JPH08160292A - Triplet lens system - Google Patents

Abstract

PURPOSE: To excellently compensate chromatic aberration and on-axis chromatic aberration and to miniaturize a lens by shortening the back focal length by limiting a composite focal distance of a first and a second lenses within a specified range. CONSTITUTION: This system comprises, in order from the object side, a first lens 1, a second lens 2 made of a plastic, a third lens 3 made of a plastic and having positive refractive power and a rear diaphragm r7. The conditions: -f<f12<-0.3f, 0.09/f<1/ν1f1+1/ν3f3<0.18/f, -3.5<f1/f2<-1.75, 0.02f<T1<0.04f, 0.01f<T4<0.04f, 0.2f<R5<0.6f are satisfied, where, f, f1, f2, f3 are focal distances of the whole system, the first lens, the second lens and the third lens, respectively. f12 is the composite focal distance of the first and the second lenses, ν1, ν3 are Abbe numbers of the first and the third lenses, respectively, R5 is a radius of curvature of the fifth surface from the object side and T1, T4 are distances between surfaces on the axis, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a triplet lens system having a rear diaphragm, and more particularly to a simple and compact photographing lens suitable for a wide-format large-format camera such as an instant camera.

[0002]

2. Description of the Related Art In the case of popular large-format cameras such as so-called instant cameras, a relatively large image circle is required as compared with a small camera, while miniaturization and lowering of the lens itself or assembly cost are required. There is a strong demand. First, in order to meet the demands for brightness of about F9 to F17 and covering power of about 55 ° in view angle while satisfying the demand for downsizing and cost reduction of the lens itself, generally, 3
A triplet lens system having a group of three elements is often selected, and in order to further reduce the assembling cost, a post diaphragm in which the diaphragm is arranged in the latter stage of the lens system is often adopted.

[0003]

The triplet lens system having the rear diaphragm as described above generally meets the demand for downsizing and cost reduction, but it is used for a popular type camera such as an instant camera. In this case, there is a strong demand for further cost reduction, and in place of the optical glass that requires grinding and molding, it is desired to use a plastic lens that can be injection-molded suitable for mass production. However, plastics that can be used as a material for forming a refractive optical system have a very narrow selection range in terms of refractive index and Abbe number, and there is a problem that it is difficult to simultaneously correct monochromatic aberration and chromatic aberration. It was

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of such problems, and a plastic lens is used as a part of an optical system in order to reduce the manufacturing cost, but each aberration is favorable. It is an object of the present invention to provide a triplet lens system having a post-correction and a compact rear diaphragm.

In summary, the triplet lens system of the present invention comprises, in order from the object side, a meniscus-shaped first lens having a positive refractive power with a convex surface facing the object side, and a second lens having a negative refractive power. By having a third lens having a positive refracting power and a rear diaphragm, including a plastic lens in a part of the lens system, and satisfying the conditions defined by the following formulas 7 to 9, Is achieved, and more preferably, the above is premised on several 10 to several 1.
By satisfying the condition specified in 2, it is intended to achieve better aberration correction.

[0006]

(Equation 7)

[0007]

(Equation 8)

[0008]

[Equation 9]

[0009]

[Equation 10]

[0010]

[Equation 11]

[0011]

(Equation 12)

However, in the above formula, f is the entire focal length, f1 is the focal length of the first lens, f2 is the focal length of the second lens, f3 is the focal length of the third lens, and f12 is the first lens. Composite focal length of the second lens, ν1 is the Abbe number of the first lens, ν3 is the Abbe number of the third lens, R5 is the radius of curvature of the fifth surface from the object side, and T1 is 1 from the object side.
The fourth axial upper surface distance, T4, represents the fourth axial upper surface distance from the object side.

[0013]

That is, when the second lens and the third lens of the triplet lens system of the rear diaphragm as the premise of the present invention are made of optical plastic, the aspherical shape can be easily adopted and the manufacturing cost can be reduced. Although it is possible to reduce the cost, color correction is generally difficult because the Abbe number and the range of refractive index of the plastic that can be selected are extremely limited. However, in the present invention, the synthetic focus of the first lens and the second lens is used. By limiting the distance to the range defined by the above formula 7, it is possible to achieve good correction of chromatic aberration, in particular correction of axial chromatic aberration, and also to shorten the back focus and achieve size reduction of the lens. It is a thing.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. 1 to 5 are cross-sectional views of optical axes of triplet lens systems according to first to fifth embodiments of the present invention. As shown in the respective drawings, the triplet lens system of the present invention is , Convex from the object side to the object side r
1, a meniscus-shaped first lens 1 having positive refractive power, a second lens 2 made of plastic and having negative refractive power, a third lens 3 made of plastic and having positive refractive power, and In addition to having the aperture stop r7, it satisfies the conditions defined in the above formulas 7 to 12.
1 to 5, r1 to r7 indicate boundary surfaces.

First, the condition defined by the equation (7) is a condition involved in correction of axial chromatic aberration and miniaturization of the lens.
If the combined focal length of the lens 1 and the second lens 2 exceeds the upper limit value defined in the formula 7, it becomes difficult to correct the axial chromatic aberration due to the restrictions of the Abbe number ν and the refractive index T of the plastic that can be adopted. If the lower limit value defined in 7 is exceeded, the back focus becomes too long and it becomes difficult to downsize the lens system.

Next, the condition defined by the equation (8) is for improving the correction of lateral chromatic aberration and chromatic aberration under the condition defined by the equation (7). Exceeding the upper limit defined in Eq. 8 causes lateral chromatic aberration of the F line to become excessive on the plus side, and exceeding the lower limit causes axial chromatic aberration of the F line to become excessive on the plus side to reduce the Petzval sum. When the negative power of the two lenses is reduced, it becomes difficult to correct chromatic aberration.

Further, the condition defined by the equation 9 is a condition relating to the astigmatic difference in the high angle of view and the miniaturization of the lens, and the ratio of the focal lengths of the first lens 1 and the second lens 2 is several. If the value exceeds the upper limit defined in 9, the Petzval sum cannot be made sufficiently small and the astigmatic difference at a high angle of view becomes large. Further, when the value goes below the lower limit, distortion aberration is corrected, the positive power of the first lens becomes weak, and it becomes difficult to downsize the lens system. After satisfying the conditions defined by these formulas 7 to 9, it is desirable to satisfy the conditions defined by formulas 10 to 12 in order to further correct each aberration.

First, the conditions defined by the equation (10) are conditions involved in downsizing of the lens system, proper correction of spherical aberration, securing of light quantity, and the like. If the axial upper surface distance T1 between the first boundary surface r1 and the second boundary surface r2 exceeds the upper limit defined by the expression 10, it becomes difficult to downsize the lens system, and the spherical aberration is insufficiently corrected. Further, if the axial upper surface interval T1 exceeds the lower limit value defined in the equation 10, spherical aberration is overcorrected, and it becomes difficult to secure the light quantity.

Next, the condition defined by the equation 11 is a condition relating to correction of field curvature, coma aberration, distortion aberration, astigmatism and the like. When the axial upper surface distance T4 between the fourth boundary surface r4 and the fifth boundary surface r5 exceeds the upper limit value defined in the equation 11, the field curvature is insufficiently corrected, and a large coma aberration occurs around the screen. Further, when the axial upper surface interval T4 exceeds the lower limit value defined by the equation 11, distortion and astigmatism increase, and coma occurs from the middle zone of the screen to the periphery.

Furthermore, the condition defined by the formula 12 is for better correcting the spherical aberration and the astigmatism under the condition defined by the formulas 7 and 8. If the radius of curvature of the fifth boundary surface r5 exceeds the upper limit value defined by the equation 12, spherical aberration and field curvature are undercorrected, and if the radius of curvature of the fifth boundary surface r5 exceeds the lower limit value defined by the equation 12. Spherical aberration is overcorrected, astigmatism is increased, and the difference between the sagittal image surface and the meridional image surface is large, making correction difficult.

Next, specific numerical values of the embodiments whose optical axis cross sections are shown in FIGS. 1 to 5 are shown in Tables 1 to 5, respectively, and their aberration diagrams are sequentially shown in FIGS. 6 to 10. Each is shown. In these tables, f is the focal length and F is F
No., r1 to r7 are the radii of curvature of each boundary surface, T1 to T
6 is the distance between the axial upper surfaces of the respective boundary surfaces, Nd1 to Nd3 are the refractive indices of the lenses, νd1 to νd3 are the Abbe numbers of the lenses, and the aspherical shape is the optical axis direction X, and the direction orthogonal to the optical axis. When Y is set, it is represented by Expression 13. Further, in the aberration diagram, d is the d line, F is the F line, SC is the sine condition, S is the sagittal image plane, and M is the meridional image plane. In Equation 13, r is a paraxial radius of curvature and ε is a conic coefficient.

[0022]

(Equation 13)

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

[0027]

[Table 5]

[0028]

As can be seen from the above embodiment and its aberration diagram, the triplet lens system of the rear diaphragm of the present invention is
Even if an inexpensive plastic is used for the second lens and the third lens, and even if an angle of view of about 57 degrees is secured, various aberrations are corrected well, and the telephoto ratio is about 1 to obtain a compact lens system. be able to. Also, with the adoption of plastic, the introduction of an aspherical surface becomes extremely easy, and it becomes possible to correct each aberration that cannot be corrected by a spherical lens.

[Brief description of drawings]

FIG. 1 is an optical axis cross-sectional view of a triplet lens system according to Example 1 of the present invention.

FIG. 2 is an optical axis sectional view of a triplet lens system according to a second example of the present invention.

FIG. 3 is an optical axis sectional view of a triplet lens system according to a third example of the present invention.

FIG. 4 is an optical axis sectional view of a triplet lens system according to a fourth example of the present invention.

FIG. 5 is an optical axis sectional view of a triplet lens system according to a fifth example of the present invention.

FIG. 6 is an aberration diagram of a triplet lens system according to Example 1 of the present invention.

FIG. 7 is an aberration diagram of a triplet lens system according to Example 2 of the present invention.

FIG. 8 is an aberration diagram of a triplet lens system according to Example 3 of the present invention.

FIG. 9 is an aberration diagram of a triplet lens system according to Example 4 of the present invention.

FIG. 10 is an aberration diagram of a triplet lens system according to Example 5 of the present invention.

[Explanation of symbols]

 1 1st lens 2 2nd lens 3 3rd lens r1, r2, r3, r4, r5, r6, r7 Boundary surface

Claims (3)

[Claims] 1. A meniscus-shaped first lens having a positive refractive power with a convex surface facing the object side, a second lens having a negative refractive power, and a third lens having a positive refractive power in order from the object side. A triplet lens system having a lens and a post diaphragm, including a plastic lens as part of the lens system, and satisfying the conditions defined in the following formulas 1 to 3. [Equation 1] [Equation 2] (Equation 3) Here, f: overall focal length f1: first lens focal length f2: second lens focal length f3: third lens focal length f12: first and second lens combined focal length ν1: first lens Abbe number ν3: Abbe number of the third lens 2. The triplet lens system according to claim 1, wherein the triplet lens system satisfies the conditions defined by the following expressions 4 to 6. [Equation 4] (Equation 5) (Equation 6) Where f: overall focal length R5: radius of curvature of the fifth surface from the object side T1: distance between the first axial upper surfaces from the object side T4: distance between the fourth axial upper surfaces from the object side 3. The triplet lens system according to claim 1, wherein the first lens is made of glass, has a meniscus shape with a convex surface facing the object side, and has a positive refractive power. The triplet lens is characterized in that the second lens is made of plastic and has a biconcave surface with a negative refracting power, and the third lens is made of plastic or glass and has a biconvex surface with a positive refracting power. system.