JP2767804B2 - Compact high-magnification zoom lens system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact zoom lens suitable for a camera having no restriction on back focus, for example, a lens shutter camera.

Conventionally, as a zoom lens for a camera having no constraint on the back focus, as proposed in JP-A-56-128911 and JP-A-57-201213, two positive and negative lenses are sequentially arranged from the object side. 2. Description of the Related Art A positive-refractive-power-type two-component zoom lens including two components is known. Further, a three-component zoom lens described in JP-A-58-138713 and JP-A-58-184915, a four-component zoom lens described in JP-A-60-57814, and other types are disclosed. Have also been proposed. However, most of these zoom lenses have a relatively small zoom ratio, and a zoom lens having a zoom ratio exceeding 3 has not been realized.

On the other hand, a relatively compact zoom lens for a single-lens reflex camera having a zoom ratio exceeding 3 is disclosed in JP-A-54-30855, JP-A-55-156912, and JP-A-57-169716. Although various proposals have been made, the overall length of the lens system (the length from the front surface of the lens closest to the object side to the film surface) is large due to the constraints of the back focus.

Therefore, an object of the present invention is to provide a compact, high-performance, high-magnification zoom lens suitable for a camera having no restrictions on the back focus.

It is a further object of the present invention to provide a zoom lens in which the zoom ratio is increased and the zoom ratio is increased, as compared with the zoom lens described in Japanese Patent Application Laid-Open No. 60-57814.

Still another object of the present invention is to provide a compact, high-performance, high-magnification zoom lens in which the miniaturization and simplification of the camera body are sufficiently considered.

In order to achieve the above object, a zoom lens according to the present invention comprises a first lens unit (I) having a positive refractive power in order from the object side as shown in FIGS. A second lens group (II) having a negative refractive power and a third lens group (III) having a negative refractive power and having at least one aspheric surface.
During zooming from the shortest focal length end to the longest focal length end, the first lens group (I) and the third lens group (III) move from the image side to the object side, and the second lens group (II) moves. The air gap between the first and second lens groups increases by moving, and the air gap between the second and third lens groups decreases, and the following condition is satisfied.

Here, X is a height Y from the optical axis represented by the following equation.
Displacement of the optical axis direction _{in, X = Xo + A 4 Y} 4 + A 6 Y 6 + A 8 Y 8 + A 10 Y 10 + ... Xo spherical shape as a reference of the aspherical surface expressed by the following formula, Xo = Coy ² / ｛1+ (1-Co ² Y ² ) ^1/2 Ａ A is the aspherical coefficient, Co is the curvature of the spherical surface as the reference of the aspherical surface,
N is the refractive index on the object side of the aspherical surface, and N 'is the refractive index on the image side of the aspherical surface.

 Hereinafter, the present invention will be described in detail.

The condition (1) is that if the surface to which the aspheric surface is applied has a positive refractive power, the aspheric surface has a surface shape in which the positive refractive power increases as the distance from the lens optical axis increases, or If the surface has a negative refractive power, it indicates that the surface has a negative refractive power that decreases as the distance from the optical axis of the aspherical lens increases. In the vicinity of the optical axis which has little effect on spherical aberration, even if the above condition (1) is deviated by only a very small amount, the present invention is substantially defined. Therefore, while the third lens group (III) as a whole has a strong negative refractive power, the aspherical surface provided therein satisfies the condition (1), so that the third lens group (III) is relatively far from the lens optical axis. It can have a loose refractive power. Therefore, the on-axis light beam passes through a relatively low position in the third lens unit (II) except for the longest focal length extremity, and the off-axis chief ray passes through a relatively high position. By applying the aspherical surface to the third lens group (III), positive distortion at the longest focal length extremity and fluctuation of coma during zooming can be satisfactorily corrected.

Furthermore, in the present invention, it is desirable that the following conditions are also satisfied.

Here, L ₂ is the distance from the vertex of the lens surface closest to the object side of the second lens unit in the shortest focal length state to the film surface, and L ₂ ′ is the closest image side of the second lens unit in the shortest focal length state. , The distance from the vertex of the lens surface to the film surface, FM is the diagonal length of the screen, fS is the focal length of the entire system at the shortest focal length end, and fL is the focal length of the entire system at the longest focal length end.

The condition (2) is a condition for maintaining the compactness of the entire system while measuring the enlargement of the zoom ratio. If the upper limit of the condition (2) is exceeded, the overall length becomes long, and the compactness targeted by the present invention is maintained. Becomes difficult. On the other hand, when the value goes below the lower limit of the condition (2), when the zoom ratio increases, the back focus decreases, the diameter of the third lens unit increases, or the second lens unit and the third lens unit become extremely thin, The degree of freedom for correcting aberration fluctuation during zooming is reduced.
When the degree of freedom is reduced, in a zoom lens including a wide angle and a relatively large zoom ratio as in the present invention, even if the aberration is corrected at the shortest focal length end and the longest focal length end, the intermediate focal length In particular, it becomes difficult to correct spherical aberration and field curvature.

Condition (3) is a condition for maintaining the compactness of the entire system while maintaining high performance under condition (2).
If the thickness of the second lens group (II) increases beyond the upper limit of, it becomes difficult to realize a compact optical system. Conversely, when the value goes below the lower limit of the condition (3), it becomes difficult to correct aberration fluctuation during zooming, in particular, spherical aberration and coma in a well-balanced manner.

Further, in the present invention, it is desirable that the following conditions be satisfied in addition to the above conditions (1) to (3).

However, where, f ₂ is the focal length of the second lens unit (II).

Condition (4) appropriately defines the refractive power of the second lens group (II). If the refractive power of the second lens group (II) becomes weaker than the upper limit of the condition (4), the third lens group The on-axis luminous flux incident on (III) is increased, so that the bag focus becomes longer, and the compactness is easily impaired. Conversely, if the refractive power of the second lens group (II) becomes too strong below the lower limit of the condition (4), the aberration generated in the second lens group (II) becomes large, and especially the spherical surface at the shortest focal length end. In addition to making it difficult to correct aberrations sufficiently, the back focus becomes too short, so that the diameter of the third lens group (III) for securing sufficient image plane illuminance at the shortest focal length end is undesirably large.

Further, in the zoom lens system according to the present invention, it is desirable to satisfy the following conditions.

(6) 0.08 <| f ₃ /fL|<0.45 (7) 0.25 <(D ₁₂ L−D ₁₂ S) / fS <0.60 where βL ₃ is the third lens group (III) at the longest focal length end. ), ΒS ₃ is the lateral magnification of the third lens group (III) at the shortest focal length extremity, and f ₃ is the third lens group (I
II) Focal length, D ₁₂ L is the first at the longest focal length end,
The axial distance between the second lens groups, D ₁₂ S, is the axial distance between the first and second lens groups at the shortest focal length end.

Condition (5) defines the zooming effect of the third lens group (III). If the upper limit of the condition (5) is exceeded, the refractive power of the third lens group (III) becomes strong or the third lens group becomes too strong. The amount of movement of the group (III) increases, and in the former case, it becomes impossible to realize the third lens group (III) with a relatively simple configuration. In the latter case, aberration fluctuation during zooming becomes large, and the total length at the longest focal length end becomes long. Therefore, it is difficult to make the entire system including the lens barrel compact. If the lower limit of the condition (5) is exceeded, the burden on the second lens group (III) for zooming will be too strong.

The condition (6) defines the refractive power of the third lens group (III), and exceeds the upper limit of the condition (6).
If the refractive power of I) becomes weak, the amount of movement of the third lens group (III) becomes large in order to obtain a predetermined zoom ratio, and it is difficult to make the entire system including the lens barrel compact. When the value goes below the lower limit of the condition (6), the third lens unit (II
When the refractive power of I) increases, the aberration generated in the third lens group (III) increases, and even if an aspherical surface is used for the third lens group (III), distortion and fluctuation of coma during zooming can be sufficiently reduced. Is difficult to correct.

Further, the condition (7) defines a change in the distance between the first lens unit (I) and the second lens unit (II) due to zooming. If the upper limit of the condition (7) is exceeded, aberrations due to zooming will occur. It becomes difficult to satisfactorily correct fluctuations, especially spherical aberration and coma, and the distance between the first and second lens groups at the shortest focal length end becomes too large, so that a sufficient image is formed at the outermost periphery of the screen. In order to secure surface illuminance, the diameter of the front lens or the diameter of the rear lens becomes too large, and it is difficult to secure compactness. If the lower limit of the condition (7) is exceeded, the change in the distance between the two lens units during zooming will be small,
It becomes difficult to increase the zoom ratio.

Further, in the present invention, it is desirable to satisfy the following conditions.

(8) Nd ₃ <1.6, νd ₃ <60 where Nd ₃ is the refractive index of the lens having an aspheric surface in the third lens group (III), and νd ₃ is the third lens group (III)
This is the Abbe number of a lens having a middle aspheric surface. By using a plastic lens satisfying the condition (8) for the lens having an aspheric surface of the third lens group (III), a large power saving can be achieved in the processing step, which is preferable in structure.

In the present invention, a specific configuration of each lens group is preferably as follows.

The first lens group (I) includes at least one positive lens and one positive lens.
By including the negative lenses, the first lens unit (I) is given a relatively strong refractive power, and the amount of movement of the first lens unit (I) due to zooming can be suppressed as small as possible. In addition, since the third lens group (III) includes at least one positive lens and one negative lens, chromatic aberration during zooming, particularly chromatic aberration of magnification, can be corrected with good balance. During zooming, the first lens group (I) and the third lens group (III) move together and the second lens group (II) also moves separately, so that each lens group is separated. This is advantageous in terms of the lens barrel configuration as compared with the configuration in which the lens barrel moves.

Although the present invention is basically a zoom lens system composed of three groups, even if a component having a relatively weak refractive power fixed to the most image side is added, the constitutional requirements of the present invention are essentially deviated. Never.

Hereinafter, examples of the present invention will be described. In each embodiment, r ₁ is the radius of curvature of the i-th lens surface in order from the object side, di is the i-th axial distance in order from the object side, and Ni and νi are the i-th lens in order from the object side. And the Abbe number. The surface marked with (*) indicates an aspherical surface, and its shape is defined as follows.

^{_{X = Xo + A 4 Y 4}} + A 6 Y 6 + A 8 Y 8 + A 10 Y 10 + ... Xo = CoY 2 / {1+ (1-Co 2 Y 2) 1/2} where where, X is from the optical axis The displacement amount in the optical axis direction at the height Y, Xo is the shape of the spherical surface serving as the reference of the aspheric surface, A is the aspheric coefficient, and Co is the curvature of the reference spherical surface of the aspheric surface. Tables 1 and 2 show the relationship between each embodiment and each condition.

[Brief description of the drawings]

1 to 9 are sectional views showing the lens arrangement at the shortest focal length end (S end) and the longest focal length end (L end) of the zoom lenses according to the first to ninth embodiments of the present invention, respectively.
FIG. 18 shows the shortest focal length end <S>, the intermediate focal length <M>, and the longest focal length end <
FIG. 7 is an aberration diagram showing various aberrations when the object distance L> is infinite. I: first lens group, II: second lens group, III: third lens group.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-58-184916 (JP, A) JP-A-61-52620 (JP, A) JP-A-62-78522 (JP, A) JP-A-63-1982 161423 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02B 15/00-15/28

Claims (4)

(57) [Claims] 1. A first lens having a positive refractive power in order from the object side.
A lens unit, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power and having at least one aspheric surface, and zooming from the shortest focal length end to the longest focal length end At this time, the first lens group and the third lens group move from the image side to the object side, and the second lens group moves, so that the air gap between the first and second lens groups increases, and the second and third lenses are also moved. A compact high-magnification zoom lens system characterized by a reduced air gap between groups and satisfying the following conditions: However, where, X: amount of displacement in the optical axis direction at a height Y of the optical axis represented by the following formula _{X = X 0 + A 4 Y} 4 + A 6 Y 6 + A 8 Y 8 + A 10 Y 10 + ... X 0 : The shape of the spherical surface that is the reference for the aspheric surface expressed by the following equation A: Aspherical surface coefficient C ₀ : Curvature of spherical surface as reference of aspherical surface N: Refractive index on the object side from aspherical surface N ′: Refractive index on the image side from aspherical surface L ₂ : Second lens group in shortest focal length Distance from the vertex of the lens surface closest to the object side to the film surface L ₂ ′: distance from the vertex of the lens surface closest to the image side of the second lens group in the shortest focal length state to the film surface FM: screen diagonal length fS: shortest The focal length of the entire system at the focal length extremity fL: The focal length of the entire system at the longest focal length extremity. 2. A compact high-magnification zoom lens system according to claim 1, further satisfying the following condition: Here, f ₂ is the focal length of the second lens group. 3. A compact high-magnification zoom lens system according to claim 2, further satisfying the following condition: However, where, .beta.L _3: the longest focal length lateral magnification of the third lens group at the end .beta.S _3: shortest focal length lateral magnification f of the third lens group at the end _3: the focal length of the third lens group D ₁₂ L: Maximum On-axis distance between the first and second lens groups at the focal length end D ₁₂ S: On-axis distance between the first and second lens groups at the shortest focal length end. 4. The compact high-magnification zoom lens system according to claim 3, further satisfying the following condition: Nd ₃ <1.6, νd ₃ <60, where Nd ₃ : third Refractive index νd _{3 of} lens having an aspheric surface in the lens group: Abbe number of lens having an aspheric surface in the third lens group.