JPH10293246A - Retrofocus type lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a retorfocus type lens having good optical performance although it is small and low cost while ensuring a sufficient back focal distance. SOLUTION: This lens is composed, in order from the object side, of five lenses of a first meniscus-shaped lens having a negative refractive power, a second lens of a biconvex lens, a third lens of a biconcave lens, a fourth lens having a positive refractive power and a fifth lens of a biconvex lens. By representing an interval between the firs lens and the second lens by d2, an interval between the second lens and the third lens by d4, the focal distance of a whole lens system by f, the air reduced length from the surface on the image plane side to the image plane of the fifth lens by bf and the diagonal length of the image plane by IS, the conditions: 0.1<d2/f<0.4, 0.3<d4/f<0.8, 0.9<bf/IS<1.2 are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus lens having a short overall length and suitable for a video camera, a digital camera and the like.

[0002]

2. Description of the Related Art In recent years, with the development of video cameras, various digital cameras have been developed. In such digital cameras, a low-pass filter is provided in front of the image sensor,
In many cases, various glass materials such as color filters are arranged. For this reason, the digital camera needs to have a longer back focus, even though the effective screen is relatively small.

For example, assuming that the diagonal length of the effective screen is L in a single-lens reflex camera for photography of 35 mm film, the back focus is about 0.8 L, while the diagonal length of the effective screen is s in a digital camera. Back focus is 1s
Degree is required. In general, a digital camera is required to have a small and simple photographing lens having a relatively small number of lenses in order to reduce the size and cost of the entire camera.

As described above, in a digital camera, the back focus can be made relatively long, and as a simple configuration, five lenses of a negative lens, a positive lens, a negative lens, a positive lens, and a positive lens have been conventionally used from the object side. Various types of retrofocus type lenses configured as described above have been proposed.

For example, Japanese Patent Publication No. 46-24194 discloses an apparatus having a back focus of about 1.3 times the focal length and an F number of 3.5. However,
Even with this, the back focus is not enough, and the F-number is also darkened to 3.5.

Accordingly, the present applicant has proposed that the back focus is made longer and the f-number is made brighter.
Japanese Patent Application Laid-Open No. 200519 proposes a photographing lens having a back focus of about twice the effective screen and an F-number of 2.0.

However, in recent years, imaging devices have also been developed, and there is a current situation in which it is no longer necessary to increase the back focus so far and brighten the F-number.

Japanese Patent Application Laid-Open Nos. 2-85816 and 3-63613 disclose a device having a back focus longer than a focal length and an F-number of about 1.4 to 1.6. The lens is enlarged in order to lengthen the focus and brighten the F-number, and the distortion is increased, which is unsatisfactory for a digital camera.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a retro-focus type lens which improves the above-mentioned conventional drawbacks, secures sufficient back focus, and is small in size and low in cost, and excellent in optical performance. To provide.

[0010]

In order from the object side, a meniscus-shaped first lens having a negative refractive power, a second lens of a biconvex lens, a third lens of a biconcave lens, The fifth lens is a fourth lens having a refractive power and the fifth lens is a biconvex lens. This shows an appropriate shape and arrangement of each lens in order to achieve a retrofocus type lens which is small and has good performance at low cost. Specifically, if the first lens is deviated from this, and the first lens is made a convex lens, the outer diameter of the lens becomes large, and miniaturization becomes difficult. The distance between the first lens and the second lens is d2, the distance between the second lens and the third lens is d4, the focal length of the entire lens system is f, and the length of the fifth lens R2 surface to the image surface in air conversion. Is the bf and the diagonal length of the image plane is IS 0.1 <d2 / f <0.4 (1) 0.3 <d4 / f <0.8 (2) 0.9 <bf / IS < 1.2 (3) The following condition must be satisfied.

Further, the fifth lens has an optical element having two flat surfaces on the image surface side, and the thickness of the element on the optical axis is dd,
When the focal length of the i-th lens is fi and the combined focal length from the i-th lens to the j-th lens is fij, 0.4 <dd / bf <1.0 (4) -1.6 <F1 / f25 <-1.0 (5) 0.9 <f2 / f <1.3 (6)

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 3, 5, and 7 are lens sectional views of Numerical Embodiments 1 to 4 of the present invention.

FIGS. 2, 4, 6, and 8 show various aberration diagrams of each numerical example.

In the sectional view of the lens, the left side is the object side, and the right side is the image plane side. In this embodiment, a low-cost retrofocus lens having good optical performance, a small size, and a simple configuration is achieved by setting each lens as described above with a five-group, five-lens configuration.

In this embodiment, the stop is arranged between the second lens and the third lens to obtain an exit pupil position suitable for a recent image pickup device.

Next, the technical meaning of each of the above conditional expressions will be described.

The conditional expression (1) is a conditional expression relating to the distance between the first lens and the second lens. If the distance is narrowed below the lower limit, it becomes difficult to obtain a sufficient back focus, and the back focus is forcibly reduced. If it is extended, the power of the first lens becomes strong, and it becomes difficult to correct aberration. Conversely, if the distance is increased beyond the upper limit, the outer diameter of the first lens becomes large, the back focus becomes too long, and a problem that the entire lens becomes large occurs.

Conditional expression (2) is a conditional expression relating to the distance between the second lens and the third lens. If the distance is narrowed below the lower limit, it becomes difficult to place an aperture between them, and a sufficient exit pupil is obtained. You will not be able to do it. Conversely, if the interval is increased beyond the upper limit, the lens diameter of the entire lens becomes large, and a large amount of off-axis light flux enters, causing a problem that coma aberration increases.

Conditional expression (3) is a conditional expression relating to the back focus. Since many retrofocus lenses have relatively wide-angle lenses, a sufficient back focus can be obtained only by increasing the back focus with respect to the focal length of the lens. Therefore, the ratio of the back focus to the diagonal length of the image plane is taken. If the back focus is longer than the upper limit of this conditional expression, the overall length becomes longer and larger, the power of the first lens becomes stronger, and it becomes difficult to correct distortion. Conversely, if the back focus is shorter than the lower limit,
It becomes difficult to insert low-pass filters and color filters.

Conditional expression (4) relates to the thickness of an optical element such as a low-pass filter or a color filter. CC
When an image sensor such as D is used, a low-pass filter or an IR cut filter adapted to the specifications of the CCD or the like is provided in front of the image sensor. When the ratio of these optical elements to the entire length increases as in the present embodiment, it is necessary to correct aberrations including these elements. If the thickness of the optical element is increased beyond this condition, there is a problem that the clearance for adjusting the focus position at the time of mounting the image sensor is lost. Conversely, when the thickness is reduced, the back focus is too long, and there is a problem that the total length is increased.

Conditional expression (5) relates to the ratio of the focal length of the first lens to the composite focal length of the second to fifth lenses. If the power of the first lens is weaker than the lower limit, sufficient back focus cannot be secured, and if the power of the first lens is stronger than the upper limit, distortion increases.

Conditional expression (6) relates to the ratio between the focal length of the second lens and the focal length relation of the entire system. If the power of the second lens exceeds the lower limit and the power of the second lens increases, the Petzval sum increases in the positive direction, and the curvature of field increases. Conversely, if the power of the second lens is weakened beyond the upper limit, it becomes difficult to correct distortion.

To improve the performance while further reducing the size, the radius of curvature of the object-side surface of the fourth lens is R1, the radius of curvature of the image-side surface is R2, and the Abbe number of the i-th lens is ν.
When i and the refractive index are ni, it is desirable to satisfy the following conditions.

0.5 <(R1−R2) / (R1 + R2) <1.2 (7) ν3 <30 (8) ν4> 45 (9) ν5> 50 (10) n3> 1.65 (11) n4 > 1.65 (12) n5> 1.55 (13)

Condition (7) relates to the shape of the fourth lens. Deviating from the conditional expression increases field curvature and astigmatism.

The conditional expressions (8) to (13) relate to the glass material of the lens. By selecting a glass material that satisfies the conditional expression, it is possible to improve the axial, lateral chromatic aberration and the field curvature in a well-balanced manner. It becomes possible.

In order to obtain good optical performance while further reducing the size, it is desirable that the above-mentioned conditional expressions (1) to (6) particularly satisfy the following conditions.

0.2 <d2 / f <0.35 0.4 <d4 / f <0.7 1.0 <bf / IS <1.15 0.6 <dd / bf <1.0 -1. 5 <f1 / f25 <-1.1 0.9 <f2 / f <1.2

Further, in this embodiment, all the lenses are constituted by spherical surfaces. However, it is possible to further improve the performance by using an aspherical surface.

Next, numerical examples of the present invention will be described. In the numerical examples, ri is the i-th radius of curvature 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 glass.

In the numerical examples, R12 to R13 indicate an optical filter, a face plate, and the like.

The aspherical shape has an X axis in the optical axis direction, an H axis in a direction perpendicular to the optical axis, a positive traveling direction of light, R represents a paraxial radius of curvature, and B, C, D, E, and F each represent Aspherical surface coefficient

[0033]

[Outside 1] It is represented by the following expression.

Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples.

[0035]

[Table 1]

[0036]

[Outside 2]

[0037]

[Outside 3]

[0038]

[Outside 4]

[0039]

[Outside 5]

[0040]

As described above, by adopting the configuration according to the present invention, it is possible to achieve good performance at low cost with a long back focus suitable for a digital camera at a low cost.

[Brief description of the drawings]

FIG. 1 is a lens cross-sectional view of Numerical Example 1 according to the present invention.

FIG. 2 is a diagram showing various aberrations of Numerical Example 1 according to the present invention;

FIG. 3 is a sectional view of a lens according to a second numerical embodiment of the present invention;

FIG. 4 is a diagram showing various aberrations of Numerical Example 2 according to the present invention.

FIG. 5 is a sectional view of a lens according to a third numerical example of the present invention.

FIG. 6 is a diagram showing various aberrations of Numerical Example 3 relating to the present invention.

FIG. 7 is a sectional view of a lens according to a numerical example 4 of the present invention.

FIG. 8 is a diagram illustrating various aberrations of Numerical Example 4 according to the present invention.

[Explanation of symbols]

 SP stop d d-line g g-line ΔM Meridional image plane ΔS Sagittal image plane

Claims (2)

[Claims] 1. A first lens having a negative refractive power of a meniscus shape having a convex surface facing the object side, a second lens of a biconvex lens, a third lens of a biconcave lens, and a positive refractive power in order from the object side. The fifth lens is composed of a fourth lens and a fifth biconvex lens. The distance between the first lens and the second lens is d2, the distance between the second lens and the third lens is d4, and the focal length of the entire lens system is f. Where bf is the air-equivalent length from the image-side surface of the fifth lens to the image surface, and IS is the diagonal length of the image surface. 0.1 <d2 / f <0.4 0.3 < A retrofocus type lens which satisfies a condition of d4 / f <0.8 0.9 <bf / IS <1.2. 2. An optical element having two flat surfaces on the image surface side of the fifth lens, wherein the thickness of the element on the optical axis is dd, and the i-th element is
When the focal length of the i-th lens is fi and the combined focal length from the i-th lens to the j-th lens is fij, 0.4 <dd / bf <1.0-1.6 <f1 / f25 < 2. The retrofocus lens according to claim 1, wherein a condition of -1.0 0.9 <f2 / f <1.3 is satisfied.