JP2590944B2 - Bifocal lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a bifocal lens, and more particularly to a bifocal lens applicable to a small camera such as a video camera or an electronic still camera.

2. Technical Background of the Invention and Prior Art In recent years, packaging of electronic components has been advanced and the integration ratio has been increased, so that the volume and weight of a lens occupying a main body of a video camera or the like have become relatively large. Also, regarding the cost, the lens system is a bottleneck in reducing the overall cost. In the case of a current video camera or the like, miniaturization, light weight, and low cost are absolute requirements, and in order to achieve this, it is important to make the optical system small and inexpensive. As a result of these pursuits, there are many cases in which a single lens is used instead of a zoom lens. However, this can certainly achieve small size, light weight and low cost, but the attractiveness as a product will be drastically reduced due to the single focus. Of course, telephoto and wide angle can be achieved using a converter or attachment, but it is necessary to carry it separately in addition to the camera, and it takes time to attach and detach it, so it is continuously zoomed in movie shooting etc. It is not easy to use as it is impossible. On the other hand, even if the specifications are slightly reduced when sticking to the zoom lens, the small size, light weight, and low cost are, in fact, incomparable when compared with the single focus lens. Therefore, a bifocal lens used in a lens shutter camera or the like in recent years can be considered. These can be zoomed in a short period of time, and their product appeal is high compared to single focus lenses. However, the method that has been put into practical use or the method that has been proposed is a type in which the main lens and the built-in converter lens are inserted in and out of the main lens, and the main lens moving mechanism and the converter lens high-precision insertion mechanism are used. Becomes very complicated, the cost becomes considerably higher than that of a single focus lens, and the accuracy control is severe due to the influence of manufacturing errors. In addition, the scaling factor is at most 1.5 to 2
In fact, the telephoto effect is still lacking.

SUMMARY OF THE INVENTION The present invention provides a bifocal lens which sufficiently satisfies small size, light weight and low cost, has a magnification of about 2 to 3 times, and can instantaneously change the focal length with a simple mechanism. The purpose is to:

SUMMARY OF THE INVENTION In order to achieve the above object, a bifocal lens according to the present invention employs a system completely different from a conventional converter system. That is, a configuration in which a certain lens block is attached and detached is not used, and the characteristics of the zoom lens are used to increase the magnification. Therefore, a method of moving a certain lens block on the optical axis is adopted. However, in a zoom lens, two or more lens units move, and a high-precision cam is required for the movement, which inevitably increases the cost. Accordingly, the present invention provides a bifocal lens that can realize zooming by simply moving only one lens group or a plurality of lens groups connected one-to-one by making good use of the features of a zoom lens. The method will be described with reference to FIGS. Generally, in a zoom lens, at least one variator lens group (V) that moves for zooming and another compensator lens group (C) that moves in a non-linear manner in order to correct the displacement of the image point due to the variator lens group exist. I do. As a simple case, FIG. 1 shows an example of the movement trajectories of a zoom lens having three groups of positive, negative, positive, and when the first group is a compensator lens group and the second group is a variator lens group. However, (a)
FIG. 4B is a schematic diagram of a telephoto end, FIG. 4B is a schematic diagram of an intermediate focal length, and FIG. The designer selects the best zoom solution to determine the movement trajectory depending on lens specifications, aberration correction, and other factors. Here, the zoom solution is devised so that the compensator lens group returns to substantially the same position at the telephoto end and the wide end as shown in FIG. This is used as a bifocal lens. That is, if only the telephoto end and the wide end are used, the compensator lens group is apparently immovable, so that simply changing the variator lens group does not cause the image point to move, and the magnification can be changed. This is the basic idea of the method of the present invention. FIGS. 1 and 2 show the case where the first unit is a compensator lens unit. However, even when the third unit is considered as a compensator lens unit, it can be constructed in exactly the same way. The same applies to a multi-component zoom lens.

As described above, according to this method, the variator lens group or the movement is moved, but the variator lens group can be easily changed simply by moving the plurality of variator lens groups connected one to one from the position corresponding to the telephoto end to the position corresponding to the wide end. Double is possible.
Since the movement is the simplest form, the movement mechanism may be very simple, and the cost is far lower than that of a zoom lens using a cam. On the other hand, since it is based on a zoom lens, it has the advantage of being resistant to high magnification, and does not require correction of aberrations in the intermediate focal length range, so that it is possible to considerably reduce the number of lenses as compared with a zoom lens. Needless to say, miniaturization is easy. In addition, the influence on the production error is considerably less than that of the zoom lens and the built-in converter attachment / detachment method, making it easy to manufacture and increasing the mass production rate, and it is possible to bring the cost close enough to the single focus lens area. is there. Since the moving mode is simple, magnification can be changed instantaneously, so that magnification can be changed even during moving image shooting.

Now, the advantage of the present invention is obvious, but in the actual design stage, the design is advanced while always fixing the zoom solution so that the compensator lens group is exactly the same at the tele end and the wide end. Is too restrictive. However, this constraint can be relaxed to the extent that the following condition is satisfied.

Here, ΔLc is a lens group corresponding to the compensator lens group, and when a zoom solution is obtained in relation to the variator lens group, the position of the compensator lens group at the tele end and the wide end in the optical axis direction is obtained. And c is the refractive power of the compensator lens group.

That is, the influence of the position error of the compensator lens group in the optical axis direction at the wide end on the fluctuation of the image point position is generally smaller than the error amount. In particular, when the first group is the compensator lens group, it is extremely difficult. Become smaller. That is, even if the position of the compensator lens group at the telephoto end and the wide end is slightly deviated from the zoom solution, even if the compensator lens group is shifted to the position at the telephoto end at the wide end, almost no image point movement occurs. It is not seen. As long as the range satisfies the expression, the movement of the image point can be sufficiently suppressed to within an allowable range, thereby greatly improving the design freedom. Conversely, if the value exceeds the range of this expression, the movement of the image point becomes large, so that some image point movement correction means is required.

Now, an example in which the present invention is realized in a specific form will be described. The present invention can be applied to any zoom lens. However, if the object is set to a 6 × zoom class, the cost and the size of the zoom lens cannot be avoided by this method as compared with a single focus lens. In addition, 6 of zoom lens from application
The magnification of 2 times is attractive, but as a bifocal lens, it changes too much and is hard to use, and has a feeling of over-spec. Here, a lens which is suitable for a bifocal lens of about two to three times as good in terms of convenience and cost and size as compared with a single focus lens will be described.

A two-component system can be considered first, but the two-component system has disadvantages in that the moving amount is large and it is difficult to realize a large magnification. An example of realizing such a bifocal lens in a two-component system is disclosed in Japanese Patent Application Laid-Open No. Sho 56-150716, but these magnifications are limited to at most twice.

Therefore, a multi-component system of three or more components can be considered. In particular, the first and second units are configured to have positive and negative refractive powers, respectively, and the second unit is a variator lens unit that moves on the optical axis during zooming. The magnification can be easily obtained, and the fluctuation of aberration due to zooming can be made sufficiently small. In order to achieve a sufficient zoom ratio and compactness, the following expression must be satisfied.

d _T ^I-II / d _W ^I-II > 3 where d _T ^I-II and d _W ^I-II are the air gaps of the first group and the second group at the telephoto end and the wide end, respectively. If the lower limit is exceeded, the zoom ratio will be small or the first lens unit will be large, and compactness cannot be achieved.

Furthermore, in practice, a three-component system is suitable for targeting a cost sufficiently close to a single focus lens. That is, the second group is a variator lens group having strong negative refracting power in order from the object side, and moves to the image side at the tele end position and to the object side at the wide end position. actually,
The first and third groups are stationary, but are to be compared with a zoom lens. Therefore, if a compensator lens group is assumed, it may be considered as the first group or the third group. Then, the following conditions are imposed on the refractive power relationship of these three groups.

_{0.28 <I / | II | <} 0.365 ( where _{II <0) 0.85 <III f} M <1.70 Here, _I , _II , and _III are the respective refractive powers of the first, second, and third groups, and f _T and f _W are the focal lengths on the tele side and the wide side, respectively.

When the zoom solution is bent by aberration correction when the solution considered in the thin system is actually made thicker, the movement locus is usually different from that in the case of the thin system. Therefore, in order to obtain a solution in which the compensator lens groups are almost at the same position at the telephoto end and the wide-angle end, the refractive power relationships of the respective groups must be within a certain range. Conversely, the refractive power relationship of a thin-walled system can be realized within a certain range, not only one certain solution is realized. This range is indicated by the condition.
It is difficult to obtain a zoom solution in which the compensator lens group is at the same position at the tele end and the wide end, and even if obtained, it is a very disadvantageous solution in terms of aberration correction and physical size. Therefore, cost increase and size increase are inevitable.

Now, the configurations of the first and second groups will be described in more detail. The first group and the second group greatly affect the aberration variation due to zooming, and in particular, the first group is large in weight.
By configuring these as follows, aberration variation is small, compact and lightweight, and very low cost.
1, the second group can be realized.

That is, in order from the object side, the first lens unit has a negative meniscus lens having a strong surface facing the image side, the biconvex positive lens having the strong surface facing the object side, and the second lens unit has the strong surface facing the image side. It is composed of a negative lens and a positive meniscus lens having a strong surface facing the object side, and satisfies the following conditions.

0.05 <R _1P / | R _2R | <0.43 (where R _2R <0) 0.38 <R _3R | _II |> 0.75 (where _II <0) (n ₁ + n ₄ ) / 2> 1.78 where R is the radius of curvature The first suffix indicates the lens number counted from the object side, and the second suffix indicates P when the surface is on the object side and R when the surface is on the image side. Further, n indicates a refractive index, and a subscript indicates a lens number counted from the object side.

Even if the first and second units are composed of only four lenses in this way, good aberrations can be obtained because both are a combination of a negative lens and a positive lens, and the negative lens is of a leading type, so that the incident angle is large. Ambient light as quickly as possible to reduce the inclination for image formation
By leading the light to the group, it is possible to sufficiently reduce the coma aberration and the field curvature of the peripheral light, and it is also useful for correcting the spherical aberration. In addition, since the peripheral light having a large incident angle is bent so as to loosen the inclination as quickly as possible, it is possible to reduce the diameter of the front lens (in particular, the effective diameter of the first-group object-side lens). Since the biconvex positive lens is used as the image side lens of the first group, the effect of reducing the diameter of the front lens is further enhanced. Further, since the color correction can be performed for each group by selecting the dispersion of the glass material, the fluctuation of the axial chromatic aberration and the chromatic aberration of magnification can be made sufficiently small.

The condition corresponds to a condition that gives a ratio between the refractive power on the object side and the refractive power on the image side when the entire first unit is regarded as one positive lens, and greatly affects the aberration of peripheral light. In particular, if the light outside the principal ray of the peripheral light at the telephoto end exceeds the lower limit, the amount of upper coma generated when the lower limit is exceeded, and the amount of lower coma generated above the upper limit is large, and sufficient performance cannot be guaranteed.

The condition is an expression indicating how much a strong refracting surface of the negative lens has a refracting power in the second group, and particularly affects spherical aberration and field curvature at the telephoto end. If the lower limit is exceeded, a large spherical aberration and curvature of field will occur on the over side, and if the upper limit is exceeded, a large spherical aberration and field curvature will occur, making it impossible to guarantee the performance.

The condition indicates that the negative lens of the first group and the positive lens of the second group need to be made of a glass material having a sufficiently high refractive index,
If this is not satisfied, the occurrence of higher-order aberrations cannot be suppressed.

In order to satisfy the conditions described above, it is inexpensive with only four sheets, lightweight and compact, and has a small aberration variation.
1, the second group can be provided, but the whole system is cheaper,
In order to provide a bifocal lens having high performance while having a large aperture, it is desirable that the third unit be configured by selecting one of the following two methods and following it.

That is, as a first method, the third lens unit is a positive lens having a strong refracting surface facing the object side in order from the object side, a biconcave lens, a positive lens having the strong refracting surface facing the image side, and a strong refracting surface facing the object side. It consists of four positive lenses and satisfies the following conditions.

_{1.0 <A 1 / A 2 0.60} <f 7 7R <1.30 _{where, 7R = (n 7 -1)} / | R 7R | 0.70 <f 8 · 8P <1.6 _{where, 8P = (n 8 -1)} / | R _8P | where A ₁ and A ₂ are the fifth and sixth lenses, respectively.
And the axial air gap between the sixth and seventh lenses, and f ₇ and f ₈ are the single focal lengths of the seventh and eighth lenses.

With such a positive, negative, positive and positive configuration, it is possible to secure a compact but sufficiently long back focus and to easily correct various aberrations.

The condition is to indicate the way of the axial air space before and after the sixth lens, which is the only negative lens in the third group. Always make the space on the object side larger and make the space on the image side smaller. Indicates that it is desirable. Distortion and chromatic aberration of magnification largely vary depending on the arrangement of the sixth lens. If the lower limit of this equation is exceeded, the distortion is greatly negative and the chromatic aberration of magnification becomes large, making it impossible to correct.

The condition indicates that the positive refractive powers of the seventh lens and the eighth lens are concentrated on the image side surface of the seventh lens and the object side surface of the eighth lens, respectively. When the shape becomes 1 or more, the shape becomes a meniscus shape. The optimal shape of both lenses is determined by the balance between spherical aberration and off-axis curvature of field, coma, and distortion. If both convex and concave are less than the lower limit, it is advantageous for spherical aberration, but off-axis aberration When the meniscus shape is strengthened beyond the upper limit, spherical aberration is worsened, and the off-axis distortion is greatly deteriorated.

Apart from these, the third group may be configured as follows as a second method. That is, the third lens unit includes a positive lens having a strong refractive surface facing the image side, a positive lens having a strong refractive surface facing the object side, a biconcave lens having a strong refractive surface facing the image side, and a biconvex lens. And the following conditions are satisfied.

_{0.05 <R 7R / | R 7P} | <0.50 ( where, R _7R <0), where _{0.20 <T 7 / f W <} 0.60, T 7 is the axial thickness of the seventh lens.

With such a positive, positive, negative, and positive configuration, the third unit can be made very compact, and the negative refracting surface can be arranged very rearward, so that it can be made very strong. Petzval sum can be controlled sufficiently well.

The condition relates to the refractive power balance of the front and rear surfaces of the biconcave seventh lens, and when the refractive power of the rear surface is increased beyond the lower limit,
If the generation of high-order coma aberration increases, and if the refractive power exceeds the upper limit and the front surface also has a large refractive power, spherical aberration will be overcorrected and distortion will be negatively generated.

The condition is related to the on-axis thickness of the seventh lens, and it is desirable that the thickness is at least a certain value in order to favorably correct off-axis coma and field curvature. If the thickness is less than the lower limit, these cannot be corrected. On the other hand, if the thickness exceeds the upper limit, the performance is improved, but sufficient back focus cannot be secured and practical use cannot be achieved.

With the above-described configuration, a bifocal lens with a magnification of about 2 to 3 times can be realized by adding a small number of lenses to a single focus lens, and as compared with a conventional built-in converter type bifocal lens, the magnification is small. Are much higher, high performance, a sufficiently bright F-number is obtained at both the wide end and the telephoto end, and the zoom mechanism is very inexpensive and loose in accuracy. Compared to a zoom lens, the number of components is much smaller, it is very compact, it does not require a high-precision mechanism such as a cam for movement, it is easy to manufacture, etc. Is several steps higher.

Embodiments of the Present Invention Hereinafter, embodiments of a large-diameter, small-sized, light-weight, and low-cost high-performance bifocal lens according to the present invention will be described.

However, in each embodiment, r _{1 to} r ₂₁ are the radii of curvature, d _{1 to} d
₂₀ shows an axial _{distance, N 1 ~N 20, ν 1} ~ν 20 shows a refractive index at the d-line, the Abbe number. In each embodiment, a flat plate corresponding to a low-pass filter or a face plate is inserted at the end.

<Example 1> <Example 2> <Example 3> <Example 4> <Example 5> <Example 6> <Example 7> As described above, FIG. 1 schematically shows a zooming state in the case of a general three-component zoom, and FIG. 2 shows a schematic diagram of a special zoom solution used in the present invention. is there.

FIG. 3 shows a schematic configuration at the telephoto end of a lens configuration corresponding to the above-described first to fifth embodiments, FIG. 4 corresponds to the sixth embodiment, and FIG. 5 corresponds to the seventh embodiment. The schematic configuration at the telephoto end of the lens configuration is shown.
FIGS. 3 to 5 show R _{1P to} R _8P , A ₁ , A ₂ , and T ₇ described in the outline of the present invention and r ₁ to r ₂₁ described in the embodiment of the present invention. , are to fill in for d ₁ ~d ₂₀ or the like.
In the second group (II), the movement from the telephoto end (T) to the wide end (W) is schematically indicated by an arrow line.
(3) shown in front of the third group (III) represents an aperture, and a flat plate (4) arranged behind the third group (III)
Is a flat plate corresponding to a low-pass filter or a face plate.

FIGS. 6 to 12 are aberration diagrams of Examples 1 to 7, wherein (a) shows various aberrations at the telephoto end and (b) shows various aberrations at the wide end. The solid line (d) represents the aberration with respect to the d-line, and the dotted line (SC) represents the sine condition. Further, a dotted line (DM) and a solid line (DS) represent astigmatism on the meridional surface and the sagittal surface, respectively.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a zooming state of a general three-component zoom system. FIG. 2 is a schematic diagram of a special zoom solution of a three-component zoom used in the present invention. FIG. 3 shows the first embodiment.
FIG. 4 is a schematic lens configuration diagram corresponding to Example 5, FIG. 4 is a schematic lens configuration diagram corresponding to Example 6, and FIG. 5 is a schematic lens configuration diagram corresponding to Example 7. FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11, and FIG.

Claims (3)

(57) [Claims] 1. 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.
The zoom lens can be zoomed with little movement of the image point by moving only the second lens unit by a fixed amount in the optical axis direction, and the first lens unit is sequentially moved from the object side to the image side. A negative meniscus lens having a strong surface facing the object, and a biconvex positive lens having a strong surface facing the object side. The second group is configured such that the strong surface faces the image side in order from the object side. A bifocal lens comprising a negative lens and a positive meniscus lens having a strong surface facing the object side, and satisfying the following conditions: d _T ^I-II / d _W ^{I- II} > 3 0.28 < _I / | _II | <0.365 ( _II <0) 0.85 < _III f _M <1.70 0.05 <R _1P / | R _2R | <0.43 (where R _2R <0) 0.38 <R _3R | _II | <0.75 (where _II <0) (n ₁ + n ₄ ) / 2> 1.78 where d _T ^{I− II:} axial air distance between the first and second lens units at the telephoto end, d _W ^I-II: axial air distance between the first and second lens units at the wide end, _I: first the refractive power of the group, _II: refractive power of the second group, _III: refractive power of the third group, f _T: the focal length of the telephoto side, f _W: focal length of the wide side, R _1P: first position from the object side radius of curvature of the object side surface of the lens, R _2R: curvature of the image side surface of the second lens from the object side radius, R _3R: curvature of the image side surface of the third lens from the object side radius, n _1: the object side , N ₄ : the refractive index of the fourth lens from the object side. 2. A third lens unit comprising, in order from the object side, a positive lens having a strong refractive surface facing the object side, a biconcave lens, a positive lens having the strong refractive surface facing the image side, and a strong refractive surface. 2. The lens system according to claim 1, wherein the first lens and the second lens are configured to include four positive lenses directed to the object side and satisfy the following conditions.
_{Lens; 1.0 <A 1 / A 2} 0.60 <f 7 7R <1.30 { where _{7R = (n 7 -1) /} | R 7R |} 0.70 <f 8 8P <1.60 { where _8P = (n ₈ -1) / | R _8P |｝ where A ₁ : axial air gap between the fifth lens from the object side and the sixth lens from the object side, A ₂ : the sixth lens from the object side On-axis air distance from the object side to the seventh lens, f ₇ : single focal length of the seventh lens from the object side, f ₈ : single focal length of the eighth lens from the object side, R _7R : radius of curvature of the image side of the seventh lens from the object side, R _8P : radius of curvature of the object side of the eighth lens from the object side, n ₇ : refractive index of the seventh lens from the object side, n ₈ is the refractive index of the eighth lens from the object side. 3. A positive lens having a strong refracting surface facing the image side, a positive lens having a strong refracting surface facing the object side, and a strong refracting surface facing the image side. 2. A biconcave lens and a biconvex lens, each comprising four lenses, satisfying the following conditions.
_{Lens; 0.05 <R 7R / | R} 7P | <0.50 ( where R _7P <0) 0.20 <where _{T 7 / f W <0.60,} R 7R: curvature of the image side surface of the seventh lens from the object side radius , R _7P: curvature of the object side surface of the seventh lens from the object side radius, T _7: is the axial thickness of the seventh lens from the object side.