JPH10186258A - (ftheta) lens system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize FNO to be <=20 while preventing the diameter of a lens from being made large and to secure the distance of about 0.314f-0.2f (f: focal distance) between a deflector and a first lens by providing a system with a first and a second lens groups in turn from the deflector side and specifying the respective constitution thereof. SOLUTION: The first lens group I is constituted of a positive meniscus lens L1 whose concave surface is turned to the deflector D side, a negative meniscus lens L2 whose concave surface is turned to the deflector D side and a negative lens L3 in turn from the deflector D side. The second lens group II is constituted of one positive lens group. Besides, it is preferable that this fθlens system is constituted so as to satisfy the relation of 1.5<|fI/fII|<2.4. Provided that (fI) is the focal distance of the first lens group I and (fII) is the focal distance of the second lens group II. Thus, the fθ lens system which is constituted so that the size of the lens is suppressed to be about the half of scanning width and which is brighter than F20 by the comparatively small number of lenses, that means, four lenses as a whole and while securing the distance of about 0.14f-0.2f between a deflector and the first lens L1 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an fθ lens system used for a scanning optical system.

[0002]

2. Description of the Related Art The fθ lens system has an angle θ formed by a light beam deflected by a deflector or the like with the lens optical axis.
(Polarization angle) and a lens system designed so that the distance between the optical axis on the imaging scan plane orthogonal to the optical axis and the imaging position is in a proportional relationship. It is used for constant speed motion. In other words, the deflection angle θ
From the optical axis of the lens system having the focal length f, fxθ
This is a lens system that has the function of focusing on the position.

Generally, in such an fθ lens system, it is necessary to increase the number of lenses or increase the diameter of the lens in order to reduce (brighten) the F _NO or increase the scanning width. A three-element fθ lens system has also been proposed (Japanese Patent Application Laid-Open No. 2-157715), but it is as dark as about F50. When miniaturization is realized, the distance from the deflector such as a polygon mirror to the first lens is 0.1 f (f; focal length)
The degree of freedom with respect to the device configuration, such as the incident angle of light, is low (Japanese Patent Publication No. 63-3283). In a telecentric fθ lens system, a plurality of lenses having a size close to the scanning width are required (Japanese Patent Laid-Open No. 4-52612), and it is unavoidable to increase the cost due to an increase in size and an increase in the number of components.

[0004]

SUMMARY OF THE INVENTION The present invention is configured number is relatively small, while keeping F _NO suppressed large diameter of the lens was reduced to 20 or less, the distance from the deflector to the first lens from 0.14f 0. An object is to obtain an fθ lens system that can secure about 2f.

[0005]

SUMMARY OF THE INVENTION The fθ lens system of the present invention has two aspects. One is a non-telecentric mode including a first lens group and a second lens group in order from the deflector side,
The other is a telecentric mode including a first lens group, a second lens, and a third lens group in order from the deflector side. In the aspect including the first lens group and the second lens group, the first lens group includes, in order from the deflector side, a positive meniscus lens having a concave surface facing the deflector side, and a negative meniscus lens having a concave surface facing the deflector side. , And a negative lens, and the second lens group includes two or less positive lenses.

The fθ lens system preferably satisfies the following conditional expression (1). (1) 1.5 <| f _I / f _II | <2.4, where f _{I is} the focal length of the first lens group, and f _{II is} the focal length of the second lens group.

Furthermore, it is preferable to satisfy the following conditional expressions (2) to (4). _{(2) 0.30 <f 1 /f<0.55} (3) -0.60 <f 23 / f II <-0.35 (4) d 4 <0.08f where, f _1: the first lens The focal length of the positive meniscus lens of the group, f ₂₃ : the combined focal length of the negative meniscus lens and the negative lens of the first lens group, d ₄ : the air gap on the optical axis of the negative meniscus lens and the negative lens of the first lens group, It is.

On the other hand, an embodiment having a first lens group, a second lens group, and a third lens group is suitable for a telecentric system. The first lens group has concave surfaces on the deflector side in order from the deflector side. The second lens group consists of two or less positive lenses, and the third lens group has a convex surface on the deflector side. It is characterized by comprising a plano-convex lens aimed at.

This telecentric fθ lens system is
It is preferable to satisfy the following conditional expressions (1 ′) and (5). (1 ′) 0.9 <| f _I / f _II | <1.5 (5) 0.35 <f _II /f<0.65, where f: focal length of the whole system.

Further, the following conditional expressions (2 '), (3'),
It is preferable to satisfy (4) and (6). (2 ′) 0.22 <f ₁ /f<0.50 (3 ′) − 0.50 <f ₂₃ / f _II <−0.30 (4) d ₄ <0.08f (6) n ₁ > 1.65 where n ₁ is the refractive index of the positive meniscus lens of the first lens group. In the fθ lens system of the present invention, all the constituent lenses can be formed of spherical lenses.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a non-telecentric embodiment, the fθ lens system of the present invention comprises, in order from the deflector side, a positive meniscus lens L1 having a concave surface facing the deflector side, and a negative meniscus lens having a concave surface facing the deflector side. Lens L2 and negative lens L3
And a second lens group consisting of one or two positive lenses, and a relatively small number of four or five lenses in total, and the distance from the deflector to the first lens While securing about 0.14f to 0.2f
It is possible to realize an fθ lens system in which the size of the lens is suppressed to about half of the scanning width and which is brighter than F20.

In the telecentric mode, a plano-convex lens having a convex surface facing the deflector is further added as a third lens group. In this mode, a lens system other than the third lens group added to realize telecentricity with a configuration of 5 to 6 lenses realizes an fθ lens system brighter than F20 while keeping its size down to about half the scanning width. it can.

Conditional expressions (1) and (1 ') are conditions relating to correction of spherical aberration. When the value exceeds the upper limit, correction of spherical aberration is insufficient. Further, astigmatism increases, and it becomes difficult to simultaneously flatten both surfaces of the meridional and the sagittal. If the lower limit is exceeded, the correction of spherical aberration becomes excessive.

Conditional expressions (2) and (2 ') are conditions relating to correction of spherical aberration. Exceeding the upper limit results in excessive correction of spherical aberration. Also, a large coma aberration occurs. Below the lower limit, the correction of spherical aberration is insufficient.

The conditional expressions (3) and (3 ') are conditions relating to correction of spherical aberration and coma. When the value exceeds the upper limit, correction of spherical aberration is insufficient. Below the lower limit, large coma aberrations occur.

Condition (4) is a condition for correcting spherical aberration and coma. If the upper limit is exceeded, spherical aberration and coma cannot be properly corrected in a well-balanced manner.
It becomes difficult to realize an fθ lens system brighter than F20.

Condition (5) is a condition for providing an appropriate distortion to satisfy the fθ characteristic in a telecentric mode. Beyond the upper limit, distortion will be excessive. If the lower limit is exceeded, distortion will be insufficient, and in any case, good fθ characteristics cannot be obtained.

Conditional expression (6) is a condition relating to correction of curvature of field in a telecentric manner.
Below the lower limit, the Petzval sum becomes large, making it difficult to simultaneously flatten both the meridional and sagittal image planes.

Next, specific numerical examples 1 to 8 will be described. Embodiments 1 and 2 are non-telecentric embodiments that include a first lens group I and a second lens group II in order from the deflector side D. Embodiments 3 to 8 are telecentric embodiments including a first lens unit I, a second lens unit II, and a third lens unit III in order from the deflector D side. i indicates an image plane. The third lens group III is located near the image plane i.

Embodiment 1 FIG. 1 is a lens configuration diagram (non-telecentric system) of a first embodiment of the fθ lens system of the present invention. The first lens group I 1 is, in order from the deflector D side,
The second lens group II is composed of one positive lens. The second lens group II includes a positive meniscus lens L1 having a concave surface facing the deflector, a negative meniscus lens L2 having a concave surface facing the deflector, and a negative lens L3. . i indicates an image plane. Table 1 shows the numerical data of this lens, FIG. 2 shows the spherical aberration of this fθ lens system, and FIG.
FIG. 9 is a graph diagram showing calculated θ characteristics. FIG. 4 is a graph showing the calculated field curvature in the main scanning direction M and the sub-scanning direction S. 3 and 4 show the position in the main scanning direction, and the horizontal axis in FIG. 3 shows the deviation (mm) from the ideal position.
The horizontal axis indicates the relative focal position (mm) (the same applies to the fθ characteristic diagram and the field curvature diagram for the following embodiments).

[0021] In the tables and the drawings, F is F number, f is the focal length of the entire system, W is the half angle of view, f _B is the back focus, d ₀ is a positive meniscus of the first lens group from the deflector D The distance to the lens L1, EA is the maximum effective diameter of the first lens group I and the second lens group II in the fθ lens system of the present invention,
SW is the scanning width, R is the radius of curvature of each lens surface, D is the lens thickness or lens spacing, N (0.488) is the refractive index for a wavelength of 488 nm, N _d is the refractive index for the d line, and ν _d is the Abbe number of the d line. Is shown. S indicates a sagittal image plane, and M indicates a meridional image plane.

[0022]

[Table 1] F = 1: 20 f = 399.78 W = 25.8 ゜ f _B = 461.28 d ₀ = 60.00 (0.15f) EA = 180.02 SW = 360 EA / SW = 0.50 Surface No. RDN (0.488) N _d ν _d 1 -398.464 18.13 1.80593 1.78472 25.7 2 -89.714 0.90---3 -83.875 6.00 1.52177 1.51633 64.1 4 -213.474 5.22---5 -110.366 10.00 1.52177 1.51633 64.1 6 953.271 56.61---7 -2486.490 32.70 1.80593 1.78472 25.7 8- 193.706----

Embodiment 2 FIG. 5 is a lens configuration diagram (non-telecentric system) of a second embodiment of the fθ lens system of the present invention. The second embodiment differs from the first embodiment in that the second lens group II includes two positive lenses. Table 2 shows the numerical data of this lens, FIG. 6 shows the spherical aberration of the fθ lens system, and FIG.
FIG. 9 is a graph diagram showing calculated θ characteristics. FIG. 8 is a graph showing the calculated field curvature in the main scanning direction M and the sub-scanning direction S.

[0024]

[Table 2] F = 1: 18 f = 399.86 W = 25.8 ゜ f _B = 464.37 d ₀ = 55.00 (0.14f) EA = 180.02 SW = 360 EA / SW = 0.50 Surface No. RDN (0.488) N _d ν _d 1 -601.420 15.37 1.80593 1.78472 25.7 2 -97.128 1.45---3 -89.649 6.00 1.52177 1.51633 64.1 4 -646.096 8.34---5 -105.959 12.80 1.52177 1.51633 64.1 6 2125.682 34.10---7 -3160.156 30.88 1.63182 1.62004 36.3 8- 174.086 7.14---9 -749.149 19.11 1.63182 1.62004 36.3 10 -293.389----

Embodiment 3 FIG. 9 is a lens configuration diagram (telecentric system) of a third embodiment of the fθ lens system of the present invention. In the third embodiment, the first lens unit I includes, in order from the deflector side D, a positive meniscus lens L1 having a concave surface facing the deflector side, a negative meniscus lens L2 having a concave surface facing the deflector side, and a negative lens L3. And the second lens group II
The third lens group III comprises a plano-convex lens having a convex surface facing the deflector. Table 3 shows the numerical data of the lens, FIG. 10 shows the spherical aberration of the fθ lens system, and FIG. 11 is a graph showing the calculated fθ characteristics of the numerical data shown in Table 3. FIG. 12 is a graph illustrating the calculated field curvature in the main scanning direction M and the sub-scanning direction S.

[0026]

[Table 3] F = 1: 20 f = 400.47 W = 25.8 ゜ f _B = 145.70 d ₀ = 78.60 (0.20f) EA = 179.98 SW = 360 EA / SW = 0.50 Surface No. RDN (0.488) N _d ν _d 1 -296.195 23.22 1.80593 1.78472 25.7 2 -81.145 0.50---3 -82.268 10.00 1.59124 1.58144 40.7 4 -263.544 18.74---5 -80.141 10.00 1.52177 1.51633 64.1 6 809.332 12.43---7 5839.249 43.38 1.80593 1.78472 25.7 8 -147.008 374.41---9 531.894 44.00 1.63182 1.62004 36.3 10 ∞----

[Embodiment 4] FIG. 13 is a lens configuration diagram (telecentric system) of a fourth embodiment of the fθ lens system of the present invention. The basic lens configuration is the same as that of the third embodiment. Table 4 shows the numerical data of this lens, FIG. 14 shows the spherical aberration of the fθ lens system, and FIG. 15 is a graph showing the calculated fθ characteristics of the numerical data shown in Table 4. FIG. 16 is a graph showing the calculated field curvature in the main scanning direction M and the sub-scanning direction S.

[0028]

[Table 4] F = 1: 20 f = 400.58 W = 25.7 ゜ f _B = 185.29 d ₀ = 75.00 (0.19f) EA = 179.98 SW = 360 EA / SW = 0.50 Surface No. RDN (0.488) N _d ν _d 1 -385.031 23.96 1.80593 1.78472 25.7 2 -77.822 1.29---3 -76.980 10.00 1.59124 1.58144 40.7 4 -314.643 19.77---5 -77.025 10.00 1.52177 1.51633 64.1 6 -756.933 22.32----7 -491.191 37.65 1.80593 1.78472 25.7 8 -138.519 341.77---9 507.569 46.00 1.63182 1.62004 36.3 10 ∞----

[Embodiment 5] FIG. 17 is a lens configuration diagram (telecentric system) of a fifth embodiment of the fθ lens system of the present invention. The basic lens configuration is the same as that of the third embodiment, except that the positive meniscus lens L1 and the negative meniscus lens L2 of the first lens group are laminated lenses. Table 5 shows
FIG. 18 is a diagram showing the spherical aberration caused by this fθ lens system, and FIG. 19 is a graph showing fθ characteristics calculated for the numerical data shown in Table 5. FIG. 20 is a graph showing the calculated field curvature in the main scanning direction M and the sub-scanning direction S.

[0030]

[Table 5] F = 1: 20 f = 400.63 W = 25.7 ゜ f _B = 144.92 d ₀ = 76.65 (0.19f) EA = 175.42 SW = 360 EA / SW = 0.49 Surface No. RDN (0.488) N _d ν _d 1 -334.643 25.90 1.80593 1.78472 25.7 2 -71.923 0.00---3 -71.923 8.00 1.59124 1.58144 40.7 4 -306.266 20.29---5 -79.687 10.00 1.52177 1.51633 64.1 6 818.689 11.94----7 4257.036 40.91 1.80593 1.78472 25.7 8 -15064 366.23---9 508.434 44.00 1.63182 1.62004 36.3 10 ∞----

Embodiment 6 FIG. 21 is a lens configuration diagram (telecentric system) of a sixth embodiment of the fθ lens system of the present invention. Example 6 is different from Examples 3, 4, and 5 in that the second lens group II includes two positive lenses. Table 6
Is numerical data of this lens, FIG. 22 is a diagram showing spherical aberration by this fθ lens system, and FIG. 23 is a graph showing fθ characteristics calculated for the numerical data shown in Table 6. FIG.
FIG. 4 is a graph showing the calculated field curvature in the main scanning direction M and the sub-scanning direction S.

[0032]

[Table 6] F = 1: 20 f = 400.17 W = 25.8 ゜ f _B = 79.49 d ₀ = 60.91 (0.15f) EA = 210.00 SW = 360 EA / SW = 0.58 Surface No. RDN (0.488) N _d ν _d 1 -177.839 16.60 1.80593 1.78472 25.7 2 -64.608 0.14---3 -65.326 8.00 1.63182 1.62004 36.3 4 -214.621 27.76---5 -71.480 10.00 1.52177 1.51633 64.1 6 -3266.662 13.75---7 -967.522 39.64 1.63182 1.62004 36.3 8 -125.096 15.80---9 1161.625 28.43 1.52177 1.51633 64.1 10 -404.781 445.51---11 548.215 43.00 1.52177 1.51633 64.1 12 ∞----

[Embodiment 7] FIG. 25 is a diagram (telecentric system) of a lens according to a seventh embodiment of the fθ lens system of the present invention. The basic lens configuration is the same as that of the sixth embodiment, except that the positive meniscus lens L1 and the negative meniscus lens L2 of the first lens group are laminated lenses. Table 7 shows
FIG. 26 is a diagram showing the spherical aberration due to this fθ lens system, and FIG. 27 is a graph showing fθ characteristics calculated for the numerical data shown in Table 7. FIG. 28 is a graph showing the calculated field curvature in the main scanning direction M and the sub-scanning direction S.

[0034]

[Table 7] F = 1: 20 f = 400.38 W = 25.8 ゜ f _B = 90.05 d ₀ = 67.50 (0.17f) EA = 209.98 SW = 360 EA / SW = 0.58 Surface No.RDN (0.488) N _d ν _d 1 -197.230 24.17 1.80593 1.78472 25.7 2 -61.747 0.00---3 -61.747 8.00 1.63182 1.62004 36.3 4 -229.920 21.76---5 -72.886 10.00 1.52177 1.51633 64.1 6 -22035.770 12.72----7 -1350.909 42.53 1.63182 1.62004 36.3 8 -128.792 10.91---9 1094.181 25.91 1.52177 1.51633 64.1 10 -505.174 438.42---11 531.358 45.00 1.52177 1.51633 64.1 12 ∞----

[Embodiment 8] FIG. 29 is a diagram (telecentric system) of an eighth embodiment of the fθ lens system according to the present invention. The basic lens configuration is the same as in the sixth embodiment. Table 8 shows the numerical data of this lens, FIG. 30 shows the spherical aberration of the fθ lens system, and FIG. 31 is a graph showing the calculated fθ characteristics of the numerical data shown in Table 8. FIG. 32 is a graph showing the calculated field curvature in the main scanning direction M and the sub-scanning direction S.

[0036]

[Table 8] F = 1: 16.5 f = 399.97 W = 25.8 ゜ f _B = 93.47 d ₀ = 66.37 (0.17f) EA = 194.50 SW = 360 EA / SW = 0.54 Surface No. RDN (0.488) N _d ν _d 1 -163.755 15.45 1.80593 1.78472 25.7 2 -72.327 0.00---3 -79.508 7.00 1.63182 1.62004 36.3 4 -194.824 16.48---5 -67.399 10.00 1.52177 1.51633 64.1 6 -8281.752 12.58---7 -1135.228 39.53 1.66091 1.64769 33.8 8 -126.189 0.00---9 1141.275 24.28 1.52177 1.51633 64.1 10 -448.497 446.00---11 510.066 45.00 1.52177 1.51633 64.1 12 ∞----

Table 9 shows the values of the conditional expressions in Examples 1 to 8.

[Table 9] Example 1 Example 2 Example 3 Example 4 Conditional expression (1) 1.99 1.53--Conditional expression (2) 0.35 0.35--Conditional expression (3) -0.42 -0.44--Conditional expression (4) 0.01f 0.02f 0.05f 0.05f Condition (1 ')--1.22 1.38 Condition (5)--0.45 0.57 Condition (2')--0.33 0.29 Condition (3 ')---0.43 -0.34 Condition Formula (6)--1.80593 1.80593 Example 5 Example 6 Example 7 Example 8 Conditional expression (1)----Conditional expression (2)----Conditional expression (3)----Conditional expression ( 4) 0.05f 0.07f 0.05f 0.04f Condition (1 ') 1.26 1.05 1.07 1.04 Condition (5) 0.45 0.42 0.43 0.40 Condition (2') 0.27 0.30 0.26 0.37 Condition (3 ') -0.38 -0.39- 0.37 -0.48 Conditional expression (6) 1.80593 1.80593 1.80593 1.80593

As is clear from Table 9, the two non-telecentric embodiments 1 and 2 consisting of the first lens unit and the second lens unit have conditional expressions (1), (2), (3) and (3). 4), the first lens group, the second lens group, and the third lens group.
The telecentric embodiments 3 to 10 of the three lens units are conditional expressions (1 ′), (5), (2 ′),
(3 ′), (4) and (6) are satisfied. In each embodiment, the spherical aberration and the curvature of field are well corrected, an appropriate amount of distortion is obtained, and the fθ error is small.

[0039]

According to the fθ lens system of the present invention, the number of lenses is relatively small, the distance from the deflector to the first lens is about 0.14f to 0.2f, and the size of the lens is large. Is reduced to about half of the scanning width, and an fθ lens system brighter than F20 can be obtained.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram of a first embodiment of an fθ lens of the present invention.

FIG. 2 is a spherical aberration diagram of the fθ lens of FIG. 1;

FIG. 3 is a graph showing an fθ error of FIG. 1;

FIG. 4 is a graph showing the same field curvature.

FIG. 5 is a lens configuration diagram of a second embodiment of the fθ lens of the present invention.

FIG. 6 is a spherical aberration diagram of the fθ lens of FIG. 5;

FIG. 7 is a graph showing the fθ error of FIG. 5;

FIG. 8 is a graph showing the same field curvature.

FIG. 9 is a lens configuration diagram of a third embodiment of the fθ lens of the present invention.

FIG. 10 is a spherical aberration diagram of the fθ lens of FIG. 9;

FIG. 11 is a graph showing the fθ error of FIG. 9;

FIG. 12 is a graph showing the same field curvature.

FIG. 13 is a lens configuration diagram of a fourth embodiment of the fθ lens of the present invention.

14 is a diagram of spherical aberration of the fθ lens of FIG. 13;

FIG. 15 is a graph showing the fθ error of FIG. 13;

FIG. 16 is a graph showing the same field curvature.

FIG. 17 is a lens configuration diagram of a fifth embodiment of the fθ lens of the present invention.

18 is a diagram of spherical aberration of the fθ lens of FIG. 17;

FIG. 19 is a graph showing the fθ error of FIG. 17;

FIG. 20 is a graph showing the same field curvature.

FIG. 21 is a lens configuration diagram of a sixth embodiment of the fθ lens of the present invention.

FIG. 22 is a diagram of spherical aberration of the fθ lens of FIG. 21;

FIG. 23 is a graph showing the fθ error of FIG. 21;

FIG. 24 is a graph showing the same field curvature.

FIG. 25 is a lens configuration diagram of a seventh embodiment of the fθ lens of the present invention.

26 is a diagram of spherical aberration of the fθ lens in FIG. 25;

FIG. 27 is a graph showing the fθ error of FIG. 25.

FIG. 28 is a graph showing the same field curvature.

FIG. 29 is a lens configuration diagram of an eighth embodiment of the fθ lens of the present invention.

30 is a diagram of spherical aberration of the fθ lens of FIG. 29;

FIG. 31 is a graph showing the fθ error of FIG. 29;

FIG. 32 is a graph showing the same field curvature.

Claims (8)

[Claims] 1. A first lens group and a second lens group in order from the deflector side.
The first lens group includes, in order from the deflector side, a positive meniscus lens having a concave surface facing the deflector side, a negative meniscus lens having a concave surface facing the deflector side, and a negative lens. An fθ lens system, wherein the lens group includes two or less positive lenses. 2. The fθ lens system according to claim 1, wherein
An fθ lens system that satisfies the following conditional expression (1). (1) 1.5 <| f _I / f _II | <2.4 where f _I : focal length of the first lens group, f _II : focal length of the second lens group. 3. The fθ lens system according to claim 2, wherein
An fθ lens system satisfying the following conditional expressions (2) to (4). _{(2) 0.30 <f 1 /f<0.55} (3) -0.60 <f 23 / f II <-0.35 (4) d 4 <0.08f where, f _1: the first lens The focal length of the positive meniscus lens of the group, f ₂₃ : the composite focal length of the negative meniscus lens and the negative lens of the first lens group, and d ₄ : the air gap on the optical axis of the negative meniscus lens and the negative lens of the first lens group. 4. The fθ lens system according to claim 1, wherein all the constituent lenses are spherical lenses. 5. A first lens group and a second lens group in order from the deflector side.
An approximately telecentric fθ lens system including a lens group and a third lens group, wherein the first lens group includes, in order from the deflector side, a positive meniscus lens having a concave surface facing the deflector side, and a concave surface facing the deflector side. The second lens group is composed of two or less positive lenses, and the third lens group is composed of a plano-convex lens having a convex surface facing the deflector. fθ lens system. 6. The fθ lens system according to claim 5, wherein
An fθ lens system satisfying the following conditional expressions (1 ′) and (5). (1 ′) 0.9 <| f _I / f _II | <1.5 (5) 0.35 <f _II /f<0.65, where f: focal length of the entire system. 7. The fθ lens system according to claim 6, wherein
An fθ lens system satisfying the following conditional expressions (2 ′), (3 ′), (4) and (6). (2 ′) 0.22 <f ₁ /f<0.50 (3 ′) − 0.50 <f ₂₃ / f _II <−0.30 (4) d ₄ <0.08f (6) n ₁ > 1.65 where n _{1 is} the refractive index of the positive meniscus lens of the first lens group. 8. The fθ lens system according to claim 5, wherein all the constituent lenses are spherical lenses.