JP2738099B2 - Rear focus zoom lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a rear focus type zoom lens,
In particular, it relates to a rear focus type zoom lens having a large aperture ratio of about 8 and an F number of about 1.8 used in a photographic camera, a video camera, a broadcast camera, and the like, and a total of four lens groups having a high zoom ratio as a whole. It is.

2. Description of the Related Art Conventionally, various types of zoom lenses such as a photographic camera and a video camera adopting a so-called rear focus type in which a lens group other than the first group on the object side is moved to perform focusing is proposed. ing.

Generally, a rear focus type zoom lens has a first lens position compared to a zoom lens which moves the first lens unit to perform focusing.
Since the effective diameter of the lens group is small, it is easy to reduce the size of the entire lens system, and it is also easy to perform close-up photography, especially very close-up photography. Has a small driving force so that quick focusing can be performed.

As such a rear focus type zoom lens, for example, in JP-A-62-24213, 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 are arranged in order from the object side. In a zoom lens composed of four lens units of a fourth group having a positive refractive power, the first unit and the third unit are fixed, and the second unit is moved in one direction to perform zooming. The image plane fluctuation is performed by moving the fourth lens unit. And the fourth
The group is moved to focus.

In Japanese Patent Application Laid-Open No. 58-160913, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power are arranged in order from the object side. The zoom lens system has four lens units, and the first and second units are moved to perform zooming, and the image plane variation accompanying zooming is performed by moving the fourth unit. Then, focusing is performed by moving one or more of these lens groups.

(Problems to be Solved by the Invention) Generally, if a rear focus method is adopted in a zoom lens, as described above, the whole lens system is reduced in size, quick focusing becomes possible, and further close-up photographing becomes easier. can get.

However, on the other hand, aberration fluctuations during focusing become large, and it becomes extremely difficult to obtain high optical performance while miniaturizing the entire lens system over the entire object distance from an object at infinity to a close object. Will occur.

For example, in the above-mentioned Japanese Patent Application Laid-Open No. 62-24213, when an attempt is made to increase the angle of view at the wide-angle end, the incident position of the off-axis light beam to the first lens unit becomes high in an intermediate zoom region. The effective diameter of the first lens group increases.

In general, as a method of reducing the effective lens diameter of the first lens unit, there is a method of shortening the distance between the first lens unit and the stop during zooming. At this time, when the stop is moved in accordance with zooming, if the amount of movement of the stop becomes large, the position of incidence of the light beam on the rear lens group, for example, the third group, greatly differs for each zoom region, and the axial position becomes large. There is a problem that the aberration of the external light beam increases, and it becomes very difficult to perform good aberration correction as a whole.

In particular, a zoom lens having a large aperture ratio and a high zoom ratio has a problem that it becomes very difficult to obtain high optical performance over the entire zoom range and over the entire object distance.

The present invention employs a rear focus system, and when aiming for a large aperture ratio and a high zoom ratio, prevents the entire lens system from being enlarged, and extends over the entire zoom range from the wide-angle end to the telephoto end. An object of the present invention is to provide a rear focus type zoom lens having excellent optical performance over the entire object distance from a distant object to a close object.

(Means for Solving the Problems) The rear focus type zoom lens according to the present invention includes: (a) a first group having a positive refractive power, a second group having a negative refractive power, an aperture, and a positive lens in order from the object side. The zoom lens has four lens units, a third unit having a refractive power and a fourth unit having a positive refractive power, and moves the second unit toward the image plane when zooming from the wide-angle end to the telephoto end. In addition, the diaphragm, the third lens unit and the fourth lens unit are moved independently of each other so as to have a locus convex toward the object side. When the distance between the rear principal point of the group and the front principal point of the third group is e23, and the distance between the stop and the front principal point of the third group is LS3, 0.15 <LS3 / It is characterized by satisfying the condition of e23 <0.7.

(B) Four lens groups of a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power, in this order from the object side. When zooming from the wide-angle end to the telephoto end, the second unit is moved to the image plane side, and the aperture, the third unit and the fourth unit are both moved along the locus convex toward the object side. The fourth group is moved when focusing, and the distance between the rear principal point of the third group and the front principal point of the fourth group is e34. When the distance between the rear principal point of the third group and the front principal point of the fourth group at the wide-angle end is ew34, the following condition is satisfied: 0.5 <e34 / ew34 <1.5 over the entire zoom range. Features.

(Embodiment) FIG. 1 is a schematic view of an embodiment showing a paraxial refractive power arrangement of a rear focus type zoom lens according to the present invention. Second
The figure is a lens sectional view of a numerical example of the present invention described later.

In the figure, I is a first group having a positive refractive power, II is a second group having a negative refractive power, III is a third group having a positive refractive power, and IV is a fourth group having a positive refractive power. SP denotes an aperture stop, a second group II and a third group II
Located between I.

When zooming from the wide-angle end to the telephoto end, the second
Move the group monotonously to the image plane side,
The group and the fourth group are moved independently of each other so as to have a locus convex toward the object side.

In this embodiment, by adopting such a zoom type, the maximum effective diameter of the first lens unit is reduced in a numerical embodiment described later.
At 3.5, this is a value that is much closer than the minimum effective diameter 2.98 (7.6 / 2.55) determined by the F-number at the telephoto end. As described above, in the present embodiment, while reducing the effective diameter of the first lens unit, it is easy to increase the angle of view to 56 ° at the wide-angle end, and to achieve good optical performance over the entire zoom range. It has gained.

In addition, a rear focus type in which the fourth unit is moved on the optical axis to perform focusing is adopted. The solid line curve 4a and the dotted line curve 4b of the fourth lens group shown in the same figure show the image plane fluctuation caused by zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at a short distance, respectively. The movement locus for correction is shown. The first unit is fixed during zooming and focusing.

In the present embodiment, zooming is performed by moving the fourth lens unit, and focusing is performed by moving the fourth lens unit. In particular, as shown by curves 4a and 4b in the same figure, the zoom lens is moved so as to have a convex trajectory toward the object side when zooming from the wide-angle end to the telephoto end. Thereby, the space between the third and fourth units is effectively used, and the overall length of the lens is effectively reduced.

In this embodiment, when focusing from an object at infinity to an object at a short distance at the telephoto end, a straight line 4c in FIG.
This is performed by turning the fourth group forward as shown in FIG.

Then, the aperture stop SP is arranged between the second and third units, and at the time of zooming, it is moved as described above to reduce aberration fluctuations due to zooming, and the distance between the lens groups ahead of the aperture stop is reduced. By shortening, the effective diameter of the first lens group can be easily reduced.

In the present invention, in order to reduce the overall size of the lens system and to correct various aberrations in a well-balanced manner over the entire zoom range while securing a predetermined zoom ratio, the rear principal point of the second group and the front side of the third group When the distance between the principal point is e23 and the distance between the stop and the front principal point of the third lens unit is LS3, the following condition is satisfied: 0.15 <LS3 / e23 <0.7 over the entire zoom range. Good.

If the lower limit of conditional expression (1) is exceeded, the distance between the stop and the first lens unit will not be long, and the lens effective diameter of the first lens unit will increase, which is not good. If the upper limit value is exceeded, the distance between the stop and the third lens unit becomes too long, and as a result, the incident height of the off-axis light beam to the third and fourth lens units becomes too high. It becomes difficult.

In the present invention, in order to favorably correct off-axis aberrations particularly over the entire zoom range, the distance between the rear principal point of the third group and the front principal point of the fourth group is e34 at the wide-angle end. The third
When the distance between the rear principal point of the group and the front principal point of the fourth group is ew34, it is preferable to satisfy the following condition: 0.5 <e34 / ew34 <1.5 over the entire zoom range.

If the lower limit or the upper limit of conditional expression (2) is exceeded, in any case, the height of the incident position of the off-axis light beam to the fourth group changes greatly with zooming, and the aberration due to the off-axis light beam accompanying zooming. The fluctuations become large, and it becomes difficult to satisfactorily correct these off-axis aberrations.

In addition, in order to correct various aberrations in a well-balanced manner particularly over the entire screen in the present invention, the third lens unit in order from the object side has a positive lens having both lens surfaces convex, a negative meniscus lens having a convex surface facing the image surface, The fourth group is composed of a meniscus-shaped positive lens with a convex surface facing the object side, and the fourth unit is a meniscus-shaped negative lens with a convex surface facing the object side, both lens surfaces are convex positive lenses, and both lens surfaces are convex surfaces. It is better to use a positive lens.

Next, numerical examples of the present invention will be described. In numerical examples
Ri is the radius of curvature of the i-th lens surface in order from the object side,
Is the i-th lens thickness and air gap from the object side, Ni and ν
i is the refractive index and Abbe number of the glass of the i-th lens in order from the object side.

R24 and R25 are parallel flat plates such as a face plate and a filter. Table 1 shows the main point interval between the i-th group and the (i + 1) -th group as ei (i + 1).

(Effects of the Invention) According to the present invention, as described above, a lens configuration is adopted in which the moving conditions of each lens group and the aperture for setting the refractive power and the magnification of the four lens groups and moving the fourth group during focusing are set. This achieves good aberration correction over the entire zoom range at a zoom ratio of about 8 while reducing the size of the entire lens system, and has a high optical performance with high optical performance with little aberration fluctuation during focusing. A rear-focus zoom lens with a wide angle of view of 56 ° and an F-number of 1.8 and a large aperture ratio can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic view of an embodiment showing a paraxial refractive power arrangement of the present invention, FIG. 2 is a lens sectional view of a numerical embodiment of the present invention, and FIG.
The figure is a diagram of various aberrations of a numerical example of the present invention. In the aberration diagrams, (A) is the wide-angle end, (B) is the middle, (C)
FIG. 9 is an aberration diagram at a zoom position at a telephoto end. In FIGS. 1 and 2, I, II, III, and IV are the first, second, third, and fourth groups, d is the d-line, g is the g-line, ΔM is the meridional image plane, and ΔS is the sagittal image plane. And SP are aperture stops.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-208724 (JP, A) JP-A-64-68709 (JP, A) JP-A-58-150926 (JP, A)

Claims (3)

(57) [Claims] 1. A first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a stop, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power are arranged in order from the object side. In the case of zooming from the wide-angle end to the telephoto end, the second unit is moved to the image plane side, and the aperture, the third unit and the fourth unit are all convex toward the object side. Move independently of each other so as to have a trajectory,
At the time of focusing, the fourth lens group is moved, and the distance between the rear principal point of the second lens group and the front principal point of the third lens group is e23. A rear-focus type zoom lens that satisfies the condition of 0.15 <LS3 / e23 <0.7 over the entire zoom range when the distance to the point is LS3. 2. The distance between the rear principal point of the third group and the front principal point of the fourth group is e34, and the rear principal point of the third group and the front principal point of the fourth group at the wide-angle end. 2. The rear focus type zoom lens according to claim 1, wherein, when the distance between the point and the point is ew34, the zoom lens satisfies the following condition: 0.5 <e34 / ew34 <1.5. 3. A first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a stop, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power are arranged in order from the object side. In the case of zooming from the wide-angle end to the telephoto end, the second unit is moved to the image plane side, and the aperture, the third unit and the fourth unit are all convex toward the object side. Move independently of each other so as to have a trajectory,
At the time of focusing, the fourth lens group is moved, and the distance between the rear principal point of the third lens group and the front principal point of the fourth lens group is e34, after the third lens group at the wide-angle end. A rear focus type zoom lens which satisfies the condition of 0.5 <e34 / ew34 <1.5 over the entire zoom range when the distance between the side principal point and the front principal point of the fourth group is ew34. .