JPH0694992A - Photographic lens - Google Patents

Abstract

PURPOSE:To provide the photographic lens which is wide in angle, small in astigmatism, small in size and inexpensive by forming an aspherical face which is heretofore used for a fourth negative lens as a spherical face and in turn forming the object side face of a first positive lens as an aspherical face. CONSTITUTION:This photographic lens is the lens of 4 elements in 4 groups consisting, successively from the object side, a positive first lens L1 of a concave face, a negative second lens L2 of a concave face, a biconvex third lens L3 and a negative meniscus fourth lens L4. The first lens L1 and the fourth lens L4 are constituted of plastic materials. The second lens L2 and the third lens L3 are constituted of glass materials. The object side face of the first lens L1 is formed to the aspherical face shape and satisfies the conditions of equation, where f1 denotes the foal length of the positive first lens L1; f4 denotes the focal length of the negative fourth lens L4: fa denotes the combined focal length of the positive first lens L1 and the negative second lens L2; S1 denotes the numerical value indicating the shape of the aspherical face of the object side face of the positive first lens L1; r1 denotes the paraxial radius of curvature of the object side aspherical face of the positive first lens L1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact and wide-angle photographing lens used for a lens shutter camera or the like.

[0002]

2. Description of the Related Art In a so-called telephoto type wide-angle lens composed of all positive, negative, and positive rear groups and a negative rear group, the total lens length can be made as small as the focal length of the lens. Widely used as a lens. In recent years, as in Japanese Patent Laid-Open No. 61-182010, a concave lens in the rear group is made of a plastic aspherical lens to improve distortion, which is a defect, and in Japanese Patent Laid-Open No. 61-138225, the number of lenses is increased. It is known that the number of lenses is increased to increase the diameter, and that the production cost is reduced by using three of the four lenses as plastic lenses as disclosed in Japanese Patent Laid-Open No. 61-6616.

[0003]

However, the conventional telephoto wide-angle lens has a field angle of at most 60 °.
There is a limit to the degree, and it is difficult to keep the astigmatic difference of the maximum angle of view small even if an attempt is made to widen the angle.
As an attempt to improve this, JP-A-58-21
It is known that a thick positive lens is inserted between the front and rear groups to reduce the astigmatism and widen the angle, as in Japanese Patent No. 9509.
However, this method inevitably causes an increase in the total length of the lens, so there is no point in using the telephoto type, which is aimed at downsizing. Furthermore, with this conventional lens,
A glass material having a high refractive index is used for all lenses, and the cost of the lens system is inevitably increased. Therefore, it is an object of the present invention to provide a small-sized and inexpensive photographing lens having an F number of about 4 and an angle of view of about 70 °, and a tele ratio of close to 1.

[0004]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention has a positive first lens L1 whose image side surface is concave toward the image side, and an object side surface is the object side. 4th group 4 including a negative second lens L2 having a concave surface on the side, a biconvex third lens L3, and a negative meniscus fourth lens L4 having a convex surface facing the image side
The first lens L1 and the fourth lens L4 are made of a plastic material, the second lens L2 and the third lens L3 are made of a glass material, and the first lens L1 and the third lens L3 are made of a glass material.
The object side surface of the lens L1 has an aspherical shape, and the following conditions are satisfied to solve the above problems. 1.3 <f1 / f <2.4 (1) -1.2 <f4 / f <-0.6 (2) -2.0 <fa / f <-0.8 (3) 0.075 < S1 / r1 <0.0825 (4) where f: focal length of the entire system, f1: positive focal length of the first lens L1, f4: negative focal length of the fourth lens L4, fa: positive first lens L1 and negative Synthetic focal length of the second lens L2, S1: Numerical value representing the shape of the aspherical surface on the object side surface of the positive first lens L1. In general, the aspherical shape has a coordinate x in the optical axis direction and a direction perpendicular to the optical axis. Where y is the coordinate of R, R is the reference radius of curvature, k is the conic coefficient, and Cn is the aspherical coefficient of the nth order, x (y) = (y ² / R) / (1+ (1-k · y ² / R ^{2) 1/2)} + of ^{C2 · y 2 + C4 · y} 4 + C6 · y 6 + C8 · y 8 + C10 · y 10 + ···· (a) represented by the formula, the near-axis of the plane When the index radius is defined as r = 1 / (2 · C2 + (1 / R)), S1 = a numerical value represented by x (0.4 · r), r1: positive object-side aspherical surface of the first lens L1 Is the paraxial radius of curvature of.

[0005]

The present invention basically comprises a front group consisting of a positive first lens L1, a negative second lens L2 and a positive third lens L3,
This is a so-called telephoto type lens including a rear group including a negative fourth lens L4. With this configuration, if the total length is kept short and the angle of view is widened, an increase in the astigmatic difference at the maximum angle of view cannot be avoided. It is considered that this cause occurs because the refractive power of each lens increases, and in particular, the refractive power of the first positive lens L1 increases. Therefore, in the present invention, it was decided to find a configuration that can be miniaturized while keeping the refractive power of the first lens as small as possible. In order to achieve downsizing while weakening the refractive power of the first lens L1, it is necessary to strengthen the refractive power of the third positive lens L3 and the fourth negative lens L4. However, with this configuration, the correction of positive distortion, spherical aberration, and coma fails.
However, here, by making the aspherical surface conventionally used for the fourth negative lens L4 a spherical surface and instead making the object side surface of the first positive lens L1 aspherical, distortion and spherical aberration,
We have found that coma can be corrected well at the same time.
When the first positive lens is an aspherical surface, it is desirable that the first lens be made of a plastic material for cost reduction. The progress of glass molding technology has been remarkable in recent years, but no matter how advanced the technology is, it cannot be considered that it can be manufactured at the same cost as a plastic molded lens.

However, there are major problems in incorporating a plastic material into a taking lens. It is performance deterioration due to temperature change. Plastic has a much larger coefficient of linear expansion than glass, and changes in its density cause a large change in refractive index due to changes in temperature. Therefore, the phenomenon that the focal length of the lens changes or the back focus changes occurs. In a camera such as a so-called compact camera that has an autofocus mechanism independent of the lens, the change in back focus is fatal. Since the lens of the present invention is also intended to be used in this type of camera, some means for compensating for the change in back focus due to this temperature change is required.
This is so-called temperature erasing. Therefore, in the present invention, the first positive lens L1 is made of a plastic material, and the fourth negative lens L4 is also made of a plastic material, so that the back focus change due to the temperature change can be offset. Further, by setting the conditional expressions (1) and (2), it is possible to achieve a good temperature elimination while widening the angle and reducing the size. If the upper limit of conditional expression (1) is exceeded, it is advantageous for correction of astigmatism, but the overall length of the lens is increased. Further, if the size is forcibly reduced, the correction of distortion and spherical aberration will fail. On the other hand, when the value goes below the lower limit, astigmatism increases and wide angle cannot be achieved. Further, even if the refractive power of the fourth negative lens L4 is increased, it becomes difficult to erase the temperature. If the upper limit of conditional expression (2) is exceeded, it is advantageous for downsizing, but
It is difficult to correct positive distortion, and if the lower limit is exceeded, size reduction and temperature elimination cannot be achieved. Conditional expression (3)
Relates to correction of spherical aberration. In the lens of the present invention, the refractive power of the first positive lens L1 is smaller than that of the conventional lens, so that the refractive power of the third positive lens inevitably becomes large. Therefore, negative spherical aberration and positive distortion tend to occur. In order to satisfactorily correct this, it is desirable that the combined focal length of the first positive lens L1 and the second negative lens L2 be an appropriate negative value. Therefore, if the lower limit is exceeded, the negative spherical aberration and the positive distortion are extremely large, while if the upper limit is exceeded, the spherical aberration is overcorrected, and the front group becomes a so-called retrofocus type, so the size is small. Is disadvantageous to Conditional expression (4) defines the aspherical shape of the object side surface of the first positive lens L1. If the upper limit is exceeded, the effect of the aspherical surface is poor, and the above-mentioned various aberrations are not effectively corrected. On the contrary, if the lower limit is exceeded, the effect of the aspherical surface is too large and the spherical aberration is overcorrected. . Thus, according to the present invention, by replacing the two lenses with a plastic material while achieving both miniaturization and wide angle,
An inexpensive wide-angle lens can be achieved.

In order to satisfactorily correct various aberrations in terms of configuration or the like, it is desirable to further satisfy the following conditions. 1.45 <n1 <1.55 (5) 1.45 <n4 <1.55 (5 ') 1.8 <n2 (6) 0.5 <Σd / f <0.7 (7) 0.05 <D5 / f <0.11 (8) 0.8 <d2 / d4 <1.4 (9) 0.22 <d6 / f <0.3 (10) -0.9 <r3 / r4 <0. 2 (11) 5 <ν1-ν3 <13 (12) where n1: refractive index of the first positive lens L1 at d line, n2: refractive index of the second negative lens L2 at d line, n4: fourth negative lens Refractive index of d-line of L4, f: focal length of the entire system, Σd: total thickness of the entire system, distance from the optical axis vertex of the object side surface of the first positive lens L1 to the image-side vertex of the fourth negative lens L4, d2 : From the image side surface of the first positive lens L1 to the second negative lens L2
Distance between the optical axes of the object side surface of d3, d4: from the image side surface of the second negative lens L2 to the third positive lens L3
Distance between the optical axes of the object side surface of d3, d5: axial thickness of the third positive lens L3, d6: image side surface of the third positive lens L3 to the fourth negative lens L4
R3: radius of curvature of object side surface of second negative lens L2, r4: radius of curvature of image side surface of second negative lens L2, ν1: Abbe number of first positive lens, ν3: first The Abbe number of a 3 positive lens is Conditional expressions (5) and (5 ') define the refractive indices of the first positive lens L1 and the fourth negative lens L4. First lens L
The first and fourth lenses L4 are made of plastic as described above, and are due to the material restrictions. However, in terms of aberration correction,
If the refractive index of the first lens L1 exceeds the lower limit, the curvature of the lens becomes tight, which is not preferable. If the refractive index of the first lens L1 exceeds the upper limit, it is difficult to set the Petzval sum to an appropriate value when the size is reduced. Fourth
The refractive index of the lens L4 is preferably as close to the upper limit as possible for correction of various aberrations. Conditional expression (6) defines the refractive index of the second negative lens L2. When the size is reduced, the refractive index of the fourth negative lens L4 increases, and the Petzval sum tends to decrease. Therefore, it is necessary to set the refractive index of the second negative lens L2 to 1.8 or more and set the Petzval sum to an appropriate value. Conditional expression (7) defines the total thickness of the lens. If the total thickness is large, it is advantageous for aberration correction, but it is against size reduction.
On the contrary, if it is made small, it leads to miniaturization, but it becomes difficult to widen the angle. Conditional expression (8) defines the lens thickness of the third lens L3. Exceeding this upper limit is advantageous for correction of astigmatism, but when a diaphragm is arranged behind the third lens L3,
It is easy to cause a decrease in peripheral light intensity. On the other hand, when the value goes below the lower limit, it is difficult to correct astigmatism and the edge thickness of the third lens L3 becomes thin, which is not preferable in terms of processing. Conditional expression (9) is
It defines the position of the second negative lens L2. Second lens L
When 2 is closer to the object side, that is, when it is below the lower limit, it is advantageous in correcting distortion, and it is difficult to balance spherical aberration and the image plane. Further, since the first lens L1 and the second lens L2 are likely to mechanically interfere with each other, the amount of peripheral light is likely to decrease. On the contrary, when the second lens L2 is closer to the image side, it becomes difficult to correct distortion. Conditional expression (10) is
The air gap between the third positive lens L3 and the fourth negative lens L4 is defined. When the diaphragm is arranged behind the third positive lens L3 as described above, the diaphragm and the fourth negative lens interfere with each other unless the distance is widened to some extent. However, if this interval is too wide, it is disadvantageous for downsizing, and it becomes difficult to correct positive distortion and coma. However, in the case of adopting a focus system in which the diaphragm and the fourth negative lens L4 are fixed and the lenses L1 to L3 are extended, if the lower limit is exceeded, the variation of spherical aberration is extremely large and not preferable, and if the upper limit is exceeded, the variation of astigmatism is increased. Is not preferable because it increases. Conditional expression (11) defines the shape of the second negative lens L2. It is difficult to correct coma aberration if the upper limit or the lower limit is exceeded. Conditional expression (12) defines the balance of chromatic aberration. If it exceeds this range, it becomes difficult to balance the axial chromatic aberration and the off-axis chromatic aberration.

[0008]

EXAMPLES Examples of the present invention will be given below. In the table below, the leftmost number is the surface number, r is the radius of curvature, d is the surface spacing,
n is the refractive index at the d line (λ = 587.6 nm), Abbe is the Abbe number, f is the focal length, FN is the F number, Bf is the back focus, and TL / f is the tele ratio. In Examples 1, 2, 3, 5 and 7, both surfaces of the first positive lens L1 are used, and in Examples 4, 6 and 8, an aspherical surface is used on the object side surface of the first positive lens L1. The aspherical surface is represented by the formula (a). When both surfaces of the first positive lens L1 are aspherical surfaces, it is particularly effective in correcting coma aberration when the total lens length is shortened.

Example 1 f = 32.00 FN = 4.0 Bf = 15.36 T
L / f = 1.030

One surface aspherical surface k = .2323E + 00 c2 = .0000 c4 = .0000 c6 = -.4060E-05 c8 = -.2622E-08 c10 = -.3367E-08

Two-sided aspherical surface k = .6965E + 00 c2 = .0000 c4 = .0000 c6 = -0.5440E-05 c8 = .6746E-07 c10 = -.6062E-08

Example 2 f = 32.00 FN = 4.0 Bf = 14.77 T
L / f = 1.068

One surface aspherical surface k =. 7693E + 00 c2 = .0000 c4 = .0000 c6 =-. 2656E-05 c8 = .8273E-07 c10 =-. 3831E-08

Two-sided aspherical surface k = .2055E + 01 c2 = .0000 c4 = .0000 c6 = .2069E-05 c8 = -.1648E-06 c10 = -.2130E-08

Example 3 f = 32.00 FN = 4.0 Bf = 14.77 T
L / f = 1.012

One surface aspherical surface k =. 2251E + 00 c2 = .0000 c4 = .0000 c6 =-. 4594E-05 c8 =-. 3415E-07 c10 =-. 3460E-08

Two-sided aspherical surface k = .2581E + 01 c2 = .0000 c4 = .0000 c6 = -.7624E-05 c8 = .2096E-06 c10 = -.1051E-07

Example 4 f = 28.80 FN = 4.0 Bf = 12.83 T
L / f = 1.046

One surface aspherical surface k =-. 1176E + 00 c2 = .0000 c4 = -0.8580E-05 c6 =-. 4156E-05 c8 =-. 7963E-08 c10 =-. 2668E-08

Example 5 f = 28.80 FN = 4.0 Bf = 12.69 T
L / f = 1.031

One surface aspherical surface k = .1581E-01 c2 = .0000 c4 = -.3971E-04 c6 = -.5137E-05 c8 = -.5333E-08 c10 = -.3207E-08

Two-sided aspherical surface k =-.5691E + 00 c2 = .0000 c4 = .0000 c6 = -.2709E-05 c8 = .1468E-06 c10 =-.5450E-08

Example 6 f = 32.00 FN = 4.0 Bf = 14.79 T
L / f = 1.041

One surface aspherical surface k = .2130E + 00 c2 = .0000 c4 = .3703E-04 c6 = -.2854E-05 c8 = 0.7544E-07 c10 = -.2273E-08

Example 7 f = 32.00 FN = 4.0 Bf = 14.95 T
L / f = 0.994

One surface aspherical surface k = .1839E-01 c2 = .0000 c4 = .0000 c6 = -.5021E-05 c8 = -.1708E-07 c10 = -.65050E-08

Two-sided aspherical surface k = .5085E + 00 c2 = .0000 c4 = .0000 c6 = -.2723E-05 c8 =-. 2723E-07 c10 = -.9000E-08

Example 8 f = 32.00 FN = 4.0 Bf = 14.43 T
L / f = 1.074

One surface aspherical surface k = .2337E + 01 c2 = .0000 c4 = -.1415E-03 c6 = -.6411E-05 c8 = .1297E-06 c10 = -.3206E-08

FIG. 1 is a sectional view of lenses of Example 1 of the taking lens of the present invention, and FIGS. 2 to 9 are aberration diagrams of the lenses of Examples 1 to 8. From each aberration diagram, it can be seen that various aberrations are well corrected.

The changes in the focal length and the back focus when the refractive index of the plastic lens of each example changes by +0.003 are shown below. Δn = + 0.003 This change in the refractive index corresponds to a temperature change of about -30 ° C, and it can be seen that the temperature is erased sufficiently well in this range. By the way, the amount of change in back focus is
I have not dared to completely correct it to zero. This is to obtain good temperature erasing over the entire screen in consideration of spherical aberration and image plane variation due to changes in the refractive index.

[0032]

As described above, according to the present invention, it is possible to obtain a small and inexpensive photographing lens having an F number of about 4 and an angle of view of about 70 °, and a tele ratio of close to 1.
By the way, the lens of the present invention is of course not only extended,
Also by retracting the fourth negative lens L4,
Focusing can also be performed by feeding 1 to L3 together.

[Brief description of drawings]

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

FIG. 2 is an aberration diagram of a lens of Example 1 of the taking lens of the present invention.

FIG. 3 is an aberration diagram of a lens of Example 2 of the taking lens according to the present invention.

FIG. 4 is an aberration diagram of a lens of Example 3 of the taking lens according to the present invention.

FIG. 5 is an aberration diagram of a lens of Example 4 of the taking lens of the present invention.

FIG. 6 is an aberration diagram of a lens of Example 5 of the taking lens of the present invention.

FIG. 7 is an aberration diagram of a lens of Example 6 of the taking lens of the present invention.

FIG. 8 is an aberration diagram of a lens of Example 7 of the taking lens of the present invention.

FIG. 9 is an aberration diagram of a lens of Example 8 of the taking lens of the present invention.

[Explanation of symbols]

 L1 Positive first lens L2 Negative second lens L3 Biconvex third lens L4 Negative meniscus fourth lens

Claims (1)

[Claims] 1. A positive first lens L1 whose image-side surface is concave on the image side, a negative second lens L2 whose object-side surface is concave on the object side, and a biconvex third lens L3 in order from the object side. And a negative meniscus fourth lens L4 having a convex surface directed toward the image side, the four lenses in four groups, wherein the first lens L1 and the fourth lens L4 are made of a plastic material, and the second lens L2 and the second lens L2. A taking lens, wherein the third lens L3 is made of a glass material, the object side surface of the first lens L1 is aspherical, and the following conditions are satisfied. 1.3 <f1 / f <2.4 (1) -1.2 <f4 / f <-0.6 (2) -2.0 <fa / f <-0.8 (3) 0.075 < S1 / r1 <0.0825 (4) where f: focal length of the entire system, f1: positive focal length of the first lens L1, f4: negative focal length of the fourth lens L4, fa: positive first lens L1 and negative Synthetic focal length of the second lens L2, S1: Numerical value representing the shape of the aspherical surface on the object side surface of the positive first lens L1. In general, the aspherical shape has a coordinate x in the optical axis direction and a direction perpendicular to the optical axis. Where y is the coordinate of R, R is the reference radius of curvature, k is the conic coefficient, and Cn is the aspherical coefficient of the nth order, x (y) = (y ² / R) / (1+ (1-k · y ² / R ^{2) 1/2)} + of ^{C2 · y 2 + C4 · y} 4 + C6 · y 6 + C8 · y 8 + C10 · y 10 + ···· (a) represented by the formula, the near-axis of the plane When the index radius is defined as r = 1 / (2 · C2 + (1 / R)), S1 = a numerical value represented by x (0.4 · r), r1: positive object-side aspherical surface of the first lens L1 Is the paraxial radius of curvature of.