JPH0430564B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to a zoom lens, and particularly to a focusing method for a zoom lens that includes a wide angle of view with a maximum angle of view exceeding 60° and has a relatively wide range of variable power from so-called wide-angle to semi-telephoto. (Background of the Invention) In recent years, various zoom lenses of this type for use in 35 mm still cameras have been proposed, such as Japanese Patent Application Laid-open No. 57-161804 and Japanese Patent Application Laid-open No. 57-161824. All of these are designed to be compact and have a high zoom ratio, and for this reason, a positive power lens group is used in the front focusing group to increase the zoom ratio. However, the positive focusing group has two drawbacks: the close distance cannot be made very short, and the aberrations vary greatly over the close distance. It is possible to shorten the close-up distance to some extent by increasing the diameter of the front lens or shortening the focal length of the front lens, but this would be at the expense of making the lens smaller and reducing fluctuations in aberrations. Therefore, a macro mechanism is usually added to expand the close-range shooting area. In addition, short-distance fluctuations in aberrations are difficult to correct because they conflict with the miniaturization of lens systems, so it is common to solve the problem by distributing residual aberrations between infinity shooting conditions and close-up shooting conditions. be. Therefore, for zoom lenses for 35mm still cameras, there is a strong demand for smaller lenses, even though aberration fluctuations at close distances are large for zoom lenses that are 3x or more, starting from a wider angle than 35mm. It has become unavoidable. Furthermore, when it comes to such high-power zoom lenses,
Not only does the close-up variation in the image plane become large, but also the variation in the image plane due to zooming becomes impossible to ignore. Therefore, in reality, both short-distance fluctuations and magnification fluctuations overlap, resulting in increasingly large image plane fluctuations.
Image performance will also become uneven within the zoom range. In order to correct image plane fluctuations due to zooming, in a limited zoom group configuration, the zoom lens system itself must be made larger, or a large amount of high refractive index glass is used to increase the radius of curvature of each lens surface. There is no other solution, but even if it were possible to suppress the image plane fluctuation due to magnification change to a good value, it would be extremely difficult to correct for short-distance fluctuations, and it would be nearly impossible to correct for both at the same time. . This short-distance fluctuation of the image plane becomes larger as the zoom lens becomes a higher magnification lens and the telephoto end becomes a longer focal length lens, and the main cause of this fluctuation is the focusing lens group (usually the first lens group). This cannot be corrected solely by improving. A significant improvement would be achieved by adopting the so-called focusing group floating method, which divides the focusing group into two lens groups and slightly changes the distance between each group to compensate, but in that case, the number of lenses would be reduced. This is likely to increase and become even bigger. If the number of lenses is limited, image performance will deteriorate due to an increase in spherical aberration and comatic aberration as the distance changes. Furthermore, in this case, the number of movable groups increases, which affects the processing accuracy of the lens barrel, which tends to cause eccentricity, tilting, etc., resulting in deterioration of image performance. (Objective of the Invention) An object of the present invention is to reduce aberration fluctuations caused by zooming and focusing, especially curvature of field aberration, while having a relatively wide magnification range including a wide angle of view and a compact shape. An object of the present invention is to provide a zoom lens that can satisfactorily correct fluctuations caused by focusing. (Summary of the Invention) As shown in FIG. 1, the present invention includes, in order from the object side, a first lens group G ₁ with positive refractive power, a second lens group G ₂ with added refractive power, and a third lens group with positive refractive power. a _fourth lens group G ₄ having positive refractive power;
In a zoom lens in which the third lens group forms an almost afocal system, as shown in Fig. 2, when focusing on a close object, the first lens group _G1 is aligned along the optical axis with respect to the image plane I. While moving toward the object side, the second lens group _G2 and the third lens group
G ₃ is moved along the optical axis toward the image side. FIG. 1 shows peripheral rays from an on-axis infinity object point and an off-axis infinity object point when the zoom lens of the present invention is focused at infinity. Also, the second
In the figure, the principal ray of the oblique beam when focusing at infinity is shown by a dotted line, and the principal ray of the oblique beam when focusing at close distance is shown by a solid line, and the first, second, and third lenses that move during focusing are shown. The arrangement of the groups (G ₁ , G ₂ , G ₃ ) when focused at infinity is shown by a two-dot chain line. According to the basic configuration of the present invention, not only short-range fluctuations in field curvature are significantly reduced, but also
Since the second lens group and the third lens group, which are close to the first lens group, move while maintaining the configuration for zooming, the structure of the lens barrel is simplified. Further, since both the second lens group and the third lens group are movable groups for changing the magnification, there is little risk of increasing mechanical errors such as eccentricity and tilting during movement for focusing. In this way, the present invention corrects the change in field curvature that occurs due to the advance of the front lens when focusing on a short-distance object, thereby maintaining good image performance over the entire image plane. However, if aberrations other than curvature of field, such as spherical aberration and coma aberration, fluctuate due to changes in the distance, the corrections made will become meaningless. Therefore, due to the change in spacing, spherical aberration,
It is important that there is no significant variation in coma aberration. It is also important that the focal length does not fluctuate, since it is inconvenient if the focal length changes due to a change in the distance. In order to satisfy these requirements at the same time, the distance between the third lens group and the fourth lens group may be set to a parallel beam system. That is, for this purpose, it is necessary to configure the first lens group to the third lens group to form an afocal system when focusing on infinity. By doing so, by moving the first lens group and the second lens group integrally, the focal length will not change even if the distance between the third and fourth lens groups changes, and the aberrations will be reduced. In this case, the salvage aberration does not change, and although there is a slight change in the coma aberration, this change follows the change in the curvature of field, so the initial objective can be fully achieved. As mentioned above, in order to create a parallel beam system between the third lens group and the fourth lens group and achieve afocal coupling, the focal length of the first lens group must be set to 1, and the focal length of the second lens group must be set to ₁ .
The focal length of the lens group is ₂ , the focal length of the third lens group is ₃ , the distance between the principal points of each group is D ₁ ,
When D ₂ , the zoom magnification system must be configured with a power arrangement that maintains the relationship _1/1 −D _{1 +1/3} −D ₂ =−1/ ₂ . It is desirable to keep this relational expression accurate, but even if it deviates slightly from the equation,
Since the amount of change in various aberrations due to the change in the spacing of the groups that are almost afocal coupled is very small, there is no large change in spherical aberration or focal length, which is sufficiently satisfactory for practical use. Here, we will add some consideration from aberration theory to the variation of field curvature due to zooming. Assuming that third-order spherical aberration, coma aberration, and astigmatism are the distance from the aperture position of the optical system to the target optical subgroup, then the aperture is now △ away from this optical subgroup. If it moves, the aberration variation caused by it is given as △=0 (1) △=-α△ (2) △=-2α△+(α△) ² (3). Here α is an almost constant proportionality constant. In the present invention, it is desirable to provide the diaphragm integrally with the third lens group, especially between the second and third lens groups. Similar aberration fluctuations occur because the distance of the first lens group from the aperture changes. In reality, in addition to this, the influence of the lens group interposed between the first lens group and the aperture diaphragm and the influence of changes in object distance result in complex aberration fluctuations, but most of the aberrations that cause image plane fluctuations is(3)
It can be seen that this is due to the formula. The present invention eliminates aberration fluctuations due to such focusing by a second method.
It has been discovered that the correction can be made by moving the lens group and the third lens group integrally. Aberration fluctuations during focusing due to movement of the first lens group cause the fourth lens group located behind the aperture to
It can be corrected by moving the lens group in the same direction at the same time, but when moving the fourth lens group, the back focus also changes, so
Not practical. Therefore, in the present invention, we have discovered that a function almost equivalent to that of the aberration correction by the fourth lens group can be achieved by integrally moving the second and third lens groups, and we have found that fluctuations in aberrations other than field curvature can be achieved by integrally moving the second and third lens groups. In order to prevent deterioration, the third lens group and the fourth lens group are afocal coupled. In the correction of aberration fluctuations during focusing according to the present invention, the amount of movement of the first lens group on the object side is S, and the integral movement of the second lens group and the third lens group on the image side is When the amount is △T, it is desirable that 0.1S≦△T≦0.35S (4). If the lower limit of the condition of equation (4) is exceeded, the effect of aberration correction during focusing becomes poor, and if the upper limit is exceeded, the imaging performance deteriorates due to excessive correction at the wide-angle end. (Example) Next, an example according to the present invention will be described. In both the first and second embodiments of the present invention, when changing the magnification from the wide-angle end w to the telephoto end _T , as shown in FIG. 3, the first lens group G ₁ and the fourth lens group G ₄
moves monotonically by almost the same amount toward the object side with respect to the image plane, and at the same time, the third lens group _G3 moves 0.4 to 0.8 times the amount of movement of the first and fourth lens groups _G1 and _G4 . While moving monotonically toward the object side, the second lens group
G ₂ is configured to move toward the object side at least near the wide-angle end. The fact that such a magnification variable system allows the diameter of the front lens to be reduced and is advantageous in downsizing the lens system is disclosed in a separate application (Japanese Patent Laid-Open No. 78114/1983) filed by the same applicant as the present application. Detailed. The first example is a 3x zoom lens for 35mm still cameras, including standard focal lengths from 35mm to 105mm, and the second example is a 3x zoom lens for 35mm still cameras.
It is a 5.7x zoom lens with a focal length of 35mm to 200mm for mm-format still cameras.

【table】

[Table] Figures 4 and 5 show the first example and the second example, respectively.
A lens configuration diagram of an example is shown, and Tables 1 and 2 below show specifications of the first and second examples. In each table, each value is shown sequentially from the object side, and the subscript number represents the order from the object side. FIG. 6 is a diagram showing variations in the meridional image plane at the 70% angle of view at each focal length at the wide-angle end _W and the intermediate _M telephoto end _T in the first embodiment. Curve a in the figure represents the variation due to magnification when the object is at infinity, and shows an inverted dogleg-shaped variation in which the image plane becomes under-image at the wide-angle end, over-image at the middle, and again under-image at the telephoto end. In this case, this fluctuation difference exists on the order of 0.3, and it is difficult to reduce it. Generally, if the aim is to make the lens system more compact, such fluctuation components will increase. Curve b moves only the first lens group toward the object side S=3.5mm
This is the variation in magnification when focusing on an object distance of 1.4 m by moving the lens by a distance of 1.4 m, and the difference between curves a and b, that is, the short-distance variation is about 0.4 to 0.5. On the other hand, curves c and d show the magnification fluctuations when the present invention is implemented in which the second lens group and the third lens group are simultaneously moved toward the image side when focusing on the same short-distance object. Curve c is the case when the second and third lens groups are moved toward the image side by △T=0.5mm (△T≒0.14S), and it becomes close to curve a, indicating that short-range fluctuations are considerably corrected. is clear. The curve d represents the second and third lens groups at △T=0.7mm (△T≒
0.2S) toward the image side, and it can be seen that the correction is even better. However, if more correction is made, the correction becomes excessive at the wide-angle end and the image performance deteriorates, so it is necessary to select an appropriate value within the range of the condition of equation (4). Figure 7 shows the variation in magnification from the wide-angle side to the telephoto side on the meridional image plane of the 70% field of view of the second embodiment, similar to Figure 6.Curve a shows the change in magnification when the object is at infinity. This changes to the state shown by curve b when only the first lens group is moved 4.4 mm toward the object side and focused at an object distance of 1.6 m. As you can see from the figure, the close-range fluctuations at the wide-angle end and the telephoto end are not uniform, and this is a phenomenon that appears due to the high zoom ratio of 5.7x. In this case, between the telephoto end and the middle, 0.6~
There is a short-range variation of about 0.9, and a variation of 0.05 at the wide-angle end. When the zoom ratio is as high as 5.7x as in this example, the fluctuations due to zooming become complicated and exhibit an inverted S-shaped fluctuation as shown in the figure. For this reason, it becomes even more difficult to correct not only magnification fluctuations of various aberrations but also short-distance fluctuations. Curve c shows the state when the second and third lens groups are simultaneously moved by 0.7 mm toward the image side according to the present invention when focusing on the same object, and while avoiding too much overcorrection at the wide-angle end, the telephoto This is an attempt to correct short-distance fluctuations at the edges as much as possible. Performing the correction according to the present invention in this way is very effective in practice, and the short-distance fluctuations of the image plane are greatly improved.
Good image performance can always be obtained across the entire screen over the entire magnification range. FIG. 8 shows various aberration diagrams when focusing on infinity in the first embodiment, and FIG. 9 shows various aberration diagrams when focusing on short distance (object distance 1.4 m). Figure 10 shows various aberration diagrams when focusing at infinity, and Figure 11 shows various aberration diagrams when focusing at close range (object distance 1.6 m). Each aberration diagram shows spherical aberration Sph, astigmatism (Ast), distortion aberration (Dis), and coma aberration (Coma) at wide-angle end A, intermediate B, and telephoto end C, respectively. The amount of violation of conditions is also indicated with a dotted line. From the respective aberration diagrams, it can be seen that in both examples according to the present invention, various aberrations are well corrected even in close-range shooting conditions, and in particular, the improvement of field curvature and astigmatism at the intermediate and telephoto ends is remarkable, and at the wide-angle end Although the astigmatism at short distances increases slightly, it is clear that the symmetry of the coma aberration is excellent and performance sufficiently good for practical use is maintained. (Effect of the invention) As described above, according to the present invention, the maximum angle of view is
It has a wide angle of view of more than 60°, an F number of around 3.5, and a high zoom ratio of 3x to 5.7x, while being compact overall and offering excellent imaging performance over the entire zoom range. Moreover, a zoom lens with less deterioration of aberrations even at short distances can be achieved. Moreover, in the focusing method according to the present invention, since the second and third lens groups that move during zooming are moved integrally, it is not only stable in terms of aberrations, but also advantageous in terms of the structure of the lens barrel.
Further, since the third lens group and the fourth lens group are parallel light beam systems, there is no change in back focus during focusing, and adverse effects on aberrations other than field curvature can be suppressed, which is extremely useful. In the above embodiment, the four lens groups constituting the zoom lens all move with respect to the image plane when changing the magnification, but the present invention provides a zoom lens in which only the second and third lens groups move when changing the magnification. It is also effective for lenses.

[Brief explanation of drawings]

1 and 2 are basic configuration diagrams according to the present invention,
FIG. 3 is a diagram showing the movement trajectory of each lens group for zooming in the embodiment according to the present invention, FIGS. 4 and 5 are lens configuration diagrams of the first and second embodiments, respectively, and FIG.
7 and 7 are diagrams showing the magnification fluctuations and short-distance fluctuations of the meridional image plane at a field angle of 70% for the first and second embodiments, respectively, and FIGS. 8 and 9 are diagrams for the first embodiment. 10 and 11 are diagrams of various aberrations during infinity focusing and short distance focusing. FIGS. 10 and 11 are diagrams of various aberrations during infinity focusing and short distance focusing in the second embodiment. Explanation of symbols of main parts, G ₁ ... first lens group,
G ₂ ...Second lens group, G ₃ ...Third lens group, G ₄
...Fourth lens group.

Claims (1)

[Scope of Claims] In order from the object side, it has a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, and a fourth lens group with positive refractive power, In a zoom lens in which the first, second, and third lens groups form an approximately afocal system, when focusing on a close object, the first lens group is aligned along the optical axis with respect to the image plane, and At the same time, the second lens group and the third lens group are integrally moved along the optical axis toward the image side, and the amount of movement of the first lens group for focusing is S, and the third lens group is integrally moved to the image side. A zoom lens including a wide angle of view, characterized in that the zoom lens satisfies the following condition: 0.1S≦ΔT≦0.35S, where ΔT is the amount of integral movement of the second lens group and the third lens group.