JP2001166359A - Reading lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reading lens whose reduction ratio is about 0.236, where chromatic aberration is extremely accurately compensated in order to excellently read a color original, which is bright such as an f-number is about 4.2, whose aperture efficiency is kept near to 100% to a peripheral part, where some aberration is excellently compensated and which has high contrast in a high spatial frequency area. SOLUTION: This reading lens is constituted of four groups being six lenses, that is, a 1st group G1 on an object side is a 1st lens L1 being a convex meniscus lens whose convex surface faces to the object side, a 2nd group G2 is a combined lens consisting of a 2nd lens L2 being a convex meniscus lens whose convex surface faces to the object side and a 3rd lens L3 being a concave meniscus lens whose convex surface faces to the object side and has negative refractive power, a 3rd group G3 is a combined lens consisting of a 4th lens L4 being a concave meniscus lens whose concave surface faces to the object side and a 5th lens L5 being a convex meniscus lens whose concave surface faces to the object side and has negative refractive power, and a 4th group G4 is a 6th lens L6 being a convex meniscus lens whose convex surface faces to an image side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a reading lens,
In particular, the present invention relates to a full-color reading lens used for a facsimile, a document reading unit of a digital copying machine, and various image scanners.

[0002]

2. Description of the Related Art In a facsimile or a document reading section of a digital copying machine or an image scanner, image information to be read is reduced by a reading lens and a CCD (Charge Coupled Device) is used.
The image information is converted into a signal by forming an image on a solid-state image sensor such as that described in (2). Further, recent three-color CCDs often employ three-line CCDs. This is because light receiving elements having red, green, and blue filters are arranged in three rows on one chip, and a color image is formed on this light receiving surface.
The color image is separated into primary colors to signal color image information.

In such a reading lens, in order to read a color original satisfactorily, it is necessary to match the image forming positions of red, green, and blue on the light receiving surface in the optical axis direction. Chromatic aberration must be corrected. Such chromatic aberration correction needs to be corrected with extremely high accuracy. Further, such a reading lens generally requires a high contrast in a high spatial frequency region on an image plane, and has an aperture efficiency of 100 to the periphery of the angle of view.
% Is required.

Conventionally, as such a reading lens, a Gauss type having four elements in six groups has been used. Japanese Patent Application Laid-Open No. 6-109971 discloses a Gauss type color reading lens.
JP, JP-A-10-68881, JP-A-10-6881
The invention according to Japanese Patent No. 253881 and Japanese Patent Application Laid-Open No. H11-109221 is disclosed.

[0005]

However, the above-mentioned conventional color reading lens has various problems. For example, the invention according to Japanese Patent Laid-Open No. 6-109971 has a dark F-number of 5. Also, the invention according to Japanese Patent Application Laid-Open No. 10-68881 has a small shrinkage of 0.189, and when the shrinkage is about 0.236, the amount of aberration becomes large and cannot be used. Further, the invention according to Japanese Patent Application Laid-Open No. H10-253881 has a large aperture with an F-number of about 4 and a half angle of view of 17
In the case of about 18 °, the shrinkage is as small as 0.189, and when the shrinkage is about 0.236, the amount of aberration becomes large and cannot be used. Conversely, when the shrinkage is more than 0.2, the image becomes darker than the F-number 6. Or the half angle of view is only as narrow as 11 °. Also, JP-A-11-10922
The invention according to the first publication has a very narrow half angle of view of about 10 °.

Accordingly, the present invention has been made in view of the above problems, and has a reduction ratio of about 0.236, chromatic aberration is corrected with extremely high precision in order to read a color original satisfactorily, and the F-number is small. It is an object of the present invention to provide a reading lens which is as bright as about 4.2, has an aperture efficiency close to 100% up to the peripheral portion, and has well corrected various aberrations, and has a high contrast in a high spatial frequency region.

[0007]

The above object is achieved by the following reading lens according to the present invention. That is, in order to solve the above problem, a reading lens according to claim 1 includes a first group to a fourth group arranged in order from the object side, and the first group is a convex meniscus lens having a convex surface facing the object side. The second group is a cemented lens of a second lens having a positive refractive power and a third lens having a negative refractive power. The third group has a negative refractive power. A cemented lens composed of a fourth lens having a positive power and a fifth lens having a positive refractive power. The fourth group is a sixth lens of a convex meniscus lens having a negative refractive power. A reading lens having a group of six,
The average of δθgd of the lenses (first, second, fifth, and sixth lenses) whose δθ convex has a positive refractive power, and the average of δθgd of the lenses (third and fourth lenses) whose δθ concave has a negative refractive power. Average, δθgd is deviation of θgd (partial dispersion ratio) from a standard line (a straight line connecting C7 and F2), θgd (partial dispersion ratio) is (ng−nd) / (nF−nC), and nd is refraction of d line. Ng is the refractive index of the g-line, nF is the refractive index of the F-line, nC is the refractive index of the C-line, f1 is the focal length of the e-line of the first lens, f
6 is the focal length of the e-line of the sixth lens, f25 is the combined focal length of the e-line from the second lens to the fifth lens, f is the combined focal length of the e-line of the entire system, and n convex is the positive refractive power. Average of the nd of the lenses (first, second, fifth, and sixth lenses), and n of the lenses (third and fourth lenses) in which the n-concave has a negative refractive power.
When d is the average, the following conditional expressions (1-1) to (4-
It is characterized by satisfying 1).

(1-1) 0.0095 <δθ convex−δθ concave <0.0203 (2-1) 0.87 <f1 / f6 <1.20 (3-1) −0.57 <f25 / f <−0.37 (4-1) −0.0223 <n convex−n concave <−0.002
7

Here, the conditional expression (1-1) is a condition for favorably correcting axial chromatic aberration. When the value exceeds the upper limit, the axial chromatic aberration is overcorrected, and the axial chromatic aberration increases toward the positive side at a wavelength shorter than the main wavelength. On the other hand, if the lower limit is exceeded, the axial chromatic aberration will be undercorrected, and the axial chromatic aberration will be larger on the negative side on the short wavelength side than the main wavelength.

The conditional expression (2-1) defines the power of the first lens unit. And when this limit is exceeded,
The power of the first lens unit becomes too weak, the lens becomes large, and causes an increase in cost. Conversely, exceeding the lower limit is advantageous for downsizing the lens, but increases the flare.

The conditional expression (3-1) represents the second lens unit and the third lens unit.
It determines the power of group synthesis. If the upper limit is exceeded, the spherical aberration becomes negative and large. Conversely, if the lower limit is exceeded, on the contrary, the spherical aberration becomes positive and large, and the coma flare also increases. If the condition is out of the range, the balance between the on-axis and off-axis aberrations will be greatly disrupted.

The conditional expression (4-1) defines the range of the refractive index of the four convex lenses and the two concave lenses constituting the reading lens according to the present invention. If the upper limit is exceeded, the Petzval sum becomes too small, the image surface falls to the positive side, and the field curvature increases. Conversely, if the lower limit is exceeded, on the contrary, the Pebbal sum becomes too large, the image plane falls to the negative side, the astigmatic difference increases, and outside this range, good imaging performance over the entire screen is obtained. You can't get it.

According to a second aspect of the present invention, there is provided a reading lens according to the first aspect, wherein f13 is a distance e from the first lens to the third lens.
When the focal length of the line, f46 is the focal length of the e-line from the fourth lens to the sixth lens, and d5 is the fifth surface interval counted from the object side, the following conditional expressions (5-1) and (6) −
It is characterized by satisfying 1).

(5-1) 1.03 <f13 / f46 <1.35 (6-1) 0.24 <d5 / f <0.26

Here, these conditional expressions (5-1) and (6-1) are conditions for obtaining even better performance in a reading lens satisfying the above condition (1). That is, the conditional expression (5-1) defines the ratio between the combined power of the first and second units and the combined power of the third and fourth units. If the upper limit is exceeded, the distortion becomes negative and large. Conversely, if the value exceeds the lower limit, the distortion becomes positive and large.

The conditional expression (6-1) represents the second and third lens units.
It determines the air spacing between groups. When the value exceeds the upper limit, the Petzval sum increases, the astigmatic difference increases, and the lens becomes longer. Conversely, when the value exceeds the lower limit, the Petzval sum becomes small, the field curvature becomes large, and the coma flare becomes large.

According to a third aspect of the present invention, there is provided a reading lens according to the third aspect, wherein f is the combined focal length of the e-line of the entire system, and ri is counted from the object side. When the i-th radius of curvature and di are the i-th surface interval counted from the object side, the following conditional expression (7-1)
To (9-1).

(7-1) 0.32 <r1 / f <0.35 (8-1) -0.21 <r5 / f <-0.17 (9-1) 0.85 <(d3 + d4) / f <0.93

Here, these conditional expressions (7-1) to (9)
Condition -1) is a condition for obtaining even better performance in a reading lens satisfying the condition according to claim 2. That is, the conditional expression (7-1) determines the radius of curvature of the object-side surface of the first lens. When the value exceeds the upper limit, the spherical aberration becomes negative and large, and the lens becomes long. Conversely, below the lower limit, the spherical aberration becomes positive and large.

Condition (8-1) defines the radius of curvature of the object-side surface of the fourth lens. When the value exceeds the upper limit, the image plane becomes positive and the spherical aberration becomes negative. Conversely, when the value goes below the lower limit, the image surface becomes negative, the spherical aberration becomes positive, and when the value is out of the range, the center and the periphery cannot be balanced.

The conditional expression (9-1) defines the lens thickness of the second group of lenses (the second lens and the third lens). If the upper limit is exceeded, the Petzval sum becomes small, and the balance between spherical aberration and curvature of field deteriorates. Conversely, if the lower limit is exceeded, the field curvature becomes too negative and large, and the off-axis performance is greatly deteriorated.

According to another aspect of the present invention, there is provided a reading lens including a first lens unit, a fourth lens unit, and a fourth lens unit arranged in order from the object side. The second group is a cemented lens of a second lens of a convex meniscus lens having a convex surface facing the object side and a third lens of a concave meniscus lens having a convex surface facing the object side. The third lens unit has a negative refractive power. The third lens unit is a cemented lens composed of a fourth lens of a concave meniscus lens having a concave surface facing the object side and a fifth lens of a convex meniscus lens having a concave surface facing the object surface. The fourth lens group is a reading lens having a four-element six-lens structure, which is a sixth lens of a convex meniscus lens having a convex surface facing the image side, wherein the δθ convex lens has a positive refractive power (first and second lenses). (2, 5th, 6th lens) δθgd average, δθ concave is negative bending Lens having a force (third, fourth
Average of δθgd of lens), deviation of δθgd from standard line (straight line connecting C7 and F2) of θgd (partial dispersion ratio),
θgd (partial dispersion ratio) is (ng−nd) / (nF−n)
C), nd is the refractive index of the d-line, ng is the refractive index of the g-line, nF
Is the refractive index of F-line, nC is the refractive index of C-line, f1 is the focal length of e-line of the first lens, f6 is the focal length of e-line of the sixth lens, and f25 is the focal length of the second to fifth lenses. e is the combined focal length of e-line, f is the combined focal length of e-line of the entire system, n is the average of nd of lenses (first, second, fifth, and sixth lenses) having positive refractive power, and n is concave. Satisfies the following conditional expressions (1-2) to (4-2), where nd is the average of nd of the lenses having the negative refractive power (the third and fourth lenses).

(1-2) 0.0108 <δθ convex-δθ concave <0.0202 (2-2) 0.89 <f1 / f6 <1.20 (3-2) -0.57 <f25 / f <−0.37 (4-2) −0.0223 <n convex−n concave <−0.002
7

As described above, the reading lens according to claim 4 is
The second, third, fourth, and fifth lenses in the reading lens according to claim 1 each have a meniscus shape,
By satisfying the conditional expressions (1-2) to (9-2) which are slightly narrower than the conditional expressions (1-1) to (9-1), a good performance utilizing the characteristics of the meniscus lens is obtained. It is intended to be.

According to a fifth aspect of the present invention, there is provided a reading lens according to the fifth aspect, wherein f13 is a distance from the first lens to the third lens.
When the focal length of the line, f46 is the focal length of the e-line from the fourth lens to the sixth lens, and d5 is the fifth surface interval counted from the object side, the following conditional expressions (5-2) and (6) −
2) is satisfied.

(5-2) 1.03 <f13 / f46 <1.35 (6-2) 0.24 <d5 / f <0.26

Here, these conditional expressions (5-2) and (6-2) are conditions for obtaining even better performance in a reading lens satisfying the condition according to the fourth aspect. That is, with respect to the reading lens according to claim 2,
It is intended to obtain good performance utilizing characteristics of the meniscus lens. However, these conditional expressions (5-
2) and (6-2) are conditional expressions (5) according to the second aspect.
-1) and (6-1), the description of each conditional expression is omitted.

According to a sixth aspect of the present invention, there is provided a reading lens according to the sixth aspect, wherein f is a combined focal length of e-line of the entire system, and ri is a number counted from the object side. When the i-th radius of curvature and di are the i-th surface interval counted from the object side, the following conditional expression (7-2)
To (9-2).

(7-2) 0.32 <r1 / f <0.35 (8-2) −0.21 <r5 / f <−0.17 (9-2) 0.85 <(d3 + d4) / f <0.93

Here, these conditional expressions (7-2) to (9)
Condition -2) is a condition for obtaining more excellent performance in a reading lens satisfying the condition according to claim 5. That is, a good performance utilizing the characteristics of the meniscus lens can be obtained with respect to the reading lens according to the third aspect. However, these conditional expressions (7-
2) to (9-2) are conditional expressions (7-) according to the third aspect.
Since the conditions are the same as 1) to (9-1), the description of each conditional expression is omitted.

[0031]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
An embodiment of the present invention will be described. FIG. 1 is a schematic sectional view showing a configuration of a full-color reading lens according to one embodiment of the present invention.

As shown in FIG. 1, in the full-color reading lens according to the present embodiment, the first group G1 to the fourth group G4 are arranged on the optical axis in order from the object side. The first group G1 includes a first lens L1 of a convex meniscus lens having a convex surface facing the object side. The second group G2 includes a second lens L2 of a convex meniscus lens having a convex surface facing the object side.
And a third lens L3 of a concave meniscus lens having a convex surface facing the object side, and has a negative refractive power. The third group G3 is a cemented lens composed of a fourth lens L4 of a concave meniscus lens having a concave surface facing the object side and a fifth lens L5 of a convex meniscus lens having a concave surface facing the object side, and having a negative refractive power. I have. The fourth unit G4 includes a sixth lens L6 of a convex meniscus lens having a convex surface facing the image side. Thus, the six first to sixth lenses L1 to L4 of the fourth group G1 to G4
L6. A contact glass CG1 is arranged on the object side of the first group G1, and a CCD cover glass CG2 is arranged on the image side of the fourth group G4. Further, an aperture stop S is arranged between the second group G2 and the third group G3.

The meanings of the reference numerals in FIG. 1 are as follows. ri (i = 1 to 10): radius of curvature of the i-th lens surface counted from the object side di (i = 1 to 9): i-th surface distance nj (j = 1 to 6) counted from the object side : Refractive index of the material of the j-th lens counted from the object side νj (j = 1 to 6): Abbe number of the material of the j-th lens counted from the object side rc1: Curvature radius of the contact glass on the object side rc2: Curvature radius on the image side of the contact glass rc3: Curvature radius on the object side of the CCD cover glass rc4: Curvature radius on the image side of the CCD cover glass dc1: Wall thickness of the contact glass dc3: Wall thickness of the CCD cover glass nc1: CCD cover glass Nc3: Refractive index of contact glass νc1: Abbe number of contact glass νc3: Abbe number of CCD cover glass

[0034]

EXAMPLES Examples 1 to 7 show various aberrations and specific numerical data of a full-color reading lens according to an embodiment of the present invention. Here, Examples 1 to 7 are examples corresponding to the inventions according to Claims 1 to 3, and
4 to 7 are examples corresponding to the inventions according to claims 4 to 6.

2 to 8 show various aberrations in Examples 1 to 7, specifically, spherical aberration, astigmatism, distortion,
FIG. 4 is an aberration curve diagram showing a coma aberration. Note that in these aberration curve diagrams of FIGS. 2 to 8, the solid line is the e-line (546.
07 nm), the dotted line indicates the g line (436.83 nm), and the broken line indicates the c line (656.27 nm). Also,
The astigmatism is due to the e-line (546.07 nm). In the figure, the solid line represents a sagittal ray, and the broken line represents a meridional ray.

The meanings of the symbols in Examples 1 to 7 are as follows. However, description of the same components as those in FIG. 1 will be omitted. f: Composite focal length of e-line of the whole system FNO: F-number m: Shrinkage ratio ω: Half angle of view (degree) Y: Object height nd: Refractive index of d-line ng: Refractive index of g-line nF: F-line of F-line Refractive index nC: Refractive index of C line δθ convex: Average of δθgd of lenses having positive refractive power (first, second, fifth, sixth lenses) δθ concave: Lens having negative refractive power (third lens) , 4th lens) Average of δθgd n-convex: lenses having first refractive power (first, second, fifth,
Average of nd of sixth lens) n-concave: lenses having negative refractive power (third and fourth lenses)
F1: focal length of e-line of the first lens f6: focal length of e-line of the sixth lens f25: composite focal length of e-line from the second lens to the fifth lens f13: focal length of the first lens F-line focal length of the e-line from the fourth lens to the sixth lens f46: focal length of the e-line from the third lens to the third lens

<Embodiment 1> The spherical aberration, astigmatism, distortion, and coma of the full-color reading lens according to Embodiment 1 are as shown in FIG. The specific numerical data is as shown in Table 1 below.

[0038]

[Table 1]

Second Embodiment The spherical aberration, astigmatism, distortion, and coma of the full-color reading lens according to the second embodiment are as shown in FIG. The specific numerical data is as shown in Table 2 below.

[0040]

[Table 2]

<Embodiment 3> The spherical aberration, astigmatism, distortion and coma of the full-color reading lens according to Embodiment 3 are as shown in FIG. The specific numerical data is as shown in Table 3 below.

[0042]

[Table 3]

<Embodiment 4> The spherical aberration, astigmatism, distortion, and coma of the full-color reading lens according to Embodiment 4 are as shown in FIG. The specific numerical data is as shown in Table 4 below.

[0044]

[Table 4]

<Embodiment 5> The spherical aberration, astigmatism, distortion and coma of the full-color reading lens according to Embodiment 5 are as shown in FIG. The specific numerical data is as shown in Table 5 below.

[0046]

[Table 5]

<Embodiment 6> The spherical aberration, astigmatism, distortion, and coma of the full-color reading lens according to Embodiment 6 are as shown in FIG. The specific numerical data is as shown in Table 6 below.

[0048]

[Table 6]

<Embodiment 7> The spherical aberration, astigmatism, distortion, and coma of the full-color reading lens according to Embodiment 7 are as shown in FIG. The specific numerical data is as shown in Table 7 below.

[0050]

[Table 7]

The values of the conditional expressions (1-1) to (9-1) and (1-2) to (9-2) in Examples 1 to 7 are as shown in Table 8 below. Become.

[0052]

[Table 8]

[0053]

As described above in detail, according to the reading lens of the present invention, the following effects can be obtained. That is, as is clear from the aberration curve diagram, at a reduction ratio of about 0.236, extremely high-precision chromatic aberration correction is performed in order to read a color original satisfactorily, and each color of red, green, and blue is adopted when a three-line CCD is adopted. Variations in the SN ratio can be kept small. At the same time, the F-number is 4.
2, a lens having a large aperture, a good balance of on-axis and off-axis aberrations, an aperture efficiency of nearly 100%, and a high contrast in a high spatial frequency region can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a configuration of a full-color reading lens according to an embodiment of the present invention.

FIG. 2 is an aberration curve diagram showing spherical aberration, astigmatism, distortion, and coma in Example 1.

FIG. 3 is an aberration curve diagram showing spherical aberration, astigmatism, distortion, and coma in Example 2.

FIG. 4 is an aberration curve diagram showing spherical aberration, astigmatism, distortion, and coma aberration in Example 3.

FIG. 5 is an aberration curve diagram showing spherical aberration, astigmatism, distortion, and coma aberration in Example 4.

FIG. 6 is an aberration curve diagram showing spherical aberration, astigmatism, distortion, and coma aberration in Example 5.

FIG. 7 is an aberration curve diagram showing spherical aberration, astigmatism, distortion, and coma aberration in Example 6.

FIG. 8 is an aberration curve diagram showing spherical aberration, astigmatism, distortion, and coma aberration in Example 7.

[Explanation of symbols]

 G1 First group G2 Second group G3 Third group G4 Fourth group L1 First lens L2 Second lens L3 Third lens L4 Fourth lens L5 Fifth lens L6 Sixth lens L1 First lens CG1 Contact glass CG2 CCD cover Glass S aperture stop

Claims (6)

[Claims] 1. A first group to a fourth group are arranged in order from the object side, the first group being a first lens of a convex meniscus lens having a convex surface facing the object side, and the second group being a positive lens. A cemented lens of a second lens having a refractive power and a third lens having a negative refractive power. The third group has a fourth lens having a negative refractive power and a positive refractive power. The fourth lens unit is a reading lens having a four-element six-lens structure, which is a sixth lens of a convex meniscus lens having a negative refractive power and a sixth lens having a convex surface facing the image side. A reading lens characterized by satisfying conditional expressions. (1-1) 0.0095 <δθ convex−δθ concave <0.020
3 (2-1) 0.87 <f1 / f6 <1.20 (3-1) -0.57 <f25 / f <-0.37 (4-1) -0.0223 <n convex-n concave <-0.002
7 However, δθ convex: average of δθgd of lenses having positive refractive power (first, second, fifth, sixth lenses) δθ concave: of lenses having negative refractive power (third, fourth lenses) Average of δθgd δθgd: Standard line of θgd (partial dispersion ratio) (C7 and F2
Θgd (partial dispersion ratio): (ng−nd) / (nF−n)
C) nd: refractive index of d-line ng: refractive index of g-line nF: refractive index of F-line nC: refractive index of C-line f1: focal length of e-line of first lens f6: e-line of sixth lens Focal length f25: Synthetic focal length of e-line from the second lens to the fifth lens f: Synthetic focal length of e-line of the whole system n-convex: Lenses having positive refractive power (first, second, fifth,
Average of nd of sixth lens) n-concave: lenses having negative refractive power (third and fourth lenses)
Average of nd 2. The reading lens according to claim 1, wherein the following conditional expression is satisfied. (5-1) 1.03 <f13 / f46 <1.35 (6-1) 0.24 <d5 / f <0.26 where f13: focal length of e-line from the first lens to the third lens. f46: focal length of e-line from the fourth lens to the sixth lens d5: fifth distance from the object side 3. The reading lens according to claim 2, wherein the following conditional expression is satisfied. (7-1) 0.32 <r1 / f <0.35 (8-1) -0.21 <r5 / f <-0.17 (9-1) 0.85 <(d3 + d4) / f <0 .93 where, f: Composite focal length of e-line of the whole system ri: i-th radius of curvature counted from the object side di: i-th surface interval counted from the object side 4. A first group to a fourth group are arranged in order from the object side, the first group being a first lens of a convex meniscus lens having a convex surface facing the object side, and the second group being a object side. This is a cemented lens composed of a second lens of a convex meniscus lens having a convex surface facing the third lens and a third lens of a concave meniscus lens having a convex surface facing the object side. The third group has a concave surface facing the object side. A fourth meniscus lens having a negative refractive power, and the fourth group is a convex meniscus lens having a convex surface facing the image side, the fourth lens being a concave meniscus lens and the fifth lens being a convex meniscus lens having a concave surface facing the object side. 6. The reading lens according to claim 6, wherein said reading lens has a four-element six-lens structure, and satisfies the following conditional expression. (1-2) 0.0108 <δθ convex−δθ concave <0.020
2 (2-2) 0.89 <f1 / f6 <1.20 (3-2) -0.57 <f25 / f <-0.37 (4-2) -0.0223 <n convex-n concave <-0.002
7 However, δθ convex: average of δθgd of lenses having positive refractive power (first, second, fifth, sixth lenses) δθ concave: of lenses having negative refractive power (third, fourth lenses) Average of δθgd δθgd: Standard line of θgd (partial dispersion ratio) (C7 and F2
Θgd (partial dispersion ratio): (ng−nd) / (nF−n)
C) nd: refractive index of d-line ng: refractive index of g-line nF: refractive index of F-line nC: refractive index of C-line f1: focal length of e-line of first lens f6: e-line of sixth lens Focal length f25: Synthetic focal length of e-line from the second lens to the fifth lens f: Synthetic focal length of e-line of the whole system n-convex: Lenses having positive refractive power (first, second, fifth,
Average of nd of sixth lens) n-concave: lenses having negative refractive power (third and fourth lenses)
Average of nd 5. The reading lens according to claim 4, wherein the following conditional expression is satisfied. (5-2) 1.03 <f13 / f46 <1.35 (6-2) 0.24 <d5 / f <0.26 where f13: focal length of e-line from the first lens to the third lens. f46: focal length of e-line from the fourth lens to the sixth lens d5: fifth distance from the object side 6. The reading lens according to claim 5, wherein the following conditional expression is satisfied. (7-2) 0.32 <r1 / f <0.35 (8-2) -0.21 <r5 / f <-0.17 (9-2) 0.85 <(d3 + d4) / f <0 .93 where, f: Composite focal length of e-line of the whole system ri: i-th radius of curvature counted from the object side di: i-th surface interval counted from the object side