JP3039044B2 - Rear focus zoom lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear focus type zoom lens, and more particularly, to a photographic camera, a video camera,
The present invention also relates to a rear focus type zoom lens suitable for a small zoom lens having a large zoom ratio of about 8 and an F-number of about 1.8 used for a broadcast camera or the like and a high zoom ratio.

[0002]

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 system in which a lens group other than the first group on the object side is moved to perform focusing is proposed. ing.

In general, a rear focus type zoom lens has a smaller effective diameter of the first lens group than a zoom lens which moves and focuses the first lens group, so that the entire lens system can be easily miniaturized, and close-up photographing can be performed. In particular, extremely close-up photographing is facilitated, and since the relatively small and light lens group is moved, the driving force of the lens group is small and quick focusing can be performed.

In such rear focus type zoom lenses, for example, Japanese Patent Application Laid-Open Nos. Sho 62-24213, Sho 63-29718, and Hei 2-48621 disclose a lens having a positive refractive power in order from the object side. The zoom lens includes four lens groups, a first group, a second group having a negative refractive power for zooming, a third group having a positive refractive power, and a fourth group having a positive refractive power. The third lens group is fixed, the second lens group is moved to perform zooming, the fourth lens group is moved to correct the image plane variation accompanying zooming, and the fourth lens group is moved to perform focusing. ing.

[0005]

Generally, when a rear focus system is employed in a zoom lens, the overall lens system can be reduced in size and quick focusing becomes possible as described above, and further advantages such as close-up photographing can be easily obtained. .

On the other hand, however, aberration fluctuations during focusing increase, and it becomes very difficult to obtain high optical performance while reducing the size of the entire lens system over the entire object distance from an object at infinity to an object at a short distance. The problem arises.

Particularly, in a zoom lens having a large aperture ratio and a high zoom ratio, it is very difficult to obtain high optical performance over the entire zoom range and the entire object distance while shortening the overall length of the lens. Come up.

When the present invention employs a rear focus method to achieve a large aperture ratio and a high zoom ratio, the entire lens system is reduced in size from the wide-angle end to the telephoto end, while the size of the entire lens system is reduced. It is another object of the present invention to provide a rear focus zoom lens having excellent optical performance over the entire object distance from an object at infinity to an object at a short distance.

[0009]

A rear focus type 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. And a fourth lens unit of a fourth group having a positive refractive power.
The lens unit is moved to the image plane side to perform zooming from the wide-angle end to the telephoto end, and the image plane fluctuation caused by zooming is corrected by moving the fourth unit, and the fourth unit is moved to focus. The first group is composed of a cemented lens in which one negative lens and one positive lens are joined, the third group is composed of one positive lens, and the fourth group is composed of one negative lens. Two of lens and one positive lens
The focal length of the first group is f1, the focal length of the entire system at the telephoto end is fT, the open F number of the entire system is FNO, and the focal lengths of the third and fourth groups are f3 and f3, respectively.
f4, when the focal length of the entire system at the wide-angle end is fw

[0010]

(Equation 3)

(Equation 4) It is characterized by satisfying certain conditions.

[0011]

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.

2 to 4 are lens sectional views of Numerical Examples 1 to 3 of the present invention described later, and FIGS. 5 to 13 are aberration diagrams of Numerical Examples 1 to 3 of the present invention. 5, 8 and 11 show the wide-angle end, FIGS. 6, 9 and 12 show the middle, and FIGS. 7, 10 and 13 show the telephoto end.

In FIG. 1, reference numeral 1 denotes a first group of positive refractive power, 2 denotes a second group of negative refractive power, 3 denotes a third group of positive refractive power, and 4 denotes a fourth group of positive refractive power. . SP denotes an aperture stop, which is arranged in front of the third lens unit 3.

In this embodiment, when zooming from the wide-angle end to the telephoto end, the second lens unit is moved to the image plane side as indicated by an arrow, and the image plane fluctuation caused by zooming is corrected by moving the fourth lens unit. ing.

Also, a rear focus system is employed in which the fourth unit is moved on the optical axis to perform focusing. A solid line curve 4a and a 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 and third units are fixed during zooming and focusing.

In this embodiment, the fourth lens unit is moved to correct the image plane fluctuation accompanying the zooming, and the fourth lens unit is moved to perform focusing. In particular, curve 4 in FIG.
When zooming from the wide-angle end to the telephoto end, as shown in FIGS.
Thus, the space between the third and fourth units is effectively used, and the overall length of the lens is effectively reduced.

In this embodiment, for example, when focusing from an object at infinity to an object at a short distance at the telephoto end, the fourth unit is moved forward as indicated by a straight line 4c in FIG.

The zoom lens in this embodiment employs a zoom system in which a virtual image formed by a combined system of the first and second units is formed on the photosensitive surface by the third and fourth units.

In this embodiment, the performance degradation due to the eccentric error of the first group is prevented by adopting the rear focus method as described above in comparison with a conventional so-called four-group zoom lens in which the first group is extended and focused. While effectively increasing the effective diameter of the lens of the first lens group.

By arranging the aperture stop immediately before the third lens unit, aberration fluctuations caused by the movable lens unit can be reduced, and the distance between the lens units in front of the aperture stop can be reduced to reduce the diameter of the front lens. Achieved easily.

As described above, by specifying the optical constants of the first lens unit, in particular, by specifying the refractive power and the lens configuration of the first lens unit, the overall length of the zoom lens can be reduced while shortening the overall length of the lens. Further, a zoom lens having a high zoom ratio and excellent optical performance over the entire object distance is obtained.

Next, the technical meaning of the above conditional expression will be described.

Conditional expression (1) is for properly correcting various aberrations while appropriately reducing the size of the entire lens system by appropriately setting the refractive power of the first lens unit.

In this embodiment, when the focal length at the telephoto end and the open F-number are kept at predetermined values, conditional expression (1)
If the refractive power of the first group becomes too weak beyond the upper limit of
Although the amount of occurrence of various aberrations is reduced, the distance between the first lens unit and the aperture stop becomes longer, the overall length of the lens increases, and the outer diameter of the first lens unit for securing off-axis light flux increases. Absent. If the refractive power of the first lens unit becomes too strong beyond the lower limit value of the conditional expression (1), the overall length of the lens becomes short.
It is not good because a large amount of chromatic aberration occurs over the entire zoom range.

[0025]

[0026]

[0027]

Conditional expression (2) sets the refractive power of the second lens unit appropriately to the product of the focal length at the wide-angle end and the focal length at the telephoto end, and effectively secures a predetermined zoom ratio. , For properly correcting aberration fluctuations caused by zooming.

If the refractive power of the second lens unit becomes too strong beyond the lower limit value of the conditional expression (2), the whole lens system becomes smaller, but the Petzval sum increases in the negative direction, and the field curvature becomes larger. Aberration fluctuation accompanying zooming increases. If the refractive power of the second lens unit becomes too weak beyond the upper limit value of the conditional expression (2), the fluctuation of aberration due to zooming decreases, but the amount of movement of the second lens unit for obtaining a predetermined zoom ratio increases. Also, the overall length of the lens becomes longer, which is not good.

Conditional expression (3) is to appropriately set the refractive power of the third lens unit when the third lens unit is composed of one lens with respect to the product of the focal length at the wide-angle end and the focal length at the telephoto end. This is mainly for shortening the overall length of the lens.

If the refractive power of the third lens unit becomes too strong beyond the lower limit value of the conditional expression (3), the amount of movement of the fourth lens unit during zooming or focusing by the fourth lens unit increases, and aberration fluctuations increase. It is becoming. If the positive refractive power of the third lens unit becomes too weak beyond the upper limit of conditional expression (3), the angle of incidence of the light beam on the fourth lens unit becomes large, and it becomes difficult to correct aberrations in the fourth lens unit. Not so good.

In addition, in order to secure high optical performance over the entire zooming range when achieving high zooming while shortening the overall length of the lens in the present invention, (a) the laminated lens of the first group Has an aspherical surface in which the positive refractive power becomes gentler toward the lens periphery.

(B) The third unit has at least one aspherical surface having a shape in which the positive refractive power becomes gentler toward the periphery of the lens, and the fourth unit has a positive surface along the periphery of the lens. It has at least one aspherical surface having a shape with a high refractive power.

(C) The cemented lens of the first group has a biconvex shape as a whole.

(D) The fourth unit is composed of a meniscus negative lens having a convex surface facing the object side and a positive lens having both lens surfaces convex. Are preferred.

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 spacing from the object side, and Ni and νi are the i-th lens surfaces in order from the object side. The refractive index and Abbe number of glass.

Note that R16, R in Numerical Examples 1 and 2
Reference numeral 17 and R17 and R18 in Numerical Example 3 indicate parallel flat plates such as a face plate and a filter.

The aspheric surface has an X-axis in the optical axis direction, an H-axis in a direction perpendicular to the optical axis, a positive traveling direction of light, R is a paraxial radius of curvature,
When A, B, C, D, and E are each aspheric coefficients

[0039]

(Equation 5) It is represented by the following equation.

Table 1 shows the relationship with each conditional expression in each numerical example. Numerical example 1 F = 1 ~ 7.603 FNo = 1: 1.85 ~ 2.62 2ω = 50.8 ° ~ 6.6 ° R 1 = 2.500 D 1 = 0.1511 N 1 = 1.84666 ν 1 = 23.8 R 2 = 1.806 D 2 = 1.0301 N 2 = 1.58313 ν 2 = 59.4 R 3 = -12.275 D 3 = Variable R 4 = 4.091 D 4 = 0.0755 N 3 = 1.88300 ν 3 = 40.8 R 5 = 0.916 D 5 = 0.4257 R 6 = -1.079 D 6 = 0.0755 N 4 = 1.51633 ν 4 = 64.1 R 7 = 1.383 D 7 = 0.2659 N 5 = 1.84666 ν 5 = 23.8 R 8 = 12.209 D 8 = Variable R 9 = (Aperture) D 9 = 0.1813 R10 = 1.472 D10 = 0.4764 N 6 = 1.58313 ν 6 = 59.4 R11 = -90.147 D11 = variable R12 = 2.012 D12 = 0.0755 N 7 = 1.84666 ν 7 = 23.8 R13 = 0.954 D13 = 0.0602 R14 = 1.207 D14 = 0.5939 N 8 = 1.58313 ν 8 = 59.4 R15 = -2.308 D15 = 0.7100 R16 = ∞ D16 = 0.8006 N 9 = 1.51633 ν 9 = 64.1 R17 =

[0041]

[Table 1]

[0042]

[Table 2] Numerical example 2 F = ~ 1 ~ 7.603 FNo = 1: 1.85 ~ 2.63 2ω = 50.8 ° ~ 6.6 ° R 1 = 2.905 D 1 = 1.0412 N 1 = 1.58913 ν 1 = 61.3 R 2 = -3.487 D 2 = 0.1511 N 2 = 1.84666 ν 2 = 23.8 R 3 = -7.381 D 3 = Variable R 4 = 2.285 D 4 = 0.0755 N 3 = 1.88300 ν 3 = 40.8 R 5 = 0.824 D 5 = 0.4892 R 6 = -1.027 D 6 = 0.0755 N 4 = 1.51633 ν 4 = 64.1 R 7 = 1.292 D 7 = 0.2690 N 5 = 1.84666 ν 5 = 23.8 R 8 = 7.722 D 8 = Variable R 9 = (Aperture) D 9 = 0.1813 R10 = 1.690 D10 = 0.4825 N 6 = 1.58313 ν 6 = 59.4 R11 = -7.810 D11 = variable R12 = 2.524 D12 = 0.0755 N 7 = 1.84666 ν 7 = 23.8 R13 = 1.048 D13 = 0.0396 R14 = 1.219 D14 = 0.5715 N 8 = 1.58313 ν 8 = 59.4 R15 = -2.335 D15 = 0.7100 R16 = ∞ D16 = 0.8006 N 9 = 1.51633 ν 9 = 64.1 R17 =

[0043]

[Table 3]

[0044]

[Table 4] Numerical Example 3 F = 1 to 7.603 FNo = 1: 1.85 to 2.53 2ω = 50.8 ° to 6.6 ° R 1 = 2.394 D 1 = 0.1511 N 1 = 1.84666 ν 1 = 23.8 R 2 = 1.770 D 2 = 1.0932 N 2 = 1.58313 ν 2 = 59.4 R 3 = -17.295 D 3 = Variable R 4 = 3.432 D 4 = 0.0755 N 3 = 1.88300 ν 3 = 40.8 R 5 = 0.856 D 5 = 0.3851 R 6 = -1.185 D 6 = 0.1057 N 4 = 1.51633 ν 4 = 64.1 R 7 = 1.586 D 7 = 0.0830 R 8 = 1.733 D 8 = 0.2403 N 5 = 1.84666 ν 5 = 23.8 R 9 = 12.209 D 9 = Variable R10 = (Aperture) D10 = 0.1813 R11 = 1.657 D11 = 0.5052 N 6 = 1.58313 ν 6 = 59.4 R12 = -7.435 D12 = Variable R13 = 2.105 D13 = 0.0755 N 7 = 1.84666 ν 7 = 23.8 R14 = 0.955 D14 = 0.0713 R15 = 1.263 D15 = 0.5760 N 8 = 1.58313 ν 8 = 59.4 R16 = -2.398 D16 = 0.7100 R17 = ∞ D17 = 0.8006 N 9 = 1.51633 ν 9 = 64.1 R18 = ∞

[0045]

[Table 5]

[0046]

[Table 6]

[0047]

[Table 7]

[0048]

According to the present invention, as described above, the refractive power of the four lens units and the moving conditions of the second and fourth units in zooming are set, and the fourth lens unit is moved during focusing. By adopting the above-mentioned formula, an F lens having high optical performance with small aberration variation during focusing while achieving good aberration correction over the entire zoom range with a zoom ratio of about 8 while miniaturizing the entire lens system. A rear focus type zoom lens having a large aperture ratio of 1.8 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 sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 3 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 4 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 5 is an aberration diagram at a wide-angle end according to Numerical Embodiment 1 of the present invention.

FIG. 6 is an intermediate aberration diagram of the numerical example 1 of the present invention.

FIG. 7 is an aberration diagram at a telephoto end in Numerical Example 1 of the present invention.

FIG. 8 is an aberration diagram at a wide angle end according to Numerical Example 2 of the present invention.

FIG. 9 is an intermediate aberration diagram of the numerical example 2 of the present invention.

FIG. 10 is an aberration diagram at a telephoto end in Numerical Example 2 of the present invention.

FIG. 11 is an aberration diagram at a wide angle end according to Numerical Example 3 of the present invention.

FIG. 12 is an intermediate aberration diagram of the numerical example 3 of the present invention.

FIG. 13 is a diagram illustrating various aberrations at the telephoto end according to Numerical Example 3 of the present invention.

[Explanation of symbols]

 Reference Signs List 1 1st group 2 2nd group 3 3rd group 4 4th group SP Aperture d d-line g g-line S Sagittal image plane M Meridional image plane

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (3)

(57) [Claims] 1. 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. The second unit is moved to the image plane side to perform zooming from the wide-angle end to the telephoto end, and the image plane fluctuation accompanying zooming is corrected by moving the fourth unit. Focusing is performed by moving the group, and the first group is 1
The third group is composed of one positive lens, and the fourth group is composed of one negative lens and one positive lens. The focal length of the first group is f1, the focal length of the entire system at the telephoto end is fT, and the open F-number of the entire system is FN.
O, when the focal lengths of the third and fourth lens units are f3 and f4, respectively, and the focal length of the entire system at the wide-angle end is fw. (Equation 2) A rear focus zoom lens that satisfies certain conditions. 2. The rear focus type zoom according to claim 1, wherein the first group of cemented lenses has an aspherical surface having a positive refractive power that becomes gentler toward the periphery of the lens. lens. 3. The third group has at least one aspherical surface having a shape in which the positive refractive power becomes gentler toward the lens periphery, and the fourth group has a positive refractive power toward the lens periphery. 2. The rear focus type zoom lens according to claim 1, wherein the zoom lens has at least one aspherical surface having a shape with a strong force.