JPH09179022A - Single focus lens - Google Patents

Abstract

(57) [Abstract] (Correction) [Problem] A single-focus lens that is bright with a large aperture ratio, wide-angle, and has a small number of lenses is made of all plastic, and various aberrations generated by an aspherical plastic lens are corrected. Provide a single focus lens. SOLUTION: A first lens 1 having a negative focal length and a second lens 2 having a positive focal length arranged with a large air gap with respect to the first lens 1 are arranged in order from the object side.
And a third lens 3 having a positive focal length and a fourth lens 4 having a negative focal length, the first lens 1
Is a plastic lens having a meniscus lens shape with a concave surface facing the image side, and aspherical surfaces provided on both surfaces of the lens, and the second lens 2 is a biconvex lens shape and plastic having an aspheric surface provided on the image side. By using a lens, and similarly, the third lens 3 and the fourth lens 4 are surface-contacted or bonded together in the same manner, various aberrations can be corrected well and a plastic lens can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single focus lens, and more particularly to a single focus lens suitable for making all plastics.

[0002]

2. Description of the Related Art A monofocal lens that can be mounted on a CCD camera is a surveillance camera in the field of security, a rear surveillance camera for automobiles, a surveillance camera for inside and outside of vehicles such as trains, aircraft and ships, indoors and outdoors. Camera for observation at
It can be widely used for security cameras. In addition, board cameras equipped with a compact single-focus lens are used not only for tools such as videophones, videoconference systems and TV intercoms, but also for fields such as built-in personal computers and workstations. It is widely available.

Conventionally, a so-called retrofocus type in which two lens groups, a front lens group having a negative refractive power and a rear lens group having a positive refractive power, are arranged in a taking lens having a relatively wide angle of view is conventionally used. Is often used. Further, even when the size of the image pickup device is reduced, the back focus of the taking lens is shortened if a constant angle of view is secured, so that it is necessary to adopt the retro focus type.

The retrofocus type photographing lens has an advantage that the back focus can be taken for a long time, but since it has a lens structure in which the light flux diverged in the front group is converged in the rear group, spherical aberration and astigmatism are obtained. A large amount of off-axis aberrations such as aberrations and distortions are generated. In general, it is very difficult to satisfactorily correct these various aberrations as compared with a Gauss type taking lens which is nearly symmetrical, because the lens configuration is asymmetric.

In particular, if the F number is made small and a large aperture ratio is attempted, a large amount of high-order spherical aberration occurs, and
The field curvature becomes large, the flatness of the image surface of the entire screen is lost, and the distortion aberration increases significantly in the negative direction.

In order to obtain good optical performance while keeping the brightness and the photographing angle of view constant, for example, there is a method of increasing the number of lenses or weakening the refractive power of both the front group and the rear group. . However, in all of these methods, the total lens length becomes long and the entire lens system becomes large.
Further, in order to obtain a sufficiently long back focus, it is sufficient to increase the distance between the front group and the rear group, but if it is increased too much, the total lens length becomes long, and it becomes difficult to miniaturize the taking lens. Is coming.

An example of a retrofocus type photographic lens having five F-numbers, a photographic field angle of 37 to 38 degrees, and a relatively small number of lenses is disclosed in, for example, Japanese Patent Laid-Open No.
It is described in JP-A-4-12723 and JP-A-57-163212. However, in these taking lenses, since the number of lenses is reduced, chromatic coma and astigmatism remain in the middle of the screen, and chromatic aberration of magnification increases as the angle of view increases.

In order to solve these problems, a wide-angle lens using an aspherical surface is disclosed in, for example, Japanese Patent Laid-Open No. 62-7852.
No. 0 publication. However, the wide-angle lens using this aspherical surface still has as many as five lenses,
Reduction in the number of lenses and miniaturization have not been achieved.

On the other hand, as a retrofocus type photographing lens having three lenses, for example, Japanese Patent Laid-Open No. 6-3
It is described in JP-A-4878, JP-A-6-34879 and JP-A-6-82690. However, JP-A-6-34878 and JP-A-6-3487
The lens described in Japanese Patent No. 9 has no aspherical surface,
The F number is as large as 4 to 5, and the increase in diameter has not been achieved. Further, the lens described in JP-A-6-82690 has an F number of 1.4, but uses three aspherical glass lens balls, and cost reduction has not been achieved. .

[0010]

The applicant proposed in Japanese Patent Application Laid-Open No. 2-208617 a wide-angle lens that achieved an F number of -1.8 by defining the focal lengths of the front and rear groups and introducing an aspherical plastic lens. doing. In this,
An example of 4 lenses including a front group of concave lenses and a convex lens / a concave lens / a rear group of convex lenses, and 3 lenses including a front group of a concave lens and a convex lens (Abbe number 70) / a rear group of convex lenses Examples of are described.

By the way, usually, when the outer diameter of the lens becomes small, the material cost and the processing cost become small, and the cost can be reduced. However, in the case of a glass lens ball, if the lens ball outer diameter becomes too small, it becomes difficult to carry out polishing, and for example, φ3 mm is more costly than lens lens outer diameter φ10 mm. Therefore, in order to further reduce the cost, it is necessary to eliminate the glass lens ball, that is, to use an all plastic lens.

The plastic lens can obtain an aspherical surface at a low cost due to its moldability, but the types of plastic lens materials are limited, and it becomes difficult to correct aberrations and chromatic aberrations due to restrictions on the refractive index and Abbe number.

For example, when the cemented lens of the convex glass lens and the concave glass lens is made of plastic, the change estimation of the radius of curvature of the lens surface will be described using equations (1) and (2). ψ convex + ψ concave = ψ'convex + ψ'concave (1) ψ convex / ν convex + ψ concave / ν concave = ψ'convex / ν'convex + ψ'concave / ν'concave (2) where ψ and ψ'is the power of the lens ball (the reciprocal of the focal length of the lens ball), and ν and ν'are the Abbe numbers of the lens ball. The left side without "" is the right side of the glass lens with "". It represents the value of a plastic lens ball. Further, the subscript convex indicates a convex lens and the concave indicates a concave lens.

Here, the refractive index of the convex glass lens is 1.7.
1300, Abbe number is 53.9, the refractive index of the concave glass lens is 1.84666, Abbe number is 23.9,
I made a quote.

At this time, the left side of the equation (1) is ψ convex = 0.
22, ψ concave = -0.13, and ν convex = 54, ν
Recess = 24. For the plastic lens material, select a material close to the Abbe number of the original glass lens, and have ν'convex = 58 (polymethylmethacrylate), ν'concave = 30.
(Polycarbonate). Therefore, when the power of the plastic lens ball is calculated using the formulas (1) and (2), ψ ′ convex = 0.27 and ψ ′ concave = −0.18.

The lens surface power ψ is defined by the equation (3) using the curvature radius r of the lens surface and the refractive index N. Therefore, the curvature of the lens surface when the glass lens ball is made into plastic The radius r changes by 0.56 times for the convex lens and 0.50 times for the concave lens. ψ = (N−1) / r (3) That is, it can be seen that the radius of curvature r of the lens surface is about half even with a simple estimation of plasticization.

The applicant previously filed Japanese Patent Application No. 6-142844.
No. (filed on June 24, 1994) proposes a single-focus lens in which the first lens and the second lens are plastic lenses, and the third lens and the fourth lens are glass lenses. The radius of curvature of the cemented surface of the third lens and the fourth lens is -4.39 mm.

The lens data including the aspherical surface (first surface) is as shown below, whereby the axial aberration is -0.017 mm and the magnification aberration is 0.013 mm (image height is 2.2 mm). Is getting Here, r (i) is the radius of curvature of the i-th lens surface S (i) in order from the object side,
d (i) is from lens surface S (i) to lens surface S (i + 1)
On the optical axis, N (j) is the refractive index of the j-th lens in order from the object side, and ν (j) is the Abbe number of the j-th lens in order from the object side. . S rd N ν 1 19.393 0.980 1.49200 57.9 2 2.525 2.621 3 (aperture) 1.780 4 -17.360 2.230 1.49200 57.9 5 -4. 203 0.900 6 10.525 2.205 1.71300 53.9 7 -4.389 0.500 1.846666 23.9 8 -13.966 3.000 9 ∞ 4.500 1.52307 58.5 10 ∞ If the third lens and the fourth lens are plastic lenses, the radius of curvature is less than half that of the glass lens, that is, −
It becomes as small as 2 mm. Furthermore, when the cemented surface is peeled off to make it a plastic lens, the third-order spherical aberration coefficient becomes -15.6 to 262.5 (image surface side of the third lens) and -278.2 (fourth lens). (The object surface side of), and the radius of curvature becomes smaller,
Aberrations are greatly increased.

As a result, various aberrations generated on the lens surface become large, and it becomes difficult to correct the aberration. Further, in reality, when the radius of curvature r of the lens surface decreases, the edge thickness of the lens ball decreases. As a result of the correction, it is necessary to further reduce the radius of curvature r of the lens surface, and further the various aberrations increase. .

An object of the present invention is to make a single-focus lens which is bright with a large aperture ratio, wide-angle, and has a small number of lenses all plastic, and to correct various aberrations caused by using an aspherical plastic lens. To provide a single focus lens.

[0021]

In order to achieve the above object, the present invention provides, in order from the object side, a first lens having a negative focal length and a large air gap with respect to the first lens. In a single-focus lens including a second lens having a positive focal length, a third lens having a positive focal length, and a fourth lens having a negative focal length, the first lens is , A plastic lens having a meniscus lens shape with a concave surface facing the image side and having aspherical surfaces on both lens surfaces, wherein the second lens has a biconvex lens shape and has an aspherical surface on the image surface side. A plastic lens is provided, the third lens is a biconvex lens-shaped plastic lens having an aspherical surface on the object side, and the fourth lens is a meniscus lens having a concave surface on the object side. form In are those with a plastic lens provided with an aspherical surface on the image side, and the third lens and the fourth lens so as to face bearing or bonding,
With such a configuration, it becomes possible to make a single-focus lens which is bright with a large aperture ratio, wide-angle, and has a small number of lenses all plastic, and to correct various aberrations caused by using an aspherical plastic lens.

In the above single focus lens, it is preferable that
Let Ψ1 be the aspheric amount of the lens surface of the first lens on the object side.
(A) and the aspherical amount of the lens surface on the image side is Ψ1 (b)
Then, the condition of Ψ1 (a)> 0 Ψ1 (b) <0 is satisfied, and further the condition of (Ψ1 (a) + Ψ1 (b)) <0 is satisfied, where The spherical quantity Ψ is
Defined by Ψ = (K / 8 / r ³ + A4) · ΔN using the conical constant K of the aspherical expression, the fourth-order coefficient A4, and the curvature radius r of the lens surface and the refractive index difference ΔN before and after the lens surface. It was made to be done.

In the above single-focus lens, it is preferable that
Aspherical surface amount Ψ1 of the lens surface on the object side of the first lens
The sum of (a) and the aspherical amount Ψ1 (b) of the lens surface on the image side is such that the condition of −0.015 <Ψ1 (a) + Ψ1 (b) <− 0.005 is satisfied. is there.

In the above single focus lens, it is preferable that
When the bending coefficient of the first lens is B1, the condition of -1.2 <B1 <-1 is satisfied, and the bending coefficient B is the curvature radius r (a) of the lens ball on the object side. Radius of curvature r on the image plane side
Using (b), it is as defined by B = [r (b) + r (a)] / [r (b) -r (a)].

In the above single-focus lens, it is preferable that
Let r be the radius of curvature of the surface of the third lens and the fourth lens which is abutting or cemented, and f be the focal length of the entire lens system constituted by the lenses from the first lens to the fourth lens. At this time, the condition of −0.65 <r / f <−0.50 is satisfied.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION First, the structure of a single-focus lens according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a lens configuration diagram of a single-focus lens according to an embodiment of the present invention.

The wide-angle single-focus lens is sequentially arranged from the object side.
The first lens 1 having a negative focal length with a plastic lens, the second lens 2 having a positive focal length with a plastic lens, the third lens 3 having a positive focal length with a plastic lens, and the negative focal length with a plastic lens It comprises a fourth lens 4 and an aperture stop (or a fixed open aperture stop) 5. An image pickup device 7 is arranged on the image plane of this single focus lens, and a parallel plane in which a filter face plate and the like are integrated between the fourth lens 4 of the single focus lens and the image pickup device 7. The face plate 6 is arranged.

In order from the object side, the first lens 1 is a meniscus lens-shaped lens having a concave surface facing the image plane side.
A large air space is formed between the first lens 1 and the second lens 2, and the diaphragm 5 is arranged in this space. The second lens 2 and the third lens 3 are biconvex lenses. 4th
The lens 4 is a meniscus lens-shaped lens with a concave surface facing the object side.

Further, aspherical surfaces are provided on both surfaces of the first lens 1, the image surface side of the second lens 2, the object side of the third lens 3, and the image surface side of the fourth lens 4. Also, the third lens 3
And the fourth lens 4 is used as the surface contact.

With the above lens structure, a wide-angle single-focus plastic lens is formed, and a single-focus plastic lens with a large aperture ratio and a simplified number of four lenses is provided, and further, various aberration performances are improved. Has achieved.

Next, the reason why the above structure is adopted will be described below.

The focal length is as small as 3.3 to 4.4 mm,
In the case of a single-focus lens having a wide angle and a small image pickup element size, in order to secure the back focus, the taking lens needs to be a retrofocus type.

In the retrofocus type photographing lens, the ray height of the axial ray is small in the negative first lens group disposed far away in front of the diaphragm, and conversely, the ray height of the chief ray. Is getting bigger. Then, the ray height of the axial ray in the second lens group arranged rearward just behind the stop is large, and conversely, the ray height of the principal ray is small.
Therefore, axial chromatic aberration mainly occurs in the second lens group in which the ray height of the axial ray is large, and the ray height of the principal ray is large in the first lens group.
Lateral chromatic aberration mainly occurs in the lens group. That is, in order to correct axial chromatic aberration and lateral chromatic aberration at the same time, it is necessary to use a concave lens and a convex lens in combination in the first lens group and the second lens group. However, as the number of lenses increases, the entire lens becomes larger and longer.

The first lens group consists of one concave lens,
A lens material having a large Abbe number is selected for the purpose of suppressing the amount of chromatic aberration of magnification. Further, in order to suppress the amount of axial chromatic aberration generated, the second lens group uses concave and convex lens balls. Further, in order to correct the amount of lateral chromatic aberration generated in the first lens group, the position of the concave lens in the second lens group is arranged at the rearmost position where the ray height of the principal ray increases away from the diaphragm. As a result, the lens configuration is such that the first lens group has one concave lens ball and the second lens group has three convex and concave lens balls.

When the third lens and the fourth lens are glass lenses, the third lens and the fourth lens are cemented to prevent total reflection in order to secure the peripheral light amount ratio. However, in the case of a plastic lens, bonding is generally difficult, and therefore, in FIG. 1, the lens surfaces of the third lens and the fourth lens are brought into contact with each other by an air layer having a space of 0 mm. There are two reasons for this.

The first reason is the problem of eccentric collapse of the lens ball. Since the radius of curvature of the lens surfaces of the third lens 3 and the fourth lens 4 facing each other is small, the amount of aberration generated on this lens surface is particularly large. Therefore, it is important to prevent the eccentric fall of the third lens 3 and the fourth lens 4. By using this lens surface as a surface contact, even if a tilt occurs, the lens surfaces can be substantially prevented from tilting because they have the same radius of curvature. At this time, considering the third lens 3 as a reference, the image-side lens surface of the fourth lens 4 is decentered. However, since the radius of curvature of this image-side lens surface is larger than the radius of curvature of the surface-contacting lens surface, that is, the object-side lens surface of the fourth lens 4, even if the same amount of eccentric tilt occurs. This is because the amount of aberration deterioration can be small.

The second reason is a ghost problem which occurs when a light ray is reflected by a lens surface or the like. The image surface side lens surface of the third lens 3 and the object side lens surface of the fourth lens 4 are lens surfaces having a small radius of curvature for chromatic aberration correction.
When the lens surfaces having the same or similar radii of curvature are made to face each other with an air gap interposed therebetween without contact, the light rays reflected by the concave surface on the object side of the fourth lens 4 are bent to the optical axis side, next,
Since the light is reflected by the lens surface on the image side of the third lens 3 in the immediate vicinity and passes just below the original unreflected light ray (on the optical axis side), the ghost is the image of the light source. It occurs closer to the center of the image plane than the position, which causes a problem. However, the ghost image can be made substantially inconspicuous by superimposing the ghost image on the image of the light source or by causing the ghost image to be formed in the immediate vicinity of the image of the light source. For this purpose, the third lens 3 and the fourth lens 4
It is effective to contact with the lens surfaces having the same radius of curvature. In the case of only the ghost, it is substantially sufficient to make the radii of curvature of the two surfaces substantially equal. However, since there is the problem of the eccentric tilt described above, the surface contact is adopted.

It is generally said that it is difficult to bond the plastic lenses, but if the plastic lenses can be easily bonded, the third lens 3 and the fourth lens 4 should be bonded together. Is a good one. That is, the third lens 3 and the fourth lens 4 are abutting or bonded.

A third lens 3 and a fourth lens 4 which are used as a surface contact
The radius of curvature r of the lens surface of is the focal length f of the entire lens system
The condition for chromatic aberration correction standardized by dividing by will be described using equation (4). −0.65 <r / f <−0.50 (4) When the upper limit of this expression (4) is exceeded, the absolute value of the radius of curvature of the lens surface becomes too small, and the light beam with the F value becomes the lens surface. Therefore, total reflection occurs, and it becomes difficult to secure the F value. On the contrary, if the lower limit of this equation is exceeded, the absolute value of the radius of curvature of the lens surface becomes too large, the effect of chromatic aberration correction becomes small, and chromatic aberration correction becomes insufficient.

Next, the condition of the arrangement of the aspherical surfaces and the amount of the aspherical surface Ψ for correcting the various aberrations which are increased due to the small radius of curvature will be described.

The aspherical surfaces are provided on both sides of the first lens 1, the image side of the second lens 2, the object side of the third lens 3, and the image side of the fourth lens 4.

In the first lens 1, the ray height of the principal ray is large, so that the aspherical surface of the first lens 1 mainly corrects coma and astigmatism. Since the object-side lens surface of the first lens 1 has a large radius of curvature, the incident angle of the principal ray with respect to the normal line becomes large, and large coma and astigmatism occur. By providing a negative aspherical surface amount Ψ on this lens surface, the radius of curvature when spherically regressed is further increased,
Alternatively, the incident angle becomes larger with respect to the normal line of the principal ray, for example, by taking a negative value. Therefore, a positive aspherical surface amount Ψ is generated on the object-side lens surface, and conversely, a negative aspherical surface amount Ψ is generated on the image-side lens surface. A spherical amount Ψ is generated to correct coma and astigmatism.

That is, the aspherical amount of the first lens is specified so as to satisfy the equations (5), (6) and (7). Ψ1 (a)> 0 …… (5) Ψ1 (b) <0 …… (6) Ψ1 (a) + Ψ1 (b) <0 …… (7) where Ψ1 (a) is the first lens 1 The aspherical amount of the lens surface on the object side, Ψ1 (b) is the aspherical quantity of the lens surface on the image side of the first lens 1, and the aspherical quantity Ψ is the conical constant K of the aspherical surface and the fourth order. Using the coefficient A4, the radius of curvature r of the lens surface, and the refractive index difference ΔN before and after the lens surface, the value is defined by equation (8). Ψ = (K / 8 / r ³ + A4) · ΔN (8) Furthermore, regarding the sum Ψ1 (a) + Ψ1 (b) of the aspheric amount on the object side of the first lens and the aspheric amount on the image side. The range of Expression (9) is considered and preferably. −0.015 <Ψ1 (a) + Ψ1 (b) <− 0.005 (9) That is, the sum Ψ1 (a) of the aspherical amount on the object side and the image side of the first lens. If + Ψ1 (b) is less than −0.015, overcorrection occurs, and if the sum of aspherical quantities Ψ1 (a) + Ψ1 (b) is greater than −0.005, undercorrection occurs. The range of (9) is preferable.

Since the second lens 2 is located near the stop and the ray height of the principal ray is small, the aspherical surface of the second lens 2 mainly corrects the spherical aberration.
Is a negative value.

On the aspherical surface of the fourth lens 4 which is far from the stop and has a large ray height of the principal ray, the fourth lens 4 mainly corrects coma and astigmatism, so that the aspherical amount Ψ is negative. Value.

By the way, the aspherical amount Ψ of the second lens 2 is
Since the ray height of the chief ray at the second lens 2 is not 0,
Affects coma and astigmatism. Similarly, the aspherical amount Ψ of the fourth lens 4 also affects the spherical aberration because the ray height of the axial ray at the fourth lens 4 is not zero. Therefore, the aspherical surface of the third lens 3 functions to balance the correction of spherical aberration by the aspherical surface amount Ψ of the second lens 2 and the fourth lens 4 with the correction of coma and astigmatism.
The amount of aspherical surface is ψ.

The above is a description of the basic lens structure and aspherical surface of the present invention.

Next, the shape of the first lens 1 will be described. Formula (10) is the bending coefficient B of the first lens 1.
It is the condition specified for 1. -1.2 <B1 <-1 (10) However, the bending coefficient B is calculated by using the curvature radius r (a) on the object side of the lens ball and the curvature radius r (b) on the image side. It is a value defined in (11). B = [r (b) + r (a)] / [r (b) -r (a)] (11) The conditions defined by the equation (10) will be described with reference to FIGS. 2 to 6.

FIG. 2 is a diagram schematically showing the lens shape when the bending coefficient B is changed.

FIG. 2 shows changes in the shape of the first lens 1 when the bending coefficient B1 = -1.01 is changed by ± 0.1. FIG. 2A shows the lens shape of the first lens 1 when the bending coefficient B1 = −0.91 and the object side is a slight concave surface. FIG. 2B shows a bending coefficient B1 = -1.01.
In this case, the lens shape of the first lens 1 is shown. When the bending coefficient B1 = -1.0, it is a plane, so when the bending coefficient B1 = -1.01, the first
The object side of the lens 1 is a slightly convex surface. FIG.
(C) shows the lens shape of the first lens 1 when the bending coefficient B1 = -1.11.
The object side of is a convex surface.

In FIG. 2, the aspherical surface is deleted and only the change due to the difference in the bending coefficient B1 is shown.

Next, the influence of the aspherical surface will be described with reference to FIG. FIG. 3 shows the bending coefficient B1 = -0.9.
It is a figure which represents typically the lens shape at the time of providing an aspherical surface in the 1st lens.

The figure shows the bending coefficient B1 = -0.91.
And the radius of curvature on the object side is −37.084 mm, and the radius of curvature on the image side is 1.7.
It is 29 mm. If an aspherical surface with an aspherical surface amount Ψ> 0 is provided on the lens surface on the object side, the lens surface will have an aspherical surface shape as shown by the dotted line (exaggerated display) in the figure. Becomes The amount of deviation from the paraxial spherical surface due to this aspherical surface becomes larger as it moves away from the optical axis from the definition of the aspherical surface expression shown in Expression (13) described later, and as a result, the lens surface as shown by the dotted line in FIG. The shape is obtained.

That is, since the side near the optical axis is concave and the edge farthest from the optical axis is convex, there is an inflection point on the lens surface in the middle, and the mold of the aspherical plastic lens is Difficult to create.

If the bending number B1 is -1.0, the inflection point does not occur. From this point, B1≤-1.0 can be set, but B1 = -1. Since the problem of ghost occurs at 0.0, if B1 <-1.0, there is no inflection point, the mold can be made, and the problem of ghost does not occur. The ghost problem will be described later with reference to FIGS. 5 and 6.

Next, the inclination of the lens surface will be described with reference to FIG. FIG. 4 shows the bending coefficient B1 = -1.
It is a figure explaining the inclination of the lens surface of the lens of 11.

When the bending coefficient B1 = -1.11, the radius of curvature on the object side is 29.241 mm and the radius of curvature on the image plane side is 1.537 mm. In this case, since the radius of curvature of the lens surface on the image side is small, the inclination θ of the lens surface (distance 1.195 mm from the optical axis) is 47.2.
It increased from 5 degrees (56.3 degrees on an actual aspherical surface) to 51.1 degrees (60.2 degrees on an actual aspherical surface). However, the aspherical surface is deleted and evaluated in order to compare the effect of bending.

When the bending coefficient B1 = -1.2, the inclination θ of the lens surface becomes 64 degrees on an actual aspherical surface,
When the bending coefficient B1 is smaller than the lower limit value, the inclination θ of the lens surface becomes large, which makes it difficult to measure the shape of the aspherical plastic lens.

On the other hand, regarding the amount of aberration, when the bending coefficient is changed from B1 = -1.01 to B1 = -1.11, the spherical aberration coefficient of the third order changes from -30 to -33 and the third order coma. The aberration coefficient increases from 1.33 to 1.61 and the third-order astigmatism coefficient increases from −0.059 to −0.080. The third-order aberration coefficient is evaluated by removing the aspherical surface in order to compare the effect of bending.

Next, the problem of ghost will be described with reference to FIGS. In FIG. 5, the first reflection surface is the image sensor 7
FIG. 3 is a diagram schematically showing the reflected light beam of the ghost when the second reflection surface is the object-side lens surface of the first lens 1 and is a flat surface.

When the light source P1 (position Y1 from the optical axis) is close to the optical axis, the position H1 of the image of the light source P1 also becomes small. In this case, the size (cross-sectional area) of the light beam that can reach the image plane without vignetting on the lens surface or the like is large. Among the light rays reflected by the image pickup surface of the image pickup device 7, the reflected light rays contained in this light flux follow the path returning to the original position Y1 of the light source P1 due to the reversibility of ray tracing. At this time, reflection occurs at the second reflecting surface (position Y0), which is assumed to be a plane, and a ghost image of the apparent light source P2 (position Y2) is formed at position H2. The reason for this is
This is because the apparent light source P2 is at the same distance as the original light source P1 on the optical axis, so that the imaging relationship between the object and the image is maintained. Further, from the symmetry of reflection, the equation (12) is established for Y1 and Y2. Y1 + Y0 = Y2-Y0 (12) Here, when Y0 is sufficiently smaller than Y1 and Y2, Y1 = Y2 is obtained from the equation (12). This is H
Since 1 = H2, it means that when the light source P1 enters the screen at the position Y1 close to the optical axis, a ghost image is always produced.

By the way, when the light source P1 is far from the optical axis, the light ray reflected by the image pickup surface of the image pickup device 7 of the first reflecting surface becomes a more upward ray, which exceeds the upper limit of the luminous flux shown in the figure. The ray cannot reach the second reflecting surface.

The example described above will be described with reference to FIG. FIG. 6 (A) is a diagram showing the state of the ghost when H1 = 0.50 mm, and FIG. 6 (B) shows H2 =
It is a figure which shows the appearance of the ghost in the case of 1.00 mm.

When H2 = 1.00 mm, it can be seen that many light beams reflected by the image pickup surface of the image pickup element 7 which is the first reflecting surface cannot pass through the lens surface apart from the optical axis.
Further, the light ray that has reached the object-side lens surface of the first lens 1 that is the second reflecting surface is also reflected at a position away from the optical axis, and therefore cannot pass through the lens surface. By the way, since an aspherical surface having a positive aspherical surface amount Ψ shown in the equation (8) is provided on the object-side lens surface of the first lens 1, the curvature radius of the spherical regression is calculated from the paraxial curvature radius. Is smaller. In this figure, the paraxial radius of curvature is 300m.
With respect to m, the radius of curvature of spherical regression is 9.3 mm, which is a shape advantageous in ghost performance as described above.

Next, assuming that the second reflecting surface is a lens surface having a positive radius of curvature, the light ray reflected by the second reflecting surface becomes more downward and goes to a position on the image plane further away from the optical axis. Make a strike statue. Therefore, the ghost in which the first reflection surface is the image pickup surface of the image pickup element 7 and the second reflection surface is the object side lens surface of the first lens 1, is the positive side of the object side lens surface of the first lens 1. It can be improved by making the radius of curvature smaller.
Actually, the anti-reflection coating on the lens surface can mitigate the ghost to some extent, so considering other performances,
The radius of curvature of the object-side lens surface of the first lens 1 is determined.

The inflection point, lens surface inclination and go
The conditions are defined in the formula (10) by integrating the strikes.

[0067]

EXAMPLES As an example of a single-focus lens according to an embodiment of the present invention, a wide-angle single-focus plastic lens having a horizontal field angle of 60 to 45 degrees at F2.8 and a 1/4 inch C
An example for a CD sensor will be described.

In the following embodiments, r (i) is the radius of curvature of the i-th lens surface S (i) in order from the object side, and d (i) is the lens surface S (i) to the lens surface S ( i +
1) is the distance on the optical axis, N (j) is the refractive index of the jth lens in order from the object side, and ν (j) is the Abbe number of the jth lens in order from the object side. Is. The angle of view represents the diagonal angle of view of the 1/4 inch CCD sensor with a real light beam. Further, the aspherical shape is represented by the sag amount Z in the optical axis direction, and the height h from the optical axis, the paraxial radius of curvature r, the conic constant K, the 4th order, the 6th order, the 8th order, and the 10th order It is defined by Expression (13) using the coefficient of the spherical term. Z = (h ² / r) / {1 + √ [{1- (K + 1) h ² / r ² ]} + A 4h ⁴ + A 6h ⁶ + A 8h ⁸ + A 10h ¹⁰ (13) [Example 1] It is a monofocal plastic lens designed to satisfy the condition of. f = 3.32 FNO. = 1: 2.94 horizontal angle of view = 60.0 ° (diagonal angle of view 73.9 °)
It is as follows. S rd N ν 1 300.000 0.900 1.49200 57.9 2 1.629 3.000 3 (aperture) 0.485 4 −29.883 1.450 1.49200 57.9 5 −2. 546 1.200 6 5.860 1.900 1.49200 57.9 7 -1.900 0.000 8 -1.900 0.900 1.58390 30.3 9 -12.500 The first surface is aspherical. And the coefficient of the equation (13) is as follows. K = 0.0 A4 = 3.067 × 10 ⁻² A6 = −7.900 × 10 ⁻³ A8 = 8.313 × 10 ⁻⁴ A10 = 2.002 × 10 ⁻⁵ The second surface is also aspherical. The coefficient of equation (13) is as follows. K = −8.514 × 10 ⁻² A4 = 4.706 × 10 ⁻² A6 = 2.893 × 10 ⁻² A8 = −3.967 × 10 ⁻² A10 = 1.67 × 10 ⁻² Fifth surface Is also an aspherical surface, and the coefficient of Expression (13) is as follows. K = −0.6592 A4 = 6.219 × 10 ⁻⁴ A6 = −1.660 × 10 ⁻³ A8 = 6.663 × 10 ⁻⁴ A10 = −1.212 × 10 ⁻⁴ The sixth surface is also aspherical. And the coefficient of the equation (13) is as follows. K = 4.826 A4 = 2.680 × 10 ⁻³ A6 = 1.677 × 10 ⁻⁶ A8 = −8.672 × 10 ⁻⁵ A10 = 6.648 × 10 ⁻⁵ The ninth surface is also aspherical. The coefficient of equation (13) is as follows. K = −26.15 A4 = 1.405 × 10 ⁻³ A6 = −1.698 × 10 ⁻⁴ A8 = 1.733 × 10 ⁻⁴ A10 = −3.369 × 10 ⁻⁵ The relationship with respect to the condition of Expression (9) for the single-focus lens configured by the four included lenses is as follows. Ψ1 (a) + Ψ1 (b) = 0.015 + (− 0.022)
= -0.007 The relationship with respect to the condition of Formula (4) is as follows. The amount of chromatic aberration is also shown because it is a condition for correcting chromatic aberration. The evaluation image height of the chromatic aberration of magnification is 1.7 to 1.8 mm, which is different for each embodiment due to the distortion aberration. r / f = −0.57 axial chromatic aberration (mm) = 0.023 lateral chromatic aberration (mm) = 0.01 The relationship with respect to the condition of the expression (10) is as follows. B1 = -1.01 By setting the bending coefficient B1 of the first lens 1 to the above value, an inflection point on the object side lens surface of the first lens 1 is prevented, and an effective diameter of the image surface side lens surface. The maximum slope θ at is as follows. θ = 56.3 ° Further, the following is a comparison of the radius of curvature of the object-side lens surface of the first lens 1 that has been spherically detoured and the paraxial radius of curvature. Curvature radius of spherical return = 9.3 mm Paraxial radius of curvature = 300 mm Fig. 7 is an aberration diagram regarding Example 1 at a shooting distance of 2 m. FIG. 7A is a coma aberration diagram, and the maximum image height 2.2.
7 mm (A-1-1), (A-1-2) for 5 mm
Represents the coma aberration at the point where the relative image height is 1.0 mm, and FIGS. 7 (A-2-1) and (A-2-2) show the coma aberration at the point where the relative image height is 0.9 mm. Represents
7 (A-3-1) and (A-3-2) show relative image heights of 0.
The coma aberration at a point of 6 mm is shown in FIG.
4-1) and (A-4-2) represent coma aberration at a point where the relative image height is 0.3 mm, and FIG.
1) and (A-5-2) represent coma aberration at a point where the relative image height is 0 mm. FIG. 7B shows the spherical aberration, and FIG. 7C shows the sine condition. Figure 7
(D) represents astigmatism, and the solid line is the spherical ray S.
, And the dotted line represents the astigmatism for the meridional ray T. FIG. 7E shows the distortion aberration.

The maximum value of the aberration diagram coordinates is ± 20 for coma.
μm, spherical aberration and sine condition ± 50 μm, distortion ±
10%. Astigmatism is ± 50 μm.

Example 2 Example 2 is a monofocal plastic lens designed to satisfy the following conditions. f = 3.68 FNO. = 1: 2.94 Horizontal angle of view = 53.8 ° (diagonal angle of view 66.1 °) The respective lens constants of the single focus lens satisfying the above conditions are as follows. S r d N ν 1 300.000 0.900 1.49200 57.9 2 1.977 3.000 3 (aperture) 0.485 4 -25.019 1.400 1.49200 57.9 5-2. 574 1.200 6 5.884 1.700 1.49200 57.9 7 -2.100 0.000 8 -2.100 0.900 1.58390 30.3 9 -16.000 The first surface is aspherical. And the coefficient of the equation (13) is as follows. K = 0.0 A4 = 3.470 × 10 ⁻² A6 = −7.420 × 10 ⁻³ A8 = 7.059 × 10 ⁻⁴ A10 = 2.523 × 10 ⁻⁵ The second surface is also aspherical. The coefficient of equation (13) is as follows. ^{K = 2.687 × 10 -2 A4 =} 5.344 × 10 -2 A6 = 2.910 × 10 -2 A8 = -3.233 × 10 -2 A10 = 1.322 × 10 -2 also fifth surface It is an aspherical surface, and the coefficient of Expression (13) is as follows. K = -0.6351 A4 = 3.863 × 10 ^-4 A6 = -1.697 × 10 ^-3 A8 = 6.615 × 10 ^-4 A10 = -1.197 × 10 ^-4 The sixth surface is also aspherical. And the coefficient of the equation (13) is as follows. K = 4.399 A4 = 2.167 × 10 ⁻³ A6 = 1.591 × 10 ⁻⁴ A8 = −1.599 × 10 ⁻⁴ A10 = 5.389 × 10 ⁻⁵ The ninth surface is also aspherical. The coefficient of equation (13) is as follows. K = −34.74 A4 = 1.859 × 10 ⁻³ A6 = 4.445 × 10 ⁻⁴ A8 = −1.044 × 10 ⁻⁴ A10 = 1.135 × 10 ^{−5 The} above constants are provided. The relationship with respect to the condition of Expression (9) for the single-focus lens composed of four lenses is as follows. Ψ1 (a) + Ψ1 (b) = 0.017 + (− 0.027)
= -0.010 The relationship with respect to the condition of Expression (4) is as follows. The amount of chromatic aberration is also shown because it is a condition for correcting chromatic aberration. The evaluation image height of the chromatic aberration of magnification is 1.7 to 1.8 mm, which is different for each embodiment due to the distortion aberration. r / f = −0.57 axial chromatic aberration (mm) = 0.022 lateral chromatic aberration (mm) = 0.01 The relationship to the condition of the expression (10) is as follows. B1 = -1.01 By setting the bending coefficient B1 of the first lens 1 to the above value, an inflection point on the object side lens surface of the first lens 1 is prevented, and an effective diameter of the image surface side lens surface. The maximum slope θ at is as follows. θ = 56.4 ° Further, the radius of curvature of the spherical surface of the object-side lens surface of the first lens 1 and the paraxial radius of curvature are shown below in comparison. Curvature radius of spherical return = 7.2 mm Paraxial radius of curvature = 300 mm FIG. 8 is an aberration diagram regarding Example 2 at a shooting distance of 2 m. FIG. 8 (A) is a coma aberration diagram, and FIG. 8 (B) is
8C shows spherical aberration, FIG. 8C shows a sine condition, FIG. 8D shows astigmatism, and the solid line shows astigmatism with respect to the spherical ray S. The dotted line represents the astigmatism with respect to the meridional ray T, and FIG.
It represents distortion. Each drawing of FIG. 8A shows coma aberration for the same image height point as described in FIG. 7.

The maximum value of the aberration diagram coordinate is ± 20 for coma.
μm, spherical aberration and sine condition ± 50 μm, distortion ±
10%. Astigmatism is ± 50 μm.

Example 3 Example 3 is a monofocal plastic lens designed to satisfy the following conditions. f = 4.42 FNO. = 1: 2.94 Horizontal angle of view = 44.5 ° (diagonal angle of view 54.4 °) The respective lens constants of the single focus lens satisfying the above conditions are as follows. S rd N ν 1 300.000 0.900 1.49200 57.9 2 2.543 3.000 3 (aperture) 0.485 4 -16.061 1.450 1.49200 57.9 5-2. 671 1.200 6 6.396 1.630 1.49200 57.9 7-2.500 0.000 8-2.500 0.900 1.58390 30.3 9-21.712 The first surface is aspherical. And the coefficient of the equation (13) is as follows. K = 0.0 A4 = 3.953 × 10 ⁻² A6 = −7.095 × 10 ⁻³ A8 = 6.225 × 10 ⁻⁴ A10 = 4.185 × 10 ⁻⁵ The second surface is also aspherical. The coefficient of equation (13) is as follows. K = 0.2553 A4 = 5.579 × 10 ⁻² A6 = 2.998 × 10 ⁻² A8 = −2.924 × 10 ⁻² A10 = 1.114 × 10 ⁻² The fifth surface is also aspherical. The coefficient of equation (13) is as follows. K = −0.6096 A4 = 1.976 × 10 ⁻⁴ A6 = −1.808 × 0 ⁻³ A8 = 6.295 × 10 ⁻⁴ A10 = −1.005 × 10 ⁻⁴ The sixth surface is also aspherical. And the coefficient of the equation (13) is as follows. K = 4.317 A4 = 2.046 × 10 ⁻³ A6 = 2.508 × 10 ⁻⁴ A8 = −1.682 × 10 ⁻⁴ A10 = 2.773 × 10 ⁻⁵ The ninth surface is also aspherical. The coefficient of equation (13) is as follows. K = −60.03 A4 = 2.362 × 10 ⁻³ A6 = 1.018 × 10 ⁻³ A8 = −3.880 × 10 ⁻⁴ A10 = 6.096 × 10 ^{−5 The} above constants are provided. The relationship with respect to the condition of Expression (9) for the single-focus lens composed of four lenses is as follows. Ψ1 (a) + Ψ1 (b) = 0.19 + (− 0.028)
= -0.009 The relationship with respect to the condition of Expression (4) is as follows. The amount of chromatic aberration is also shown because it is a condition for correcting chromatic aberration. The evaluation image height of the chromatic aberration of magnification is 1.7 to 1.8 mm, which is different for each embodiment due to the distortion aberration. r / f = −0.57 axial chromatic aberration (mm) = 0.023 lateral chromatic aberration (mm) = 0.0012 The relationship with respect to the condition of Expression (10) is as follows. B1 = -1.02 By setting the bending coefficient B1 of the first lens 1 to the above value, an inflection point on the object side lens surface of the first lens 1 is prevented, and the effective lens surface diameter on the image side. The maximum slope θ at is as follows. θ = 53.8 ° Further, the following is a comparison of the radius of curvature of the object-side lens surface of the first lens 1 that is spherically regressed and the paraxial radius of curvature. Curvature radius of spherical return = 6.1 mm Paraxial radius of curvature = 300 mm FIG. 9 is an aberration diagram regarding Example 3 at a shooting distance of 2 m. FIG. 9A is a coma aberration diagram, and FIG. 9B is
9C shows the spherical aberration, FIG. 9C shows the sine condition, FIG. 9D shows the astigmatism, and the solid line shows the astigmatism with respect to the spherical ray S. The dotted line represents the astigmatism with respect to the meridional ray T, and FIG.
It represents distortion. Each figure in FIG. 9A shows coma aberration for the same image height point as described in FIG. 7.

The maximum value of the aberration diagram coordinates is ± 20 for coma.
μm, spherical aberration and sine condition ± 50 μm, distortion ±
10%. Astigmatism is ± 100 μm.

Example 4 Example 4 is a monofocal plastic lens designed to satisfy the following conditions. f = 4.41 FNO. = 1: 2.94 Horizontal angle of view = 44.6 ° (diagonal angle of view 54.4 °) The respective lens constants of the single focus lens satisfying the above conditions are as follows. S r d N ν 1 150.000 0.900 1.49200 57.9 2 2.585 3.000 3 (aperture) 0.485 4 -16.578 1.450 1.49200 57.9 5-2. 658 1.200 6 6.298 1.630 1.49200 57.9 7 -2.500 0.000 8 -2.500 0.900 1.58390 30.3 9 -24.330 The first surface is aspherical. And the coefficient of the equation (13) is as follows. K = 0.0 A4 = 3.974 × 10 ⁻² A6 = −6.887 × 10 ⁻³ A8 = 6.153 × 10 ⁻⁴ A10 = 3.315 × 10 ⁻⁵ The second surface is also aspherical. The coefficient of equation (13) is as follows. K = 0.2846 A4 = 5.708 × 10 ^-2 A6 = 2.828 × 10 ^-2 A8 = -2.646 × 10 ^-2 A10 = 1.010 × 10 ^-2 The fifth surface is also aspherical. The coefficient of equation (13) is as follows. K = −0.5909 A4 = 2.406 × 10 ⁻⁵ A6 = −1.796 × 10 ⁻³ A8 = 6.101 × 10 ⁻⁴ A10 = −9.318 × 10 ⁻⁵ The sixth surface is also aspherical. And the coefficient of the equation (13) is as follows. K = 3.779 A4 = 1.683 × 10 ^-3 A6 = 9.991 × 10 ^-5 A8 = -1.480 × 10 ^-4 A10 = 3.339 × 10 ^-5 The ninth surface is also aspherical. The coefficient of equation (13) is as follows. K = −45.54 A4 = 2.277 × 10 ⁻³ A6 = 1.012 × 10 ⁻³ A8 = −4.314 × 10 ⁻⁴ A10 = 6.514 × 10 ^{−5 The} above constants are provided. The relationship with respect to the condition of Expression (9) for the single-focus lens composed of four lenses is as follows. Ψ1 (a) + Ψ1 (b) = 0.020 + (− 0.029)
= -0.009 The relationship with respect to the condition of Expression (4) is as follows. The amount of chromatic aberration is also shown because it is a condition for correcting chromatic aberration. The evaluation image height of the chromatic aberration of magnification is 1.7 to 1.8 mm, which is different for each embodiment due to the distortion aberration. r / f = −0.57 axial chromatic aberration (mm) = 0.023 lateral chromatic aberration (mm) = 0.01 The relationship with respect to the condition of the expression (10) is as follows. B1 = -1.04 By setting the bending coefficient B1 of the first lens 1 to the above value, an inflection point on the object side lens surface of the first lens 1 is prevented, and an effective diameter of the image surface side lens surface. The maximum slope θ at is as follows. θ = 53.7 ° Further, the following is a comparison between the radius of curvature of the object-side lens surface of the first lens 1 and the paraxial radius of curvature. Curvature radius of spherical return = 5.9 mm Paraxial radius of curvature = 150 mm FIG. 10 is an aberration diagram regarding Example 4 at a shooting distance of 2 m. FIG. 10A is a coma aberration diagram, and FIG.
10B shows the spherical aberration, FIG. 10C shows the sine condition, FIG. 10D shows the astigmatism, and the solid line shows the non-correction for the spherical ray S. Represents point aberration,
The dotted line represents the astigmatism with respect to the meridional ray T, and FIG. 10 (E) represents the distortion aberration. Each drawing of FIG. 10A shows coma aberration for the same image height point as described in FIG. 7.

The maximum value of the aberration diagram coordinates is ± 20 for coma.
μm, spherical aberration and sine condition ± 50 μm, distortion ±
10%. Astigmatism is ± 100 μm.

As described above, according to the present embodiment, the F number is 2.8, the aperture ratio is large, the brightness is wide, the horizontal angle of view is 60 to 45 degrees, and the number of lenses is 4.
The number of single-focus lenses can be made entirely of plastic,
Moreover, it is possible to effectively correct various aberrations generated by the aspherical plastic lens used for making all plastic.

Further, the cost of the single focus lens can be reduced by making it all plastic.

[0078]

According to the present invention, a single focus lens that is bright with a large aperture ratio, wide angle, and has a small number of lenses is made of all plastic, and various aberrations caused by using an aspherical plastic lens are eliminated. Can be corrected.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram of a single-focus lens according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a lens shape when a bending coefficient B is changed.

FIG. 3 is a diagram schematically showing a lens shape when an aspherical surface is provided on a lens having a bending coefficient B1 = −0.91.

FIG. 4 is a diagram illustrating a tilt of a lens surface of a lens having a bending coefficient B1 = −1.11.

FIG. 5 is a schematic view of the reflected light beam of the ghost when the first reflecting surface is the image pickup surface of the image pickup device and the second reflecting surface is the object-side lens surface of the first lens and is a flat surface. FIG.

FIG. 6 is a graph showing the case where the first reflecting surface is the image pickup surface of the image pickup device and the second reflecting surface is the object side lens surface of the first lens.
It is a figure showing a situation of a strike.

FIG. 7 is an aberration diagram of Example 1 of the single-focus lens according to the embodiment of the present invention.

FIG. 8 is an aberration diagram for Example 2 of the single-focus lenses according to the embodiment of the present invention.

FIG. 9 is an aberration diagram of Example 3 of the single-focus lenses according to the embodiment of the present invention.

FIG. 10 is an aberration diagram for Example 4 of the single-focus lenses according to the embodiment of the present invention.

[Explanation of symbols]

 1 ... 1st lens 2 ... 2nd lens 3 ... 3rd lens 4 ... 4th lens 5 ... Aperture 6 ... Parallel plane plate 7 ... Image sensor

Claims (5)

[Claims] 1. A first lens having a negative focal length in order from the object side, and a second lens having a positive focal length arranged with a large air gap with respect to the first lens, In the monofocal lens including a third lens having a focal length of 4 and a fourth lens having a negative focal length, the first lens has a meniscus lens shape with a concave surface facing the image side, and A plastic lens having aspherical surfaces on both sides of the lens, the second lens is a biconvex lens shape, and a plastic lens having an aspherical surface on the image side, and the third lens is a biconvex lens. And a plastic lens having an aspherical surface on the object side, wherein the fourth lens is a meniscus lens shape having a concave surface on the object side and an aspherical surface on the image side. , Single focus lens, characterized in that the third lens and the fourth lens as a face bearing or bonding. 2. The single-focus lens according to claim 1, wherein the aspherical amount of the lens surface of the first lens on the object side is Ψ1.
(A) and the aspherical amount of the lens surface on the image side is Ψ1 (b)
Then, the condition of Ψ1 (a)> 0 Ψ1 (b) <0 is satisfied, and further the condition of (Ψ1 (a) + Ψ1 (b)) <0 is satisfied, where The spherical quantity Ψ is
Defined by Ψ = (K / 8 / r ³ + A4) · ΔN using the conical constant K of the aspherical expression and the fourth-order coefficient A4 A single-focus lens that is characterized. 3. The single focus lens according to claim 2, wherein the aspherical surface amount Ψ1 of the lens surface of the first lens on the object side.
The sum of (a) and the amount of aspherical surface Ψ1 (b) of the lens surface on the image plane side is characterized by satisfying the condition of −0.015 <Ψ1 (a) + Ψ1 (b) <− 0.005. Single focus lens. 4. The monofocal lens according to claim 1, wherein when the bending coefficient of the first lens is B1, the condition of −1.2 <B1 <−1 is satisfied, where the bending coefficient B is , The radius of curvature r (a) on the object side of the lens and the radius of curvature r on the image side
(B) is used, B = [r (b) + r (a)] / [r (b) -r (a)] is defined, The single focus lens characterized by the above-mentioned. 5. The single-focus lens according to claim 1, wherein a radius of curvature of a surface of the third lens and the fourth lens which is abutting or cemented is r, and the first lens to the fourth lens A single focus lens characterized by satisfying the condition of -0.65 <r / f <-0.50, where f is the focal length of the entire lens system formed by the lenses.