens in which a zooming unit is constituted by three or more lens groups, and a part of the lens groups is moved to perform focusing. However, in these zoom lenses, the diameter of the front lens becomes small, but the movement locus of the focusing lens group changes greatly between an object at infinity and an object at a close distance, so extra space is secured in the lens system in advance. And the overall length of the lens tends to increase. In Japanese Utility Model Laid-Open No. 59-63314, the fourth lens group of the four-group zoom lens is divided into two lens groups, and one of the lens groups is moved to perform focusing. However, this zoom lens has a total of 3
One lens group must be moved, and the lens barrel tends to be complicated. SUMMARY OF THE INVENTION An object of the present invention is to provide a zoom lens using a rear focus system having a high zoom ratio and a simple configuration that achieves miniaturization of the entire lens system having a large aperture ratio. I do. A zoom lens according to the present invention comprises, in order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power. A lens group, and a fourth lens group having a positive refractive power,
An aperture is provided in or near the third lens group, the first and third lens groups are fixed, and the second lens group is moved in one direction to perform zooming, and the fourth lens is The first lens is moved in a non-linear manner so as to correct the image plane variation accompanying zooming, and the fourth lens group is moved to the object side to focus from an object at infinity to a close object. The group has at least two positive lenses and one negative lens, and the second lens group has at least one positive lens and two negative lenses.
The third lens group has at least one negative lens.
The fourth lens group includes at least one positive lens and at least one negative lens. Other features of the present invention are described in the embodiments. FIG. 19 is a schematic diagram showing the basic structure of an optical system according to an embodiment of the present invention. In the figure, I is the first positive refractive power.
A lens group, II is a second lens group having a negative refractive power, III is a third lens group having a positive refractive power, and IV is a fourth lens group having a positive refractive power. Zooming is performed by moving the second and fourth lens groups in the directions of the arrows. Of these, the second lens group mainly performs zooming, and the fourth lens group corrects image plane fluctuation due to zooming. At this time, the fourth lens group is moved non-linearly so as to have a locus convex toward the object side as shown in FIG. Further, the fourth lens group is moved to the object side to perform focusing from an object at infinity to a close object. In addition, the first,
The third lens group is fixed during zooming and focusing. In this embodiment, the second lens unit having a negative refractive power disposed between the first lens unit having a positive refractive power and the third lens unit corresponding to the relay lens unit is moved in one direction. As a result, zooming is efficiently performed together with the image formation of the first and third lens groups. At the time of zooming, the fourth lens group is non-linearly moved toward the third lens group as shown in the figure to correct the image plane fluctuation, and the space between the third lens group and the fourth lens group. To reduce the overall length of the lens. In FIG. 19, a curve L2 represents a moving trajectory of the fourth lens group when the object is at infinity, and a curve L3 represents a moving trajectory accompanying zooming of the fourth lens group when the object is a close object. In the present embodiment, the first lens unit is always fixed without being extended at the time of focusing, and an increase in the lens diameter caused by extending forward is prevented. At the time of focusing, the number of movable lens groups is reduced by moving the fourth lens group, which is a part of the variable power system, instead of the first lens group, thereby simplifying the mechanism and improving the overall lens system. Is being downsized. That is, while the fourth lens group has both functions of zooming and focusing, and is moved in the space between the third lens group and the fourth lens group, effective use in the space in the lens system is achieved. To shorten the overall length of the lens. The first and third lens groups adjust the paraxial amounts such as the focal length, the angle of view, and the back focus of the entire system, and satisfactorily correct the aberration fluctuation caused by the movable lens group. In this embodiment, the diaphragm is disposed in or near the third lens group to reduce the variation in aberration due to the movable lens group and to balance the lens diameters of the first lens group and the fourth lens group. Good for good maintenance. In this embodiment, the first lens group is composed of at least two positive lenses and one negative lens, the second lens group is composed of at least one positive lens and two negative lenses, and the third lens group is composed of at least one positive lens and two negative lenses. At least one positive lens and one negative lens, and the fourth lens group is configured to have at least one positive lens and one negative lens, thereby satisfactorily correcting chromatic aberration in each lens group. The first lens group favorably corrects spherical aberration on the telephoto side, and the second lens group favorably corrects astigmatism and distortion on the wide-angle side. Further, in the present invention, in order to reduce aberration fluctuations caused by zooming and to reduce the size of the entire lens system, the focal length of the i-th lens unit is fi and the focal length at the wide-angle end of the entire system is fw. 0.7 <| f2 / fw | <2.1 (1) 1.2 <f4 / fw <4.0 (2) 1.2 <| f4 / f2 | <3.6 (3) 0.72 <f1 / f3 <2.9 (4) 1.8 <f3 / fw <4.2 ... (5) It is preferable to configure so as to satisfy the following condition. Next, the technical meaning of each of the above conditional expressions will be described. Conditional expression (1) relates to the refractive power of the second lens group. If the refractive power of the second lens group is increased beyond the lower limit, it is desirable to reduce the size of the lens system, but the Petzval sum is negative. And the curvature of field increases. When the refractive power of the second lens group becomes weaker than the upper limit, the fluctuation of aberration due to zooming is reduced, but the amount of movement for obtaining a predetermined zooming ratio must be increased, and the overall length of the lens becomes longer. It is becoming. Conditional expression (2) relates to the refractive power of the fourth lens unit. If the refractive power of the fourth lens unit exceeds the lower limit and the refractive power of the fourth lens unit becomes stronger, the axial spherical aberration on the wide-angle side will be insufficiently corrected, and the entire zoom ratio will be reduced. Extro-coma aberration occurs frequently over the region. If the refractive power of the fourth lens group is weakened beyond the upper limit, the amount of movement accompanying zooming is increased, the overall length of the lens is increased, and the fluctuation of aberrations during focusing increases. Conditional expression (3) relates to the refractive power ratio between the second lens unit and the fourth lens unit. If the refractive power of the fourth lens unit exceeds the lower limit and the refractive power of the fourth lens unit becomes too strong, the aberration variation during zooming will be reduced. If it is difficult to satisfactorily correct it, and if the refractive power of the fourth lens group exceeds the upper limit and the refractive power of the fourth lens group becomes too weak, the amount of movement increases, and the back focus becomes longer than necessary. This is not preferable because the total optical length up to the surface becomes too long. Conditional expression (4) relates to the refractive power ratio between the first lens unit and the third lens unit. If the refractive power of the first lens unit exceeds the lower limit and becomes too strong, the axial aberration on the telephoto side is corrected. If the refractive power of the first lens unit becomes too weak beyond the upper limit, the distance between the first lens unit and the third lens unit becomes too large than the movement amount of the second lens unit, and it becomes unnecessary. It is not preferable because a complicated space is generated. Conditional expression (5) relates to the refractive power of the third lens unit. If the refractive power of the third lens unit exceeds the lower limit and the refractive power of the third lens unit increases, spherical aberration on the wide-angle side tends to be insufficiently corrected. Must be reduced, and the amount of movement of the fourth lens group increases accordingly. On the other hand, if the refractive power of the third lens group is too weak beyond the upper limit, the spherical aberration on the wide-angle side tends to be overcorrected, which is not preferable. Next, numerical examples of the present invention will be described. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap from the object side, and Ni and νi are the i-th lens in order from the object side. Are the refractive index and Abbe number of the glass. Table 1 shows the focal length of each lens group in each numerical example. [Table 1] [Table 2] [Table 3] [Table 4] [Table 5] [Table 6] [Table 7] [Table 8] [Table 9] According to the present invention, aberration correction is favorably achieved while miniaturizing the entire lens system having a high zoom ratio of about 6 and an F number of about 1.2 and a large aperture ratio. A zoom lens using the rear focus method can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a lens according to Numerical Embodiment 1 of the present invention. FIG. 2 is a diagram illustrating various aberrations of Numerical Embodiment 1 of the present invention. FIG. 3 is a lens of Numerical Embodiment 2 of the present invention. FIG. 4 is a sectional view of various aberrations of Numerical Embodiment 2 of the present invention. FIG. 5 is a lens sectional view of Numerical Embodiment 3 of the present invention. FIG. 6 is a diagram of various aberrations of Numerical Embodiment 3 of the present invention. FIG. 8 is a sectional view of a lens according to a numerical example 4 of the present invention. FIG. 8 is a diagram showing various aberrations of a numerical example 4 of the present invention. FIG. 9 is a sectional view of a lens according to a numerical example 5 of the present invention. Various aberration diagrams of Example 5 [FIG. 11] Lens sectional view of Numerical Example 6 of the present invention [FIG. 12] Various aberration diagrams of Numerical Example 6 of the present invention [FIG. 13] Lens of Numerical Example 7 of the present invention FIG. 14 is a sectional view of various aberrations of Numerical Embodiment 7 of the present invention. FIG. 15 is a lens cross-sectional view of Numerical Embodiment 8 of the present invention. FIG. 17 is a sectional view of a lens according to a numerical example 9 of the present invention. FIG. 18 is an aberration diagram of a numerical example 9 according to the present invention. FIG. 19 is an optical system according to an example of the present invention. [Description of symbols] d d-line g g-line M meridional image plane S sagittal image plane I first lens group II second lens group III third lens group IV fourth lens group

Claims (1)

(57) [Claims] Four lens groups of 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 are arranged in order from the object side. Have
An aperture is provided in or near the third lens group, the first and third lens groups are fixed, and the second lens group is moved in one direction to perform zooming, and the fourth lens is The first lens is moved in a non-linear manner so as to correct the image plane variation accompanying zooming, and the fourth lens group is moved to the object side to focus from an object at infinity to a close object. The group has at least two positive lenses and one negative lens, and the second lens group has at least one positive lens and two negative lenses.
The third lens group has at least one negative lens.
A zoom lens having a positive lens and a negative lens for each lens, and wherein the fourth lens group includes at least one positive lens and a negative lens.