JP3919301B2 - Rear focus zoom lens and imaging apparatus using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a rear focus type zoom lens and an image pickup apparatus using the same, and in particular, a back focus that is long enough to provide a color separation prism on an image plane side used in an image pickup apparatus such as a video camera and a broadcast camera. In addition, the present invention relates to a small rear focus zoom lens having a large aperture ratio and a high zoom ratio and a short overall lens length, and an image pickup apparatus using the same.
[0002]
[Prior art]
Recently, with the reduction in size and weight of home video cameras and the like, remarkable progress has been made in reducing the size of zoom lenses for imaging, particularly in reducing the overall length of the lens, reducing the front lens diameter, and simplifying the configuration. It has been poured.
[0003]
As one means for achieving these objects, a so-called rear focus type zoom lens that performs focusing by moving a lens group other than the first group on the object side is known.
[0004]
In general, a rear focus type zoom lens has a smaller effective diameter of the first lens unit than a zoom lens that focuses by moving the first lens unit, which makes it easy to reduce the size of the entire lens system. Close-up photography is facilitated, and the relatively small and light lens group is moved, so that the lens group has a small driving force and can be focused quickly.
[0005]
As such a rear focus type zoom lens, Japanese Patent Application Laid-Open No. 3-15881 discloses, in order from the object side, a first group having a positive refractive power, a second group having a negative refractive power, and a third group having a positive refractive power. And having four lens groups of the fourth group having positive refractive power, changing the distance between the second group and the third group, and moving a part of the lenses of the fourth group. A zoom lens that performs focusing is disclosed.
[0006]
In Japanese Patent Laid-Open Nos. 61-296317 and 61-296318, the first group having a positive refractive power, the second group having a negative refractive power, and the second group having a positive refractive power are sequentially arranged from the object side. There are three lens groups, and four lens groups having a positive refractive power and a fourth lens group. The zooming is performed by changing the distance between the second group and the third group, and between the second group and the third group. Discloses a zoom lens in which an aperture stop is disposed and focus is performed by moving the first lens group.
[0007]
On the other hand, with the recent high performance (digitalization) of video decks, various improvements in the image quality of video cameras have been made. One of them is color separation of an image by a color separation optical system. Such zoom lenses for video cameras have been proposed in, for example, Japanese Patent Laid-Open Nos. 5-72474, 6-511199, 7-199069, and 7-270684.
[0008]
[Problems to be solved by the invention]
In general, when a rear focus method is used in a zoom lens, the entire lens system can be miniaturized, quick focusing can be performed, and close-up photography can be facilitated.
[0009]
However, on the other hand, while securing a long back focus that can arrange the color separation prism, reducing aberration fluctuations during focusing and trying to obtain high optical performance over the entire object distance from an infinite object to a close object The lens configuration becomes very difficult.
[0010]
In particular, in a zoom lens that secures a high zoom ratio with a large aperture ratio, it becomes very difficult to obtain high optical performance over the entire zoom range and the entire object distance while securing a long back focus.
[0011]
In the above-mentioned Japanese Patent Application Laid-Open No. 3-158813, the rear lens type is used to reduce the size of the entire lens system. However, an IG meter that drives and controls the diaphragm in order to move the diaphragm integrally with the third group. However, there was a tendency for the mechanism to become complicated due to the movement with zooming.
[0012]
In general, if the refracting power of each lens unit is increased in a zoom lens, the amount of movement of each lens unit to obtain a predetermined zoom ratio is reduced, and high zooming can be achieved while shortening the overall lens length. . However, when the refractive power of each lens unit is simply increased, aberration fluctuations accompanying zooming increase, and it becomes difficult to obtain good optical performance over the entire zooming range, particularly when zooming in at high magnifications is attempted.
[0013]
The present invention employs a rear focus system, has a long back focus that allows a color separation prism, an optical filter, and the like to be disposed on the image surface side, and has a large aperture ratio and a zoom ratio of 14 to 16 times. And a high zoom ratio, a rear-focus zoom lens with excellent optical performance over the entire zoom range from the wide-angle end to the telephoto end, and over the entire object distance from the infinity object to the very close object. And an imaging apparatus using the same.
[0014]
An imaging apparatus having a rear focus type zoom lens according to the first aspect of the invention includes, in order from the object side, a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, Then, only the fourth lens group of the fourth group having a positive refractive power is provided as a lens group, and the second group and the third group are monotonically moved in opposite directions to perform zooming. part or move all have line focus, the
An aperture stop is disposed between the second group and the third group;
The distance between the second group and the third group at the zoom position at the wide-angle end, and the distance between the second group and the aperture stop are D23W, D2SW,
When focusing on an object at infinity at the wide angle end, the air equivalent amount from the fourth group to the image plane is BFW, the focal length of the entire system at the wide angle end, the F number, and the half angle of view in order of fW, FNW, ωw. When
0.3 <D2SW / D23W <0.65 ( 1 )
[Expression 4]
It is characterized by satisfying the following conditions .
[0015]
An imaging apparatus having a rear focus type zoom lens according to a second aspect of the invention includes, in order from the object side, a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, Then, only the fourth lens group of the fourth group having positive refractive power is provided as a lens group, and the second group and the third group are monotonously moved in opposite directions to perform zooming, and the fourth group is negative. The lens group has only two lens groups, ie, a forty-first lens group having a refractive power of 42 and a forty-second lens group having a positive refractive power, and focusing is performed by moving one or both of the forty-first group and the forty-second group. Yes ,
An aperture stop is disposed between the second group and the third group;
The distance between the second group and the third group at the zoom position at the wide-angle end, and the distance between the second group and the aperture stop are D23W, D2SW,
When focusing on an object at infinity at the wide angle end, the air equivalent amount from the fourth group to the image plane is BFW, the focal length of the entire system at the wide angle end, the F number, and the half angle of view in order of fW, FNW, ωw. When
0.3 <D2SW / D23W <0.65 ( 1 )
[Equation 5]
It is characterized by satisfying the following conditions .
[0016]
An image pickup apparatus having a rear focus type zoom lens of the present invention is characterized by having a color separation optical system to the image plane side of the Zumuren's of rear focus type according to claim 1 or 2.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a cross-sectional view of an essential part of Embodiment 1 of an image pickup apparatus having a rear focus zoom lens according to the present invention, and FIGS. 2, 3 and 4 are zoom positions at the wide-angle end, intermediate and telephoto ends of Embodiment 1. FIG.
[0018]
FIG. 5 is a cross-sectional view of an essential part of Embodiment 2 of an imaging apparatus having a rear focus type zoom lens according to the present invention, and FIGS. 6, 7, and 8 are zoom positions at the wide-angle end, the middle, and the telephoto end of Embodiment 2. FIG.
[0019]
FIG. 9 is a cross-sectional view of an essential part of Embodiment 3 of an imaging apparatus having a rear focus type zoom lens according to the present invention, and FIGS. 10, 11, and 12 are zoom positions at the wide-angle end, intermediate, and telephoto ends of Embodiment 3. FIG.
[0020]
FIG. 13 is a cross-sectional view of an essential part of Embodiment 4 of an image pickup apparatus having a rear focus type zoom lens according to the present invention, and FIGS. 14, 15 and 16 are zoom positions at the wide-angle end, intermediate and telephoto ends of Embodiment 4, respectively. FIG.
[0021]
In the figure, L1 is a first group having a positive refractive power, L2 is a second group having a negative refractive power, L3 is a third group having a positive refractive power, and L4 is a fourth group having a positive refractive power. It has a forty-first group of refractive power and a forty-second group of positive refractive power. SP is an aperture stop, which is disposed in front of the third lens unit L3. GA is a protective glass provided as needed for the purpose of protecting the zoom lens, and GB is a glass block such as a color separation prism, a face plate, or a filter. IP is an image plane, and an image pickup device such as a CCD is disposed.
[0022]
The first lens unit L1 to the fourth lens unit L4 constitute an element of the zoom lens (zoom lens unit) ZL. The glass block GB and the image sensor are housed in the camera body CB. The zoom lens unit ZL is detachably attached to the camera body CB via the mount member C.
[0023]
In this embodiment, when zooming from the wide-angle end to the telephoto end, the second group is monotonously moved to the image plane side as indicated by an arrow, and the image plane variation accompanying zooming is shifted to the object side opposite to the third group. It is corrected by moving monotonously.
[0024]
In addition, a rear focus type is employed in which focusing is performed by moving part or all of the fourth group (the 41st group L41 in the present embodiment) on the optical axis.
[0025]
In the figure, the forty-first lens unit L41 is composed of negative lenses having both concave lens surfaces, and the negative lens is moved to the image plane side as indicated by an arrow 4a to focus from an infinite object to a close object. Yes. In the present embodiment, the forty-second group L42 is fixed at the time of focusing, but may be moved together with the forty-first group L41 or independently at a different speed.
[0026]
The video camera (imaging device) according to the present invention includes at least the zoom lens, a color separation element, an imaging element corresponding to each color divided by the color separation element, an imaging signal processing circuit, and the like.
[0027]
In this embodiment, the second group and the third group sandwiching the stop SP are monotonically moved in the opposite directions when zooming, so that the second group has a zooming effect and the zooming effect is also applied to the third group. High magnification is achieved by having it. In addition, the space between the second group and the third group is effectively used, and the total movement amount on the optical axis of the second group and the third group is reduced to shorten the overall lens length. .
[0028]
In a general zoom lens, the first group occupies 50 to 80% of the weight of the lens. Therefore, in order to reduce the weight of the zoom lens, it is effective to reduce the volume by reducing the material of the first lens group or reducing the lens diameter of the first lens group. Therefore, in this embodiment, the lens diameter of the first group is reduced to reduce the weight of the entire zoom lens.
[0029]
That is, as compared with a zoom lens in which the stop SP is arranged behind the third group, the stop is arranged at a substantially intermediate position of the optical system between the second group and the third group close to the first group. The lens diameter of one group is reduced. In the present embodiment, the effective lens diameter is the lens diameter of both the first group lens diameter determined by the incident light at the wide-angle end side and the first group lens diameter determined by the on-axis light beam (F number light beam) at the telephoto end. An aperture and a lens group are arranged so as to be small.
[0030]
In order to lengthen the back focus, the fourth group is composed of two lens groups, a 41th group having a negative refractive power and a 42nd group having a positive refractive power. This ensures a long back focus by using the fourth group as a retrofocus type. The first group is fixed during zooming and focusing.
[0031]
As described above, the zoom lens according to the present embodiment is composed of four lens groups as a whole, and is predetermined by appropriately setting the moving conditions of each lens group during zooming and focusing, the lens configuration of the fourth group, and the like. High optical performance is obtained over the entire zoom range and over the entire object distance while ensuring the back focus.
[0032]
Next, features of other lens configurations of the rear focus type zoom lens of the present invention will be described.
[0033]
[A1] 0.3 <D2SW / D23W <0.65 when the distance between the second group and the third group at the zoom position at the wide-angle end, and the distance between the second group and the aperture stop are D23W and D2SW, respectively. (1)
Is satisfied.
[0034]
This effectively achieves a reduction in the lens diameter of the first lens group, a reduction in the total lens length, and good aberration correction.
[0035]
In particular, by arranging the second group, the third group, and the stop so as to satisfy the conditional expression (1), each of the lenses can be shortened and the first lens can be shortened while achieving high zooming without causing mechanical interference. The lens diameter of one group is reduced.
[0036]
If the aperture is moved closer to the first lens group, the lens diameter of the first lens group becomes smaller, but conversely, the final lens group that is farther from the aperture becomes larger. Conditional expression (1) is mainly for reducing the overall lens diameter while maintaining a good balance between the lens diameter of the first lens group and the lens diameter of the final lens group.
[0037]
When the lower limit of conditional expression (1) is exceeded, the lens diameter of the first lens group decreases, but conversely, the lens diameter of the final lens group increases. Then, since the position of the off-axis light beam incident on the fourth group is greatly changed by moving the third group by zooming, it becomes difficult to correct aberration variation satisfactorily. Further, if the stop position is far from the first group beyond the upper limit, the lens diameter of the first group becomes large, which is not preferable.
[0038]
[A2] The focal length of the second lens unit is f2, the F number and focal length of the entire system at the wide angle end are FNW and fW, respectively, and the focal length of the entire system at the telephoto end is fT.
[0039]
[Equation 3]
0.53 <| f2 | × FNW / fM <0.84 (2)
Is satisfied.
[0040]
This effectively corrects fluctuations in coma generated by zooming. Conditional expression (2) regulates the focal length of the second group and is largely related to the F-number FNW at the wide-angle end. Since the second group mainly has a zooming function, it moves on the optical axis by zooming. Therefore, it is necessary to satisfactorily correct aberration fluctuations that occur. In particular, coma greatly varies with zooming. Conditional expression (2) is for correcting this well.
[0041]
If the lower limit of conditional expression (2) is exceeded and the FNW at the wide-angle end is brightened or the focal length f2 of the second group is shortened, higher order coma flare will occur and correction will be difficult. On the other hand, if the focal length of the second lens group is increased unnecessarily beyond the upper limit value or the FNW at the wide-angle end is darkened, the optical performance increases, but the overall lens length becomes longer, making it impossible to achieve downsizing.
[0042]
[A3] The air equivalent amount from the fourth group to the image plane when focusing on an object at infinity at the wide-angle end is BFW, the focal length of the entire system at the wide-angle end, the F-number, and the half field angle are fW and FNW in this order. , Ωw [0043]
[Expression 4]
Is satisfied.
[0044]
As a result, a back focus of a predetermined length is secured. If the F-number at the wide-angle end is made brighter beyond the lower limit value of conditional expression (3), many higher-order spherical aberrations and coma occur, and it is difficult to correct them well. Conversely, when the F-number becomes darker than the upper limit value, the axial ray bundle becomes thin, and the color separation prism arranged between the final lens surface of the fourth group and the image surface can be reduced in size. Although it is not necessary to lengthen the back focus, it must be lengthened, which leads to an increase in the overall length of the lens.
[0045]
[A4] One of the objects of the present invention is to obtain a zoom lens having a high zoom ratio. For this reason, it is desirable to cancel the chromatic aberration caused by zooming mainly in the first group and the second group.
[0046]
However, the generation of chromatic aberration of magnification accompanying zooming differs greatly between the first group and the second group, and tends to be overcorrected at the wide-angle end. Therefore, the balance of the chromatic aberration as a whole is maintained by making the correction of the chromatic aberration of the magnification of the fourth group insufficient.
[0047]
In addition, axial chromatic aberration can be corrected without greatly losing balance when the zoom ratio is small. However, when aiming at high zoom ratio and large aperture as in the present invention, axial chromatic aberration is not fully corrected. It becomes difficult to maintain high optical performance.
[0048]
Therefore, in the present invention, the third lens group has a positive lens having an appropriate refractive power and Abbe number and a negative meniscus lens having a strong concave surface facing the object side, so that chromatic aberration can be corrected optimally over the entire zoom range. In spite of its simple lens configuration, it has a zoom ratio of 14 or higher, a high zoom ratio, an F number of about FNW 1.6, a large aperture, and high optical performance.
[0049]
Basically, if the lens is joined in the configuration of each lens group, the intra-group eccentricity can be effectively suppressed and the product performance can be stabilized. However, the degree of design freedom is one. It becomes difficult to achieve sufficient initial performance while satisfying the specifications of large aperture and small zoom.
[0050]
Therefore, in the present embodiment, by having a cemented lens in the third group and further adopting an aspheric surface in the third group, as shown in Numerical Examples 2 to 3, it is possible to effectively suppress in-group eccentricity and the like. A large-aperture zoom lens with higher optical performance has been obtained.
[0051]
The aspheric surface provided in the third lens group is mainly used to correct higher-order flare components of spherical aberration at the wide-angle end, and it is effective to apply it to a stronger convex surface. Therefore, it is best to use an aspherical surface for the positive lens having the largest positive refractive power in the third group.
[0052]
[A5] By adopting an aspherical surface in the fourth lens group, as shown in Numerical Example 3, a zoom lens having a large aperture and an ultra-high magnification zoom lens with high optical performance is achieved.
[0053]
The aspherical surface provided in the fourth group is mainly used for correcting higher-order flare components and astigmatism of spherical aberration. For this purpose, it is effective to apply it to a stronger convex surface. Therefore, it is best to use an aspherical surface for the positive lens having the largest positive refractive power in the fourth group.
[0054]
[A6] In the 42nd group from the object side, a positive 421 lens having a convex surface on the image side, a meniscus negative 422 lens having a convex surface directed toward the object side, and a positive surface having both convex surfaces When the radius of curvature of the lens surface on the image plane side of the 422 lens and the lens surface on the object side of the 423 lens are Ra and Rb, respectively, and the focal length at the wide angle end of the entire system is fW. 0 ≦ | 1 / Ra−1 / Rb | · fW <0.11 (4)
Is satisfied.
[0055]
Conditional expression (4) is intended to suppress high-order astigmatism and spherical aberration components occurring in the fourth group between the positive lens and the negative lens. The lower limit value has a cemented lens or an equivalent effect, and is in a very stable state. If the upper limit is exceeded, correction of higher-order flare components concentrates on higher-order terms of the aspheric surface provided in the fourth group, and therefore, it is likely to become very unstable in consideration of manufacturing errors.
[0056]
The aspherical surface provided in the fourth group is basically intended for correction of spherical aberration, and therefore it is desirable that the aspherical surface has a shape in which the positive refractive power becomes weaker toward the periphery of the lens.
[0057]
Next, numerical examples of the present invention will be shown. 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 in order from the object side, and Ni and νi are the i-th lens in order from the object side. The refractive index and Abbe number of the glass. The aspherical surface has a radius of curvature R at the center of the lens surface, the optical axis direction (light traveling direction) is the X axis, the vertical direction to the optical axis is the Y axis, and B, C, D, and E are respectively non-circular. When using spherical coefficient [0058]
[Equation 5]
It is expressed by the following formula. “E-0X” means “× 10 ^−X ”. Table 1 shows the relationship between the conditional expressions described above and the numerical values in the numerical examples.
[0059]
In addition, R25 to R28 in Numerical Examples 1 to 3 and R28 to R31 in Numerical Example 4 indicate glass blocks such as a color separation prism, an optical filter, and a face plate.
[0060]
[Table 1]
[0061]
[Table 2]
[0062]
[Table 3]
[0063]
[Table 4]
[0064]
[Table 1]
[0065]
【The invention's effect】
As described above, according to the present invention, by setting each element, the rear focus method is adopted, and the back focus is long enough to arrange a color separation prism, an optical filter, or the like on the image plane side. In addition, with a large aperture ratio, it has a high zoom ratio of 14 to 16 times, covers the entire zoom range from the wide-angle end to the telephoto end, and the overall object distance from an infinite object to an extremely close object Thus, a rear focus type zoom lens having good optical performance and an image pickup apparatus using the same can be achieved.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a main part of an imaging apparatus having a rear focus zoom lens according to a first embodiment of the present invention. Fig. 3 is an aberration diagram at the wide-angle end. Fig. 3 is an aberration diagram in the middle of Embodiment 1 of the imaging device having the rear focus zoom lens of the present invention. Fig. 4 is an imaging device having the rear focus zoom lens of the present invention. FIG. 5 is a cross-sectional view of the main part of Embodiment 2 of the imaging apparatus having the rear focus zoom lens of the present invention. FIG. 6 is a rear focus zoom lens of the present invention. FIG. 7 is an aberration diagram at the wide-angle end of Embodiment 2 of the image pickup apparatus having a zoom lens. FIG. 7 is an aberration diagram in the middle of Embodiment 2 of the image pickup apparatus having the rear focus zoom lens of the present invention. Focus type Aberration diagram at the telephoto end of Embodiment 2 of the imaging apparatus having a zoom lens. FIG. 9 is a cross-sectional view of a main part of Embodiment 3 of the imaging apparatus having a rear focus zoom lens according to the present invention. FIG. 11 is an aberration diagram at the wide-angle end of Embodiment 3 of the imaging device having the rear focus zoom lens. FIG. 11 is an aberration diagram in the middle of Embodiment 3 of the imaging device having the rear focus zoom lens of the present invention. FIG. 12 is an aberration diagram at the telephoto end of Embodiment 3 of the imaging apparatus having the rear focus zoom lens of the present invention. FIG. 13 is an aberration chart of Embodiment 4 of the imaging apparatus having the rear focus zoom lens of the present invention. FIG. 14 is an aberration diagram at the wide-angle end of Embodiment 4 of the imaging apparatus having the rear focus zoom lens of the present invention. FIG. 15 has the rear focus zoom lens of the present invention. Aberration diagram at the telephoto end in the fourth embodiment of an imaging device having a rear focus type zoom lens aberration view of the intermediate [16] The present invention according to a fourth embodiment of an imaging apparatus [Description of symbols]
L1 1st group L2 2nd group L3 3rd group L4 4th group L41 41st group L42 42nd group SP Aperture IP Image plane d d line g g line S Sagittal image plane M Meridional image plane GB Glass block

Claims (6)

In order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a positive fourth lens group only four lens groups group refractive power as a, a third group second group by monotonously moving in opposite directions do scaling, have rows focusing by moving a part or all of the fourth group,
An aperture stop is disposed between the second group and the third group;
The distance between the second group and the third group at the zoom position at the wide-angle end, and the distance between the second group and the aperture stop are D23W, D2SW,
When focusing on an object at infinity at the wide angle end, the air equivalent amount from the fourth group to the image plane is BFW, the focal length of the entire system at the wide angle end, the F number, and the half angle of view in order of fW, FNW, ωw. When
0.3 <D2SW / D23W <0.65
An imaging apparatus having a rear focus type zoom lens, characterized in that: In order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a positive fourth lens group only four lens groups group refractive power The second group and the third group are monotonically moved in opposite directions to perform zooming, and the fourth group is a negative power of the 41st group and a positive power of the 42nd group. one of a lens group only as a lens group, have line focus by moving one or both of said 41 groups or 42 groups,
An aperture stop is disposed between the second group and the third group;
The distance between the second group and the third group at the zoom position at the wide-angle end, and the distance between the second group and the aperture stop are D23W, D2SW,
When focusing on an object at infinity at the wide angle end, the air equivalent amount from the fourth group to the image plane is BFW, the focal length of the entire system at the wide angle end, the F number, and the half angle of view in order of fW, FNW, ωw. When
0.3 <D2SW / D23W <0.65
An imaging apparatus having a rear focus type zoom lens, characterized in that: The focal length of the second group is f2, the F number and focal length of the entire system at the wide angle end are FNW and fW, respectively, and the focal length of the entire system at the telephoto end is fT.
0.53 <| f2 | × FNW / fM <0.84
The imaging apparatus having a rear focus zoom lens according to claim 1 or 2 , wherein the following condition is satisfied. The imaging apparatus having a rear focus type zoom lens according to claim 2 , wherein the forty-first group comprises a negative lens having both concave lens surfaces, and focusing is performed by moving the forty-first group. The forty-first group consists of negative lenses whose concave surfaces are concave, and the forty-second group consists of a positive lens whose convex surface is the image surface side, a meniscus negative lens whose convex surface faces the object side, and both lens surfaces The imaging apparatus having a rear focus type zoom lens according to claim 2 or 4 , comprising a positive lens having a convex surface, and focusing by moving the forty-first group. The rear focus type image pickup apparatus having a rear focus type zoom lens according to any one of claims 1 5, characterized in that it has a color separation optical system to the image plane side of the Zumuren's of.