JPH07333501A - Bifocal changeover lens system - Google Patents

Abstract

PURPOSE:To provide a lens system capable of mutually changing over two focal distances and perform focusing operation with a simple mechanism by composing the system with two lenses and satisfying a specific condition. CONSTITUTION:A first lens L1 having a negative refractive power and a second lens L2 having a positive refractive power are arranged in order from the object side. By fixing the first lens L3, moving the second lens L2 along the optical axis and adjusting the interval between the lenses L1, L2, the focal distance of the lens system is changed from either one of the wide-angle side and the telescopic side to the other and focusing at each focal distance is performed. Here, two conditions: 0.6<f2/fw<0.8, 4.0<fw/R3<7.5 are satisfied. In the relations, f2 represents the focal distance of the second lens L2, fw the focal distance of the lens system at the wide-angle side and R3 the radius of curvature of the second lens L2 on the surface on the object side. In this case, the position of a diaphragm is arranged on the image side of the second lens L2 and the position of the diaphragm is preferably moved together with the second lens L2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bifocal switching lens system suitable for use in still cameras, electronic still cameras and the like.

[0002]

2. Description of the Related Art Conventional bifocal switching lens systems have been popularized mainly for compact cameras as a camera for easily enjoying a change in angle of view. But,
In order to increase the wide-angle / telephoto ratio, such a bifocal switching lens system adopts a converter system that switches to the telephoto side by adding a converter lens to the master lens system used only on the wide-angle side. Was the most. Among them, the type in which a master lens system having a positive refractive power is arranged on the object side is the mainstream.

[0003]

However, such a problem
Since the master lens system itself moves when the focus switching lens system is switched to the telephoto side, it is necessary to install a complicated moving mechanism inside the camera. In addition to the lens system, it was necessary to install a focus mechanism inside the camera. As described above, when the conventional bifocal switching lens system is used, it is necessary to install a complicated mechanism in the camera,
There was also a limit to miniaturization.

The present invention has been made in order to solve the above-mentioned problems, and a two-focal point capable of switching between the two focal lengths of the wide-angle side and the telephoto side with a simple mechanism and also performing a focusing operation. It is intended to provide a switching lens system.

[0005]

In order to solve the above-mentioned problems, the bifocal switching lens system of the present invention comprises, in order from the object side, a first lens having a negative refracting power and a positive refracting power. The second lens has a configuration in which the first lens is fixed and the second lens is moved along the optical axis to adjust the distance between these lenses, whereby the focal length of the lens system is Focusing is performed at each focal length while switching from either the wide-angle side or the telephoto side to the other,
The following conditional expression (a) 0.6 <f ₂ / f _W <0.8 (b) 4.0 <f _W / R ₃ <7.5 (where f ₂ is the focal length of the second lens, f _W is the focal length on the wide angle side of the lens system, and R ₃ is the radius of curvature of the object side surface of the second lens.).

In the bifocal switching lens system of the present invention, the diaphragm position is located on the image side of the second lens, and this diaphragm position should move together with the second lens. Also, the second
Both the object side surface and the image side surface of the lens are preferably aspherical.

[0007]

FIG. 1 is a diagram for explaining the switching and focusing of the two focal points on the wide-angle side and the telephoto side in the two-focus switching lens system of the present invention. As shown in this figure, in the lens system of the present invention, when the focal point is switched, the negative first lens L ₁ arranged on the object side is fixed, and only the positive second lens L ₂ arranged on the image side is light. Move along an axis.

When the second lens L ₂ moves toward the first lens L ₁ , the combined focal length f of the lens system, that is, the focal length f of the zoom lens gradually increases. In addition, the first
Since the lens L ₁ is fixed, the focal position of the lens system moves according to the movement of the second lens L ₂ .

Here, the following expression f ₁ =-(f _W · f _T ) ^1/2 (1) (f ₁ is the focal length of the first lens L ₁ , and f _W is the focal length on the wide angle side of the lens system. , F _T is the focal length on the telephoto side of the lens system.), The second lens L ₂ is at a different position and the focal length is different, as shown in FIG. There are two lens states in which the focal positions are the same (hereinafter, the one with a long focal length is called telephoto and the one with a short focal length is called wide angle).

When the second lens L ₂ is moved to the first lens L ₁ side starting from the wide angle, the focal length of the lens system monotonically increases. On the other hand, as shown by the alternate long and short dash line in FIG. 1, the focus position on the image forming screen is shifted toward the first lens L ₁ side, and then passes through the position where the distance from the first lens is minimum, and then the image forming surface. Approach. When the focal point is located on the image plane again, the lens state is telephoto.

By arranging the image forming positions at the wide angle and the telephoto on the image forming screen, the present invention has a simple structure in which two lenses are arranged, but the two focal points on the wide angle side and the telephoto side are in focus. The bifocal lens system is realized. Then, by fixing the first lens L ₁ and moving only the second lens L ₂ , these two focal points can be switched to each other. However,
In the process of transitioning from wide angle to telephoto from one to the other
Equation (1) is not satisfied, and the focus position will deviate from the image formation screen.

Further, according to the bifocal switching lens system of the present invention, focusing (focusing) can be performed by moving the second lens L ₂ . This will be explained as follows.

As described above, when the lens system of the present invention has a wide angle, the focus position, which is the image forming position of the object point at infinity, is
By the movement of the second lens L ₂ , the second lens L ₂ is gradually moved to the object point side.
On the other hand, when the object point distance becomes shorter, the image forming position moves toward the image side, opposite to the direction of focus movement. By using this relationship, when the second lens L ₂ is moved to the object side from the arrangement at the wide angle, an object point having a shorter finite distance than the object point is imaged on the imaging plane.

Therefore, by moving the second lens L ₂ from the arrangement at the wide angle and adjusting the position thereof, a finite distance (closest point) determined from various conditions of the lens from infinity.
An object point located up to can be imaged on the imaging plane, that is, a focusing operation can be performed.

When the lens system of the present invention is in the telephoto state, focusing is performed by moving the second lens L ₂ and bringing it closer to the first lens L ₁ depending on the image forming relationship between the first lens L ₁ and the second lens L _2. Is possible, and there is a case where focusing can be performed by moving the second lens L ₂ away from the first lens L ₁ . In any case, as in the case of the wide angle, the first lens L ₁ is fixed and the second lens L ₁ is fixed.
Focusing operation is realized by moving only ₂ .

As described above, the bifocal switching lens system of the present invention has a simple construction in which two lenses are arranged, but the first lens L ₁ is fixed and only the second lens L ₂ is fixed. By moving along the optical axis, the two focal points on the wide-angle side and the telephoto side can be switched. Moreover, by moving the second lens L ₂ from the lens arrangement at the time of wide angle or telephoto, focusing can be performed so that the object point located at the distance from infinity to the closest point is imaged on the imaging screen. it can.

Next, conditional expression is satisfied the lens system of the present invention _{(a) 0.6 <f 2 /} f W <0.8 f 2: the focal length f _W of the second lens: the lens system at the wide-angle side of the Focal length is a formula related to optical performance and compactness of a lens system. If f ₂ / f _W exceeds the upper limit of equation (a), good optical performance can be maintained from wide-angle to telephoto even during focusing, but the overall length of the lens system becomes large and a compact lens system is obtained. Difficult to do. f ₂
When / f _W falls below the lower limit of the expression (a), the distance between the first lens L ₁ and the second lens L ₂ becomes small and the lens system becomes compact, but the refracting power of the second lens L ₂ becomes large. The optical performance in wide-angle and telephoto, as well as the optical performance during focusing, deteriorates significantly.

Further, the conditional expression (b) satisfying the lens system of the present invention is 4.0 <f _W / R ₃ <7.5 f _W : focal length on the wide angle side of the lens system R ₃ : second lens L ₂ The radius of curvature of the object side of is related to the optical performance of the lens system during focusing. When f _W / R ₃ exceeds the upper limit of the equation (b), spherical aberration and field curvature largely change due to focusing, which is not preferable. If f _W / R ₃ is below the lower limit of equation (b),
Although the fluctuation of aberration due to focusing becomes small, the curvature of field itself becomes large and the optical performance deteriorates.

Further, in the present invention, the diaphragm position is the second position.
If it is arranged on the image side of the lens and the diaphragm position is moved together with the second lens, the first and second lenses can be made into a meniscus lens having a convex surface facing the object side. It is possible to excellently correct field curvature, coma and distortion.

If the object side surface and the image side surface of the second lens are both aspherical, the second lens can have a strong refractive power even though it is a single lens.
The optical performance of the lens system is improved. Specifically, if the object side surface of the second lens is an aspherical surface, spherical aberration is corrected,
Further, when the image side surface is aspherical, coma aberration, lateral chromatic aberration, and the like caused by the object side surface being aspherical surface are favorably corrected.

The method of deriving equation (1) is shown below.

When the distance between the principal points of the first lens L ₁ and the second lens L ₂ is d _P , the synthetic focal length f of the lens system is
It is shown as follows.

1 / f = 1 / f ₁ + 1 / f ₂ −d _P / (f ₁ · f ₂ ) (2) where f ₁ is the focal length of the first lens L ₁ and f ₂ is the second
It is the focal length of the lens L ₂ .

When the equation (2) is modified, d _P = (1 / f ₁ + 1 / f ₂ −1 / f) · f ₁ · f ₂ (3) On the other hand, the back focus of the entire system in paraxial calculation and the length and l, l = f (1- d P / f 1) is a ... (4).

Next, assuming that the total length of the zoom lens in paraxial calculation is L, and using equations (3) and (4), L = l + d _P = f ₁ + 2f ₂ − (f ₁ ² + f ² ) · f _It is expressed as ₂ / (f ₁ · f) (5).

The above is the relational expression in the thin lens system, but the difference from the actual thick lens system is slight, so in practice there is a problem even if discussion is made using L in Eq. (4). Absent.

Since the total length L _{W at} wide angle and the total length L _{T at} telephoto are the same, L _W = L _T is solved with the focal length at wide angle as f _W and the focal length at telephoto as f _T. Then, the following formula is obtained. f ₁ = − (f _W · f _T ) ^1/2 (1) This is a conditional expression for making the total length L _W in the wide angle and the total length L _T in the telephoto the same (of the formula (1)). Derivation end.).

[0028]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

Example 1 FIG. 2 is a sectional view showing a bifocal switching lens system of Example 1. As shown in FIG. The lens system includes, in order from the object side, a first lens group G ₁ first lens L _1, and the as second lens group G ₂ and the second lens L ₂ is arranged, further diaphragm position D ₁ of the lens L ₂ It is arranged on the image forming surface S ₁ (film surface) side.

In FIG. 2, R _{1 to} R ₄ indicate the radii of curvature of the respective lens surfaces of the lenses L ₁ and L ₂ , and the surface numbers at the lower right of R are the surface numbers on the respective lens surfaces in order from the object side. Is. d _{1 to} d ₃ indicate the distances between the lens surface having the lower right number as the surface number and the lens surface having the next number as the surface number. For example, d ₁ indicates the distance between lens surface 1 and lens surface 2, that is, the lens thickness of lens L ₁ , and d ₂ indicates the distance between lens surface 2 and lens surface 3, that is, between lens L ₁ and lens L ₂ . The distance between lenses is shown.

The lens L ₁ is a negative meniscus lens having a convex surface facing the object side, and the lens L ₂ is a positive meniscus lens having a convex surface facing the object side. The focal length f ₁ of the lens L ₁ is -27.11Mm, the focal length of the lens L ₂ f ₂
Is 15.99 mm, and the focal length ratio (f ₁ / f ₂ ) is
Is -1.695.

The diaphragm position D ₁ is arranged at a position of 0.70 mm from the image side surface (surface number 4) of the lens L ₂ toward the image forming surface S ₁ side. The image forming surface S ₁ is concave, and its radius of curvature is −400.0 mm.

The numerical values of the respective parameters of the lens system of this embodiment are as follows.

[0034]

[Table 1]

The numbers at the left end of the table indicate numbers attached to the lower right of the radius of curvature R and the surface distance d, and the right end of the table corresponds to which lens the refractive index N _d and the Abbe number ν _d correspond to. Is showing.

In this embodiment, _both the object side surface (surface number 3) and the image side surface (surface number 4) of the lens L ₂ are aspherical surfaces. Its shape is

[0037]

[Equation 1]

In, each coefficient c, k, a ₁ , a ₂ , a
The values of ₃ and a ₄ are shown in the table below.

[0039]

[Table 2]

Further, the focal length f of the lens system at the wide angle and the telephoto, the back focus length l, the F number, the angle of view 2ω
Etc. are shown below.

[0041]

[Table 3]

The lens system of the present embodiment, the focal length at the wide-angle lens system f _W, when the focal length of the f _T telephoto lens _{system, (f W · f T)} 1/2 = -27 .11 and f ₁ = −27.11 as described above, the above formula (1) f ₁ = − (f _W · f _T ) ^1/2 (1) is satisfied.

Further, in this embodiment, f ₂ / f _W = 0.7
614, f _W / R ₃ = 4.4823, and the above (a)
And equation (b) is satisfied.

When switching from wide-angle to telephoto, lens L
₁ is fixed, the lens L ₂ and the diaphragm position D ₁ move integrally toward the object side, and the inter-lens distance d ₂ between the lenses L ₁ and L ₂ gradually decreases. As a result, the focal length of the lens system gradually increases, and the lens system switches from wide angle to telephoto. To switch from telephoto to wide-angle, the lens L ₂ may be moved in the opposite direction. The telephoto / wide-angle ratio (f _T / f _W ) of the lens system of this example is 1.66.
7

In this embodiment, in wide angle or telephoto,
The focusing operation is performed by moving the second lens L ₂ toward the object side along the optical axis. The movement amount of the second lens L ₂ for obtaining the focus amount of 1 m is 1.
It is 547 mm and 1.546 mm at telephoto.

FIG. 3 is an aberration diagram showing spherical aberration, field curvature and distortion in the lens system of this example. As shown in this aberration diagram, the lens system of the present example has various aberrations corrected well from wide angle to telephoto, and can be suitably used as a photographic lens.

Example 2 FIG. 4 is a sectional view showing a bifocal switching lens system of Example 2. As shown in FIG. In the same manner as in Example 1, in this lens system, the first lens L ₁ and the second lens L ₂ are arranged in order from the object side, and the diaphragm position D ₂ is the imaging plane S ₂ (film surface) of the lens L _2. ) Side is arranged and configured.

The lens L ₁ is a negative meniscus lens having a convex surface facing the object side, and the lens L ₂ is a positive meniscus lens having a convex surface facing the object side. The focal length f ₁ of the lens L ₁ is -30.17Mm, the focal length of the lens L ₂ f ₂
Is 17.36 mm, and the focal length ratio (f ₁ / f ₂ ) is
Is -1.7379.

Similar to the first embodiment, the diaphragm position D ₂ is arranged at a position of 0.70 mm from the image side surface (surface number 4) of the lens L ₂ toward the image forming surface S ₂ side. The image plane S ₂ is concave, and its radius of curvature is −400.0 mm.

The numerical values of the parameters of the lens system are as follows.

[0051]

[Table 4]

_Both the object side surface (surface number 3) and the image side surface (surface number 4) of the lens L ₂ are aspherical surfaces. The shape is obtained by using the above-mentioned aspherical expressions in which the respective coefficients c, k, a ₁ , a ₂ ,
the value of a _3, a ₄ are represented by those shown in the table below.

[0053]

[Table 5]

Further, the focal length f of the lens system at the wide angle and the telephoto, the back focus length l, the F number, the angle of view 2ω
Etc. are shown below.

[0055]

[Table 6]

In the lens system of the present embodiment, (f _W · f _T ) ^1/2 = −30.17, and as described above, f ₁ = −30.17. Therefore, the above formula (1) is used. f ₁ =-(f _W · f _T ) ^1/2 (1) is satisfied.

In this embodiment, f ₂ / f _W = 0.6
677, f _W / R ₃ = 6.7958, and the above (a)
And equation (b) is satisfied.

Also in the lens system of this embodiment, the focus switching operation is the same as in the first embodiment. The telephoto / wide-angle ratio (f _T / f _W ) of the lens system is 1.346. On the other hand, the wide-angle focusing operation is performed by moving the second lens L ₂ toward the object side, and the focusing operation in the telephoto state is performed by moving toward the image side. Also,
The amount of movement of the second lens L ₂ to obtain a focus amount of 2.5 m is 1.84 mm in wide angle and 2.84 in telephoto.
It is 21 mm.

FIG. 5 is an aberration diagram showing spherical aberration, field curvature and distortion in the lens system of this example. As shown in this aberration diagram, also in this embodiment, various aberrations are satisfactorily corrected from the wide angle to the telephoto.

Example 3 FIG. 6 is a sectional view showing a bifocal switching lens system of Example 3. Similar to the above embodiment, this lens system has a negative meniscus lens L whose convex surface faces the object side in order from the object side.
₁ and a positive meniscus lens lens L ₂ having a convex surface facing the object side is arranged, and the diaphragm position D ₃ is set to the lens L _2.
Is arranged on the image forming surface S ₃ (film surface) side. The focal length f ₁ of the lens L ₁ is −34.21 mm,
The focal length f ₂ of the lens L ₂ is a 19.17Mm, the ratio of the focal length (f ₁ / f ₂₎ is -1.7846.

The diaphragm position D ₃ is arranged at a position of 0.50 mm from the image side surface (surface number 4) of the lens L ₂ toward the image forming surface S ₃ side. The image plane S ₃ is a flat surface.

The numerical values of each parameter of the lens system are as follows.

[0063]

[Table 7]

[0064] In this embodiment, the object side surface (surface number 1) of the lens L ₁ and the image side surface (surface number 2), and the lens L ₂
The object side surface (surface number 3) and the image side surface (surface number 4) of are all aspherical surfaces. Its shape is the above-mentioned aspherical expression,
The values of the respective coefficients c, k, a ₁ , a ₂ , a ₃ and a ₄ are represented by the values shown in the table below.

[0065]

[Table 8]

Further, the focal length f of the lens system at the wide angle and the telephoto, the back focus length l, the F number, and the angle of view 2ω.
Etc. are shown below.

[0067]

[Table 9]

In the lens system of this example, (f _W · f _T ) ^1/2 = −34.21, and as described above, f ₁ = −34.21. Therefore, the above formula (1) is used. f ₁ =-(f _W · f _T ) ^1/2 (1) is satisfied.

In this embodiment, f ₂ / f _W = 0.7
373, f _W / R ₃ = 6.2228, and the above (a)
And equation (b) is satisfied.

Also in the lens system of this embodiment, the focus switching operation is the same as in the first and second embodiments. The telephoto / wide-angle ratio (f _T / f _W ) of the lens system is 1.7307.
In addition, the focusing operation in the wide angle is performed by the second lens L.
₂ is moved to the object side, and the focusing operation at telephoto is performed by moving it to the image side. The amount of movement of the second lens L ₂ to obtain a focus amount of 1 m is 2.463 mm in wide angle and 3.646 mm in telephoto.

FIG. 7 is an aberration diagram showing spherical aberration, field curvature and distortion in the lens system of this example. As shown in this aberration diagram, also in this embodiment, various aberrations are satisfactorily corrected from the wide angle to the telephoto.

[0072]

As described above in detail, according to the present invention,
The focus switching lens system has a simple structure including two lenses, and moreover, the focus switching operation as well as the focusing operation can be performed only by fixing the first lens and moving only the second lens. For this reason, the mechanism for switching the focus only needs to regulate the cams in the rear group, and the number of parts can be reduced to simplify the mechanism. Furthermore, since the lens system of the present invention satisfies the above formulas (a) and (b), it is compact and has good optical performance, and is suitable for use as a photographic lens. is there. Therefore, by using the bifocal switching lens system of the present invention, an inexpensive and compact bifocal switching still camera can be easily realized.

[Brief description of drawings]

FIG. 1 is a diagram for explaining switching and focusing of bifocals on a wide angle side and a telephoto side in a bifocal switching lens system of the present invention.

2 is a cross-sectional view showing a bifocal switching lens system of Example 1. FIG.

FIG. 3 is an aberration diagram of a bifocal switching lens system of Example 1.

FIG. 4 is a sectional view showing a bifocal switching lens system of Example 2.

FIG. 5 is an aberration diagram of a bifocal switching lens system of Example 2.

FIG. 6 is a sectional view showing a bifocal switching lens system of Example 3.

FIG. 7 is an aberration diagram of a bifocal switching lens system of Example 3.

[Explanation of symbols]

L ₁ ... the first lens, L ₂ ... the second lens, R ₁ to R ₄ ... curvature radius of each lens surface, the distance between each side of d ₁ to d ₃ ... lens, D ₁ to D ₃ ... throttle position, S _{1 to} S ₃ ... Image plane.

Claims (3)

[Claims] 1. A structure in which a first lens having a negative refractive power and a second lens having a positive refractive power are arranged in this order from the object side, and the first lens is fixed and the second lens is fixed. By moving the lens along the optical axis and adjusting the distance between these lenses, the focal length of the lens system can be switched from one of the wide-angle side and the telephoto side to the other and focusing at each focal length can be achieved. The following conditional expression (a) 0.6 <f ₂ / f _W <0.8 (b) 4.0 <f _W / R ₃ <7.5 (where f ₂ is the second lens The focal length, f _W is the focal length on the wide angle side of the lens system, and R ₃ is the radius of curvature of the object side surface of the second lens.). 2. The bifocal switching lens system according to claim 1, wherein a diaphragm position is arranged on the image side of the second lens, and the diaphragm position moves together with the second lens. 3. The bifocal switching lens system according to claim 1, wherein both the object side surface and the image side surface of the second lens are aspherical surfaces.