Optical system and imaging device

Abstract

To provide an optical system that is compact and lightweight and exhibits high performance.SOLUTION: Provided is an optical system (LO) composed of a front group (FG) and a rear group (RG). The front group comprises a first lens (Lp1) having positive refractive power, a second lens (Lp2) arranged closer to an image side than the first lens and having positive refractive power, and a third lens (Ln1), the refractive power of each of the first lens and the second lens on a d line being 1.7 or greater. The rear group includes a fourth lens (A) composed of two to four lenses and having positive refractive power and a fifth lens (B) arranged adjacent to the fourth lens and having negative refractive power, the fourth lens and the fifth lens being made from an organic material. When it is assumed that vdA represents the Abbe number of the fourth lens, vdB represents the Abbe number of the fifth lens, fA represents the focal distance of the fourth lens, and fB represents the focal distance of the fifth lens, then the conditional expressions 0.75<νdA / νdB<1.30 and 0.75<-fA / fB<1.30 are satisfied.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical system and an imaging device. [Background technology]

[0002] In recent years, there has been a demand for optical systems that are small and lightweight yet have high performance (high imaging performance). Patent Document 1 discloses an imaging lens (optical system) that is composed of, in order from the object side to the image side, a positive, negative, negative, negative, and positive lens. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-35781 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the optical system disclosed in Patent Document 1, all the lenses are made of organic materials (resin) and have a larger linear expansion coefficient than inorganic materials (glass), so that the thickness and curvature, i.e., the aberration, change significantly when the temperature or humidity changes. In particular, the R1 surface of the first lens corresponds to the aperture surface, that is, the light beam entering the first lens is thick, and because the material is organic, the spherical aberration and coma aberration change significantly when the temperature or humidity changes, which causes a decrease in the imaging performance of the entire screen. In addition, because the refractive index of the first lens is low, the central thickness increases, making it difficult to reduce the size.

[0005] SUMMARY OF THE PRESENT EMBODIMENTS An object of the present invention is to provide a compact, lightweight, high-performance optical system and imaging device. [Means for solving the problem]

[0006] An optical system according to one aspect of the present invention is an optical system including a front group and a rear group, the front group including a first lens having a positive refractive power, a second lens arranged closer to the image side than the first lens and having a positive refractive power, and a third lens, and the refraction of each of the first lens and the second lens at the d-line is rate is 1.7 or more, the rear group is composed of two to four lenses, and includes a fourth lens having positive refractive power and a fifth lens arranged adjacent to the fourth lens and having negative refractive power, the fourth lens and the fifth lens are made of an organic material, and when the Abbe number of the fourth lens is νdA, the Abbe number of the fifth lens is νdB, the focal length of the fourth lens is fA, and the focal length of the fifth lens is fB, 0.75<νdA / νdB<1.30 0.75<-fA / fB<1.30 The following condition is satisfied.

[0007] Other objects and features of the present invention are illustrated in the following examples. Effect of the Invention

[0008] According to the present invention, it is possible to provide a small, lightweight, high-performance optical system and imaging device. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a cross-sectional view of an optical system in the first embodiment. [Diagram 2] 4A to 4C are aberration diagrams of the optical system in Example 1. [Diagram 3] FIG. 11 is a cross-sectional view of an optical system in Example 2. [Figure 4] 10A to 10C are aberration diagrams of the optical system in Example 2. [Diagram 5] FIG. 11 is a cross-sectional view of an optical system according to a third embodiment. [Figure 6] 11A to 11C are aberration diagrams of the optical system in Example 3. [Figure 7] FIG. 11 is a cross-sectional view of an optical system in Example 4. [Figure 8]11A to 11C are aberration diagrams of the optical system in Example 4. [Figure 9] FIG. 2 is a schematic diagram of an imaging device including an optical system in each embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0011] 1, 3, 5, and 7 are cross-sectional views of the optical system LO of Examples 1 to 4 when focused at infinity. The optical system of each Example is an imaging optical system used in imaging devices such as digital video cameras, digital still cameras, broadcast cameras, silver halide film cameras, and surveillance cameras. In each cross-sectional view, the left side is the object side (front) and the right side is the image side (rear).

[0012] In Fig. 1, LO is the optical system (the entire imaging optical system), FG is the front group, and FG is the rear group. In the rear group FG, A is a lens (fourth lens) made of an organic material (resin) with positive refractive power, B is a lens (fifth lens) made of an organic material (resin) with negative refractive power adjacent to lens A, SP is the aperture (aperture stop), and IP is the image plane. When used as an imaging optical system for a video camera or digital still camera, the imaging surface of a solid-state imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor, or a photosensitive surface such as the film surface in the case of a silver halide film camera, is disposed on the image plane IP.

[0013] 2, 4, 6, and 8 are aberration diagrams of the optical systems LO of Examples 1 to 4 when focused at infinity. In the spherical aberration diagrams, Fno is the F-number, and shows the amount of spherical aberration for the d-line (wavelength 587.6 nm) and the g-line (wavelength 435.8 nm). In the astigmatism diagrams, S shows the amount of astigmatism in the sagittal image plane, and M shows the amount of astigmatism in the meridional image plane. In the distortion diagrams, the amount of distortion for the d-line is shown. In the chromatic aberration diagrams, the amount of chromatic aberration for the g-line is shown. ω is the half angle of view (degrees).

[0014] Next, a configuration common to the optical systems of each embodiment will be described with reference to FIG. 1. The optical system OL of each embodiment is composed of 5 to 7 lenses. By being composed of 5 or more lenses, it is possible to effectively suppress the occurrence of various aberrations. Furthermore, by being composed of 7 or less lenses, it is possible to realize a compact and lightweight system. Note that elements that do not have refractive power, such as flat glass, are not considered in the number of lenses.

[0015] The optical system LO in each embodiment is composed of a front group FG and a rear group RG. The front group FG is composed of three lenses: a lens (first lens) Lp1 having a positive refractive power, a lens (second lens) Lp2 having a positive refractive power, and a lens (third lens) Ln1 having a negative refractive power. The front group FG only needs to have at least two lenses having a positive refractive power, and a lens having a positive refractive power may be arranged as the third lens. In the present invention, the first lens to the third lens do not necessarily need to be arranged in order from the object side according to the ordinal numbers. The lens corresponding to the third lens may be arranged closest to the object side, or the lens corresponding to the third lens may be arranged between the lenses corresponding to the first lens and the second lens. In other words, the conditions that are preferably satisfied by the first lens described later may be satisfied by any positive lens arranged closer to the object side than the positive lens arranged closest to the image side among the positive lenses included in the front group FG, unless otherwise specified. Furthermore, unless otherwise specified, the conditions that are preferably satisfied by the second lens, which will be described later, need only be satisfied by any positive lens that is located on the image side of the positive lens located furthest to the object side among the positive lenses in the front group FG.

[0016] The refractive index at the d-line of at least two lenses (first lens, second lens) having positive refractive power in the front group FG is 1.7 or more. By having at least two lenses having positive refractive power in the front group FG, the marginal light of the axial light can be gently bent in the convergence direction while being guided to the rear group RG, thereby achieving telephoto (compactness) and good imaging performance. In addition, by making the refractive index of each of the at least two lenses 1.7 or more, the central thickness of the lens can be made thin, and the imaging optical system LO can be made compact.

[0017] The rear group RG is composed of two to four lenses, including a lens A having positive refractive power and a lens B having negative refractive power arranged adjacent to lens A. Lenses A and B are made of an organic material. Lenses A and B satisfy the following conditional expressions (1) and (2).

[0018] 0.75<νdA / νdB<1.30 (1) 0.75<-fA / fB<1.30 (2) In conditional expressions (1) and (2), νdA is the Abbe number of lens A having positive refractive power, νdB is the Abbe number of lens B having negative refractive power, fA is the focal length of lens A having positive refractive power, and fB is the focal length of lens B having negative refractive power.

[0019] In particular, in order to reduce the size of a wide-angle system in which the aperture stop SP is disposed in the front group FG, it is necessary to dispose the rear group RG at a position close to the image plane IP. Therefore, the rear group RG is disposed at a high position of off-axis light rays, and the diameter of the rear group RG increases relative to the front group FG. Therefore, if an inorganic material (glass) is disposed in the rear group RG, the weight increases. Therefore, the optical system of each embodiment is made lighter by disposing an organic material (resin). Here, the organic material (resin) means a material whose linear expansion coefficient ( / °C) is in the range of 5.0×10^-5 to 10.0×10^-5.

[0020] Furthermore, the ratio of the Abbe numbers of lens A having positive refractive power and lens B having negative refractive power, and the absolute value of the ratio of the focal lengths of lenses A and B are set to values ​​including approximately 1, as in conditional expressions (1) and (2), respectively. This makes it possible to suppress fluctuations in axial chromatic aberration and lateral chromatic aberration when temperature and humidity change. This will be described in detail below.

[0021] Here, the height from the optical axis of the on-axis ray of the i-th lens in paraxial tracking is h_i, and the height from the optical axis of the off-axis chief ray of the i-th lens in paraxial tracking is h_bar_i. In addition, the refractive power of the i-th lens in paraxial tracking is φ_i, and the Abbe number of the i-th lens in paraxial tracking is ν_i. In this case, the on-axis chromatic aberration coefficient L and the lateral chromatic aberration coefficient T of the optical system are expressed as the following formulas (A) and (B).

[0022] L=Σ(h_i×h_i×φ_i / ν_i) ···(A) T=Σ(h_i×h_bar_i×φ_i / ν_i) ···(B) From equations (A) and (B), we can see that the axial chromatic aberration is proportional to the square of the height h_i, and the lateral chromatic aberration is proportional to the heights h_i and h_bar_i. The axial chromatic aberration Δf and the lateral chromatic aberration ΔY of the optical system are given by the following equations (C) and (D).

[0023] Δf = -f × L (C) ΔY = -Y × T (D) In formulas (C) and (D), the focal length of the optical system (total system) is f, and the image height is Y. In other words, the axial chromatic aberration coefficient L and the lateral chromatic aberration coefficient T of each lens are proportional to the refractive power / Abbe number of each lens.

[0024] In general, when organic materials are used, the linear expansion coefficient is larger than that of inorganic materials, so the variation in refractive power when the temperature changes is large. In other words, the variation in chromatic aberration is larger with organic materials. Therefore, in order to reduce weight while suppressing the variation in chromatic aberration when the temperature changes, adjacent lenses with positive refractive power and negative refractive power are made of organic materials, and the ratio of the absolute values ​​of the Abbe number and power (refractive power) is set to about 1. Since they are adjacent lenses, the height of the axial ray from the optical axis and the height of the off-axis principal ray from the optical axis in paraxial tracing of a lens with positive refractive power and a lens with negative refractive power are relatively close. Therefore, by reversing the sign of the numerical value of the refractive power / Abbe number and making it approximately the same, the effect of canceling chromatic aberration works at room temperature. In addition, because the Abbe numbers of the lenses with positive and negative refractive powers are close to each other, the change in refractive index at each wavelength is similar even when the temperature changes, and the change in refractive power is also similar, so the chromatic aberration cancellation effect is maintained. This allows chromatic aberration to be well corrected even when the temperature changes, while the aspheric effect of each lens allows astigmatism and curvature of field to be corrected, resulting in high performance. With the above configuration, a small, lightweight, high-performance optical system can be realized.

[0025] Preferably, the numerical ranges of conditional expressions (1) and (2) are set as shown in the following conditional expressions (1A) and (2B).

[0026] 0.77<νdA / νdB<1.27 (1A) 0.80<-fA / fB<1.25...(2A) More preferably, the numerical ranges of conditional expressions (1A) and (2A) are set as shown in the following conditional expressions (1B) and (2B).

[0027] 0.80<νdA / νdB<1.25 (1B) 0.85<-fA / fB<1.20 (2B) Preferably, the front group FG has a lens having a negative refractive power. This reduces axial chromatic aberration and Petzval sum (field curvature), and achieves even higher performance. Also, preferably, the lens closest to the object side of the front group FG has a positive refractive power. This makes it possible to realize a telephoto lens, shorten the distance from the most object-side surface to the image plane, and achieve a compact optical system.

[0028] It is preferable that the optical system of each embodiment satisfies at least one of the following conditional expressions (3) to (20).

[0029] αp1<100×10^-7 (3) 25<νdp1<60 (4) αp2<100×10^-7 (5) 25<νdp2<60 (6) αn<100×10^-7 (7) 1.60 <N<2.10 ···(8) 15<νdn<35 (9) 1.4 <fA / f<3.5 ···(10) 1.00 <f / BF<3.00 ···(11) 0.5<|Fu| / Fm<25.0 (12) 0.10 <dA / BF<0.60 ···(13) 0.05 <dB / BF<0.40 ···(14) 0.50 <du / BF<1.50 ···(15) 50<νdA<60 (16) 1.45 <NA<1.60 ···(17) 15<νdA<40 (18) 1.55 <NA<1.75 ···(19) 0.20<-fn / f<0.70 (20) Here, αp1 is the linear expansion coefficient ( / °C) of the lens Lp1 in the front group FG that is closest to the object and has positive refractive power. νdp1 is the Abbe number of the lens Lp1. αp2 is the linear expansion coefficient of the lens Lp2 in the front group FG that is second closest to the object and has positive refractive power. νdp2 is the Abbe number of the lens Lp2. αn is the linear expansion coefficient of the lens Ln1 in the front group FG that has negative refractive power. N is the refractive index of the lens Ln1. νdn is the refractive index of the lens Ln1. Abbe number fA is the focal length of lens A. f is the focal length of the optical system (entire system) LO. BF is the back focus of the optical system LO (the air-equivalent length from the image side surface of the final lens to the image surface IP when focused at infinity). Fu is the focal length (composite focal length) of the rear group RG. Fm is the focal length (composite focal length) of the front group FG. dA is the central thickness of lens A. dB is the central thickness of lens B. du is the center distance from the vertex of the surface of the rear group RG closest to the object to the vertex of the surface of the rear group RG closest to the image. νdA is the Abbe number of lens A. NA is the refractive index of lens A. νdB is the Abbe number of lens B. NB is the refractive index of lens B. fn is the focal length of the lens having negative refractive power in the front group FG.

[0030] Next, the technical meaning of each conditional expression will be explained.

[0031] Conditional formula (3) means the linear expansion coefficient of the lens Lp1 having positive refractive power closest to the object in the front group FG. If the upper limit of conditional formula (3) is exceeded, the linear expansion coefficient becomes too large, and when the temperature changes, spherical aberration occurs, which is particularly undesirable as it reduces the imaging performance. In particular, when the front group FG has a stop SP, the on-axis / off-axis light beam becomes thick near the lens Lp1 having positive refractive power closest to the object in the front group FG, which reduces the performance of the entire screen, which is undesirable. Note that the linear expansion coefficient here is a value at a temperature of 25 degrees.

[0032] Conditional expression (4) represents the Abbe number of the lens Lp1 having positive refractive power and closest to the object in the front group FG. If the upper limit of conditional expression (4) is exceeded, the axial chromatic aberration of the F-line will be over-corrected (over-corrected) relative to the C-line, which is not preferable. On the other hand, if the lower limit of conditional expression (4) is exceeded, the axial chromatic aberration of the F-line will be under-corrected (under-corrected) relative to the C-line, which is not preferable.

[0033] Conditional formula (5) means the linear expansion coefficient of the lens Lp2 having the second most positive refractive power in the front group FG from the object side. If the upper limit of conditional formula (5) is exceeded, the linear expansion coefficient becomes too large, and when the temperature changes, spherical aberration occurs, which degrades the imaging performance, which is not preferable. In particular, in a configuration in which the front group FG has a stop SP, the on-axis / off-axis light flux becomes thick near the lens Lp2 having the second most positive refractive power in the front group FG from the object side, which degrades the performance of the entire screen, which is not preferable. Note that the linear expansion coefficient here is the value at a temperature of 25 degrees.

[0034] Conditional expression (6) represents the Abbe number of the lens Lp2 having positive refractive power that is the second most positive in the front group FG from the object side. If the upper limit of conditional expression (6) is exceeded, the axial chromatic aberration of the F-line will be over-corrected (over-corrected) relative to the C-line, which is not preferable. On the other hand, if the lower limit of conditional expression (6) is exceeded, the axial chromatic aberration of the F-line will be under-corrected (under-corrected) relative to the C-line, which is not preferable.

[0035] Conditional expression (7) means the linear expansion coefficient of the lens Ln1 having negative refractive power in the front group FG. If the upper limit of conditional expression (7) is exceeded, the linear expansion coefficient becomes too large, and when the temperature changes, spherical aberration occurs, which degrades the imaging performance, which is not preferable. In particular, in a configuration in which the front group FG has a stop SP, the on-axis / off-axis light flux becomes thick near the lens Ln1 having negative refractive power in the front group FG, which degrades the performance of the entire screen, which is not preferable. Note that the linear expansion coefficient here is the value at a temperature of 25 degrees.

[0036] Conditional expression (8) indicates the refractive index at the d-line of the lens Ln1 having negative refractive power in the front group FG. If the upper limit of conditional expression (8) is exceeded, the refractive index becomes too large, the Petzval sum (field curvature) becomes large, and the imaging performance decreases, which is undesirable. On the other hand, if the lower limit of conditional expression (8) is exceeded, the refractive index becomes too small, the curvature of the lens having negative refractive power becomes large, and the thickness in the thrust direction increases, resulting in a large size.

[0037] Condition (9) represents the Abbe number of the lens Ln1 having negative refractive power in the front group FG. If the upper limit of condition (9) is exceeded, the axial chromatic aberration of the F-line will be under-corrected relative to the C-line, which is not preferable. On the other hand, if the lower limit of condition (9) is exceeded, the axial chromatic aberration of the F-line will be over-corrected relative to the C-line, which is not preferable.

[0038] Conditional expression (10) expresses the ratio of the focal length of lens A having positive refractive power to the focal length of optical system LO. If the upper limit of conditional expression (10) is exceeded, the refractive power of lens A becomes too small, and in particular field curvature and astigmatism cannot be corrected well. On the other hand, if the lower limit of conditional expression (10) is exceeded, the refractive power of lens A becomes too large, and the amount of change in refractive power when the temperature changes becomes too large, which undesirably results in large fluctuations in field curvature and astigmatism in particular.

[0039] Conditional expression (11) represents the ratio of the focal length of the optical system LO to the back focus of the optical system LO when focused at infinity. If the upper limit of conditional expression (11) is exceeded, the back focus becomes too short, making it easier for ghosting to occur and making it impossible to achieve high performance. Also, the angle of incidence to the sensor becomes too large at the periphery of the screen, causing color shading. On the other hand, if the lower limit of conditional expression (11) is exceeded, the back focus becomes too long and the optical system LO becomes large.

[0040] Conditional expression (12) expresses the ratio of the absolute value of the composite focal length of the rear group RG to the composite focal length of the front group FG. If the upper limit of conditional expression (12) is exceeded, the composite focal length of the front group FG becomes too short, and the refractive power becomes too strong, which undesirably results in large spherical aberration and coma in particular. On the other hand, if the lower limit of conditional expression (12) is exceeded, the composite focal length of the rear group RG becomes too short, and the refractive power becomes too strong, which undesirably results in large astigmatism and curvature of field in particular.

[0041] Condition (13) expresses the ratio of the central thickness of lens A having positive refractive power to the back focus. If the upper limit of condition (13) is exceeded, the central thickness of lens A becomes too large, which increases the weight, which is not preferable. On the other hand, if the lower limit of condition (13) is exceeded, the back focus becomes too long, which increases the overall size of the optical system LO, which is not preferable.

[0042] Conditional expression (14) expresses the ratio of the central thickness of lens B having negative refractive power to the back focus. If the upper limit of conditional expression (14) is exceeded, the central thickness of lens B becomes too large, which increases the weight, which is not preferable. On the other hand, if the lower limit of conditional expression (14) is exceeded, the back focus becomes too long, which increases the overall size of the optical system LO, which is not preferable.

[0043] Conditional expression (15) expresses the ratio of the center distance from the vertex of the surface of the rear group RG closest to the object side to the vertex of the surface of the rear group RG closest to the image side to the back focus. If the upper limit of conditional expression (15) is exceeded, the weight of the rear group RG increases, which is undesirable. On the other hand, if the lower limit of conditional expression (15) is exceeded, the back focus becomes too long, which undesirably increases the overall size of the optical system LO.

[0044] Conditional expressions (16) and (17) respectively represent the Abbe number and the refractive index of lens A having positive refractive power. By increasing the refractive index and the Abbe number within ranges that satisfy both conditional expressions (16) and (17), it is possible to reduce the Petzval sum (curvature of field) and the lateral chromatic aberration.

[0045] Conditional expressions (18) and (19) respectively represent the Abbe number and the refractive index of lens A having positive refractive power. Conditional expressions (18) and (19) are to be satisfied when an organic material on the high dispersion side is used in relation to conditional expressions (16) and (17). In conditional expressions (18) and (19), the Abbe number is large compared to conditional expressions (16) and (17), so the lateral chromatic aberration is not good. However, the refractive index can be increased, so the Petzval sum (field curvature) can be further reduced.

[0046] Conditional expression (20) expresses the ratio of the focal length of the lens Ln1 having negative refractive power in the front group FG to the focal length of the optical system LO. If the upper limit of conditional expression (20) is exceeded, the absolute value of the focal length of the lens Ln1 becomes too long, causing insufficient correction of spherical aberration and an increase in the Petzval sum (curvature of field). On the other hand, if the lower limit of conditional expression (20) is exceeded, the absolute value of the focal length of the lens Ln1 becomes too short, causing the front principal point to move toward the image side, resulting in an increase in overall length and size.

[0047] More preferably, the numerical ranges of conditional expressions (3) to (20) are set as shown in the following conditional expressions (3A) to (20A), respectively.

[0048] αp1<90×10^-7 (3A) 27<νdp1<55 (4A) αp2<90×10^-7 (5A) 30<νdp2<55...(6A) αn<95×10^-7 (7A) 1.65 <N<2.05 ···(8A) 16<νdn<32 (9A) 1.5 <fA / f<3.0 ···(10A) 1.50 <f / BF<2.90 ···(11A) 0.7<|Fu| / Fm<21.0 (12A) 0.12 <dA / BF<0.50 ···(13A) 0.08 <dB / BF<0.35 ···(14A) 0.55 <du / BF<1.30 ···(15A) 52<νdA<58...(16A) 1.48 <NA<1.58 ···(17A) 17<νdA<30 (18A) 1.60 <NA<1.72 ···(19A) 0.25<-fn / f<0.68 (20A) It is even more preferable that the numerical ranges of conditional formulas (3A) to (20A) are set as shown in the following conditional formulas (3B) to (20B), respectively.

[0049] αp1<80×10^-7 (3B) 30<νdp1<50 (4B) αp2<80×10^-7 (5B) 35<νdp2<50 (6B) αn<85×10^-7 (7B) 1.73 <N<2.00 ···(8B) 17<νdn<28...(9B) 1.6 <fA / f<2.8 ···(10B) 1.80 <f / BF<2.85 ···(11B) 0.9<|Fu| / Fm<17.0...(12B) 0.15 <dA / BF<0.45 ···(13B) 0.10 <dB / BF<0.30 ···(14B) 0.60 <du / BF<1.20 ···(15B) 54<νdA<57 (16B) 1.52 <NA<1.56 ···(17B) 19<νdA<25 (18B) 1.62 <NA<1.70 ···(19B) 0.30<-fn / f<0.65 (20B) With the above configuration, it is possible to realize a small, lightweight, high-performance imaging optical system. The optical systems LO of Examples 1 to 4 will now be described in detail. EXAMPLES

[0050] First, referring to FIG. 1, the optical system LO in the first embodiment will be described. In the optical system LO in this embodiment, the front group FG is composed of three lenses. In order from the object side to the image side, a lens Lp1 having a positive refractive power, a diaphragm SP, a lens Lp2 having a positive refractive power, and a lens Ln1 having a negative refractive power are arranged. This allows the front principal point to be moved to the object side, making it a telephoto lens and realizing a compact size. In addition, all the lenses are made of inorganic materials. The front group FG has a thick on-axis / off-axis light beam, and when the curvature or thickness changes, the spherical aberration and coma aberration change significantly, which affects the imaging performance of the entire screen. By making the front group FG out of inorganic materials, the changes in the curvature, thickness, and refractive index due to temperature changes are suppressed, improving the imaging performance of the entire screen.

[0051] The rear group RG is composed of three lenses, in order from the object side to the image side, a lens B having a negative refractive power, a lens A having a positive refractive power, and a lens having a positive refractive power. By inverting the arrangement of the lenses having positive, positive, and negative refractive powers in the front group FG, it is made symmetrical (concentric), and off-axis aberrations are corrected well. In addition, the diameter of the rear group RG is large because it is close to the sensor, but since all the lenses are made of organic materials, it can be made lightweight. In addition, by satisfying conditional expressions (1) and (2) for the lens B having a negative refractive power and the lens A having a positive refractive power, which are adjacent to each other, it is possible to effectively correct the fluctuation of chromatic aberration due to temperature changes. In addition, by satisfying the above-mentioned conditional expressions (3) to (20), it is possible to achieve a small, lightweight, and high-performance optical system. EXAMPLES

[0052] Next, referring to FIG. 3, an optical system LO in Example 2 will be described. Note that from this Example onward, descriptions that overlap with those in Example 1 will be omitted. The rear group RG is made up of four lenses, and lenses having negative, positive, positive, and negative refractive powers are arranged in this order from the object side to the image side. By configuring the rear group RG with four lenses, it is possible to effectively correct non-aberration and curvature of field. In addition, two lenses (a negative lens and a positive lens) on the object side of the rear group RG are paired, and two lenses (lens A and lens B) on the image side are paired, and each satisfies conditional expressions (1) and (2). As a result, chromatic aberration during temperature changes can be effectively corrected between the paired lenses, and fluctuations in chromatic aberration as a whole can be suppressed. In addition, by satisfying the above-mentioned conditional expressions (3) to (20), it is possible to achieve a small, lightweight, and high-performance optical system. EXAMPLES

[0053] Next, referring to FIG. 5, an optical system LO in Example 3 will be described. In the optical system LO of this example, the refractive power of the front group FG and the refractive power of the rear group RG are approximately equal, and the refractive powers are appropriately shared. This makes it possible to suppress bending of light rays emerging from the front group FG with respect to light rays entering the front group FG, and in particular to reduce spherical aberration and coma aberration. In addition, by satisfying the above-mentioned conditional expressions (1) to (20), a small, lightweight, and high-performance imaging optical system can be achieved. EXAMPLES

[0054] Next, the optical system LO in the fourth embodiment will be described with reference to Fig. 7. The front group FG is composed of three lenses, and arranged in order from the object side to the image side are a lens Lp1 having a positive refractive power, a lens Ln1 having a negative refractive power, a stop SP, and a lens Lp2 having a positive refractive power. By arranging the lens Ln1 closer to the object side than in the first embodiment, the axial chromatic aberration and spherical aberration can be corrected more effectively.

[0055] The rear group RG is composed of two lenses, in that order from the object side to the image side: a lens B having negative refractive power and a lens A having positive refractive power. The two-lens configuration allows for a lighter weight than Example 1. Furthermore, by satisfying the above-mentioned conditional expressions (1) to (20), a small, lightweight, and high-performance imaging optical system can be achieved.

[0056] Next, an imaging device using the optical system LO of each embodiment as an imaging optical system will be described with reference to Fig. 9. Fig. 9 is a schematic diagram of an imaging device (digital camera) 10. In Fig. 9, 100 is a camera body (imaging device body), and 101 is an imaging optical system corresponding to the optical system LO of each embodiment. 102 is an imaging element (photoelectric conversion element) that receives (images) a subject image (optical image) formed by the imaging optical system 101. The imaging device 10 includes a display device (not shown) such as a liquid crystal panel, and the subject image formed on the imaging element 102 is displayed on the display device. By applying the optical system LO of each embodiment in this way, an imaging device with high optical performance can be realized.

[0057] Numerical Examples 1 to 4 corresponding to Examples 1 to 4, respectively, are shown below. In the surface data of each numerical example, r represents the radius of curvature of each optical surface, and d (mm) represents the axial distance (distance on the optical axis) between the mth surface and the (m+1)th surface. Here, m is the surface number counted from the light incident side. In addition, nd represents the refractive index of each optical member with respect to the d-line, and νd represents the Abbe number of the optical member. Note that the Abbe number νd of a certain material is given by Nd, NF, and NC, respectively, when the refractive indices at the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) of the Fraunhofer lines are Nd, NF, and NC, respectively. νd=(Nd-1) / (NF-NC) It is expressed as:

[0058] In each numerical example, d, focal length (mm), F-number, and half angle of view (degrees) are all values ​​when the optical system of each example is focused on an object at infinity. BF (back focus) is the distance on the optical axis from the final lens surface (the lens surface closest to the image) of the optical system to the paraxial image surface expressed as an air-equivalent length. The total lens length is the distance on the optical axis from the foremost lens surface (the lens surface closest to the object) of the optical system to the final lens surface plus the back focus. The lens group is not limited to being composed of multiple lenses, but may also be composed of a single lens.

[0059] If the optical surface is aspheric, an asterisk (*) is added to the right of the surface number. The aspheric shape is expressed by the following equation, where x is the displacement from the vertex of the surface in the optical axis direction, h is the height from the optical axis in a direction perpendicular to the optical axis, R is the paraxial radius of curvature, k is the conic constant, and A4, A6, A8, and A10 are the aspheric coefficients of each order.

[0060]

number

[0061] In addition, "e±XX" in each aspheric coefficient is "×10 ±XX " It means.

[0062] (Numerical Example 1) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1 6.027 0.71 1.80400 46.5 4.56 2 13.112 0.91 4.41 3 (Aperture) ∞ 0.39 4.13 4 5.762 0.88 1.90043 37.4 3.87 5 276.886 0.22 1.76182 26.5 3.57 6 4.087 2.12 3.28 7* -8.544 0.66 1.67070 19.3 4.78 8* -25.539 0.54 6.02 9* -7.071 1.28 1.63560 23.9 7.73 10* -5.018 0.48 8.42 11* 7.499 1.90 1.53500 56.0 11.27 12* 9.164 11.82 Image plane ∞ Aspheric Data Side 7 K = 0.00000e+000 A 4=-1.30571e-002 A 6= 1.07357e-003 A 8= 1.13910e-006 A10=-3.23336e-006 A12=-1.01049e-006 Side 8 K = 0.00000e+000 A 4=-1.05055e-002 A 6= 1.04691e-003 A 8=-3.89159e-005 A10=-5.44432e-007 A12= 2.59956e-008 9th page K = 0.00000e+000 A 4= 8.11468e-003 A 6=-6.75729e-004 A 8= 3.07488e-005 A10=-1.29401e-007 A12=-2.06278e-008 Side 10 K = 0.00000e+000 A 4= 3.89314e-003 A 6= 1.26901e-004 A 8=-2.57098e-005 A10= 1.67415e-006 A12=-3.50220e-008 Page 11 K = 0.00000e+000 A 4=-6.73863e-003 A 6= 2.57932e-004 A 8=-4.04258e-006 A10=-1.83896e-008 A12= 1.37821e-009 A14=-1.37275e-011 Side 12 K = 0.00000e+000 A 4=-6.31402e-003 A 6= 2.43612e-004 A 8=-8.06405e-006 A10= 1.82462e-007 A12=-2.06718e-009 A14= 7.83470e-012 Various data Zoom ratio 1.00 Focal length 13.12 F-number: 2.88 Half angle of view 31.02 Image height 7.89 Lens length 15.26 BF 5.19 d12 5.19 Entrance pupil position 1.51 Exit pupil position -7.70 Front principal point position 1.27 Back principal point position -7.93 Single lens data Lens starting surface focal length 1 1 13.28 2 4 6.52 3 5 -5.45 4 7 -19.45 5 9 21.91 6 11 55.22 (Numerical Example 2) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1 6.021 0.61 1.81600 46.6 4.55 2 9.865 1.00 4.40 3(Aperture) ∞ 0.75 4.19 4 6.384 0.91 1.90043 37.4 3.89 5 -35.714 0.22 1.75211 25.0 3.64 6 5.076 1.18 3.37 7* -16.521 0.87 1.65010 21.5 4.17 8* -21.442 1.18 5.05 9* -7.208 0.99 1.63560 23.9 7.03 10* -6.993 0.17 7.61 11* 5.928 0.92 1.53110 55.9 8.26 12* 11.152 0.65 9.04 13* 16.662 0.75 1.53500 56.0 9.47 14* 7.305 10.33 Image plane ∞ Aspheric Data Side 7 K = 0.00000e+000 A 4=-6.66583e-003 A 6= 9.69171e-004 A 8=-4.03272e-004 A10=8.79337e-005 A12=-6.17014e-006 Side 8 K = 0.00000e+000 A 4=-4.77623e-003 A 6= 4.25204e-004 A 8=-1.64811e-004 A10=3.15736e-005 A12=-1.59846e-006 9th page K =-1.41036e+001 A 4= 8.94812e-003 A 6=-1.68586e-003 A 8= 1.57237e-004 A10=-6.42994e-006 A12= 8.59335e-008 Side 10 K =-4.59613e+000 A 4= 5.05734e-003 A 6=-7.80669e-004 A 8= 6.43193e-005 A10=-2.10462e-006 A12= 1.30611e-008 Page 11 K = 0.00000e+000 A 4=-1.05053e-002 A 6= 4.81059e-004 A 8=-1.61741e-005 A10=-3.75795e-007 A12= 4.18961e-008 A14=-7.14086e-010 Side 12 K = 0.00000e+000 A 4=-5.20687e-003 A 6= 1.44080e-004 A 8=-1.64974e-006 A10=-2.80569e-007 A12= 1.62436e-008 A14=-2.08518e-010 Page 13 K = 0.00000e+000 A 4=-6.27407e-003 A 6= 9.12346e-005 A 8= 8.33783e-006 A10=-2.48711e-007 A12=-1.08995e-009 A14= 5.50477e-011 Side 14 K = 0.00000e+000 A 4=-8.64112e-003 A 6= 3.52520e-004 A 8=-1.18418e-005 A10= 2.99352e-007 A12=-4.50466e-009 A14= 2.14589e-011 Various data Zoom ratio 1.00 Focal length 13.09 F-number: 2.88 Half angle of view 31.08 Image height 7.89 Lens length 15.08 BF 4.88 d14 4.88 Entrance pupil position 1.51 Exit pupil position -6.27 Front principal point position -0.76 Back principal point position -8.21 Single lens data Lens starting surface focal length 1 1 17.67 2 4 6.08 3 5 -5.90 4 7 -119.01 5 9 132.23 6 11 22.46 7 13 -25.02 (Numerical Example 3) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1 6.027 0.92 1.91082 35.3 4.41 2 13.112 0.91 4.16 3 (Aperture) ∞ 0.39 3.86 4 6.564 0.64 1.77250 49.6 3.60 5 276.886 0.18 3.41 6 276.886 0.22 1.80810 22.8 3.25 7 4.087 1.82 3.01 8* -12.997 0.85 1.63560 23.9 4.45 9* -25.539 0.57 5.70 10* -5.176 1.28 1.67070 19.3 6.83 11* -4.825 0.36 7.57 12* 10.575 1.82 1.53500 56.0 9.85 13* 41.456 10.16 Image plane ∞ Aspheric Data Side 8 K = 0.00000e+000 A 4=-8.23905e-003 A 6=-1.85106e-004 A 8=-2.80429e-004 A10= 1.33504e-004 A12=-1.72217e-005 9th page K = 0.00000e+000 A 4=-5.05212e-003 A 6=-3.36300e-004 A 8= 6.08201e-005 A10=9.33005e-007 A12=-6.60549e-007 Side 10 K = 0.00000e+000 A 4= 8.48361e-003 A 6=-2.49024e-004 A 8=-2.47053e-006 A10=8.63458e-007 A12=-2.45232e-008 Page 11 K = 0.00000e+000 A 4= 3.76269e-003 A 6= 1.03039e-004 A 8=-2.36342e-006 A10=-3.45118e-008 A12= 5.29686e-009 Side 12 K = 0.00000e+000 A 4=-3.67350e-003 A 6= 1.30651e-004 A 8=-1.97917e-006 A10= 1.19013e-008 A12= 8.59667e-010 A14=-1.37275e-011 Page 13 K = 0.00000e+000 A 4=-2.93441e-003 A 6= 6.87192e-005 A 8=-7.91416e-007 A10=-3.13873e-008 A12= 1.57545e-009 A14= 7.83470e-012 Various data Zoom ratio 1.00 Focal length 15.44 F-number 3.50 Half angle of view: 27.07 Image height 7.89 Lens total length 18.00 BF 8.03 d13 8.03 Entrance pupil position 1.68 Exit pupil position -9.29 Front principal point position 3.36 Back principal point position -7.41 Single lens data Lens starting surface focal length 1 1 11.53 2 4 8.69 3 6 -5.14 4 8 -42.76 5 10 43.11 6 12 26.00 (Numerical Example 4) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1 3.882 1.02 1.95375 32.3 3.87 2 7.138 0.22 1.94594 18.0 3.57 3 3.795 0.74 3.28 4 (Aperture) ∞ 0.36 4.18 5* 5.552 0.50 1.76450 49.1 3.98 6* 8.765 3.27 3.81 7* -4.324 1.31 1.54390 56.0 5.69 8* -7.294 0.36 7.51 9* 7.042 2.11 1.54390 56.0 12.02 10* 11.987 12.52 Image plane ∞ Aspheric Data 5th page K = 0.00000e+000 A 4= 2.81554e-005 A 6= 1.84850e-004 A 8=-3.15509e-005 Side 6 K = 0.00000e+000 A 4= 1.44367e-003 A 6= 1.74832e-004 A 8=-7.82800e-006 Side 7 K = 0.00000e+000 A 4= 7.70082e-003 A 6=-1.14006e-003 A 8= 1.49727e-004 A10=-1.18323e-005 A12= 4.67624e-007 A14=-3.71569e-010 Side 8 K = 0.00000e+000 A 4= 4.07615e-003 A 6=-5.26957e-004 A 8= 7.96433e-005 A10=-6.17853e-006 A12= 2.32990e-007 A14=-3.35783e-009 9th page K = 0.00000e+000 A 4=-5.14713e-003 A 6= 3.05044e-004 A 8=-1.15182e-005 A10=2.36679e-007 A12=-2.10950e-009 Side 10 K = 0.00000e+000 A 4=-3.90330e-003 A 6= 1.84171e-004 A 8=-6.54476e-006 A10= 1.46975e-007 A12=-1.40581e-009 Various data Zoom ratio 1.00 Focal length 13.83 F-number: 3.58 Half angle of view: 29.71 Image height 7.89 Lens length 14.82 BF 4.92 d10 4.92 Entrance pupil position 1.82 Exit pupil position -6.69 Front principal point position -0.82 Back principal point position -8.90 Single lens data Lens starting surface focal length 1 1 7.74 2 2 -8.85 3 5 18.55 4 7 -23.11 5 9 27.29 Table 1 shows the values ​​of each conditional expression in each numerical example.

[0063] [Table 1]

[0064] According to each embodiment, it is possible to provide a small, lightweight, high-performance optical system and imaging device.

[0065] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist of the present invention.

[0066] For example, the aperture diaphragm SP may be arranged closest to the object side to form a front aperture configuration. Alternatively, the aperture diaphragm SP may be arranged closest to the image side of the front group FG. In addition, when combined with an imaging device equipped with an imaging element that converts an optical image formed on a light receiving surface into an electrical signal, electrical correction may be added depending on the amount of distortion aberration and lateral chromatic aberration. In addition, the aperture diaphragm SP may be configured to adjust the F-number (aperture value). [Explanation of symbols]

[0067] LO optical system FG front group RG rear group Lp1 lens (first lens) Lp2 lens (second lens) Ln1 lens (third lens) A lens (fourth lens) B lens (5th lens)

Claims

1. An optical system consisting of a front group and a rear group, the front group includes a first lens having positive refractive power, a second lens arranged closer to the image side than the first lens and having positive refractive power, and a third lens, the first lens and the second lens each have a refractive index of 1.7 or more at the d line; the rear group is composed of two or more and four or less lenses, and includes a fourth lens having positive refractive power and a fifth lens arranged adjacent to the fourth lens and having negative refractive power, the fourth lens and the fifth lens are made of an organic material; When the Abbe number of the fourth lens is νdA, the Abbe number of the fifth lens is νdB, the focal length of the fourth lens is fA, and the focal length of the fifth lens is fB, 0.75<νdA / νdB<1.30 0.75<-fA / fB<1.30 An optical system characterized in that the following condition is satisfied:

2. When the linear expansion coefficient ( / °C) of the first lens is αp1, αp1<100×10^-7 2. The optical system according to claim 1, wherein the following condition is satisfied:

3. When the Abbe number of the first lens is νdp1, 25<νdp1<60 3. The optical system according to claim 1, wherein the following condition is satisfied:

4. When the linear expansion coefficient ( / °C) of the second lens is αp2, αp2<100×10^-7 4. The optical system according to claim 1, wherein the following condition is satisfied:

5. When the Abbe number of the second lens is νdp2, 25<νdp2<60 5. The optical system according to claim 1, wherein the following condition is satisfied:

6. The optical system according to claim 1 , wherein the third lens has a negative refractive power.

7. When the linear expansion coefficient ( / °C) of the third lens is αn, αn<100×10^-7 7. The optical system according to claim 6, wherein the following condition is satisfied:

8. When the refractive index of the third lens is N, 1.60<N<2.10 8. The optical system according to claim 6, wherein the following condition is satisfied:

9. When the Abbe number of the third lens is νdn, 15<νdn<35 9. The optical system according to claim 6, wherein the following condition is satisfied:

10. When the focal length of the optical system is f, 1.4<fA / f<3.5 10. The optical system according to claim 1, wherein the following condition is satisfied:

11. When the focal length of the optical system is f and the back focus of the optical system is BF, 1.00<f / BF<3.00 11. The optical system according to claim 1, wherein the following condition is satisfied:

12. Let Fm be the focal length of the front group and Fu be the focal length of the rear group. 0.5<|Fu| / Fm<25.0 12. The optical system according to claim 1, wherein the following condition is satisfied:

13. When the central thickness of the fourth lens is dA and the back focus of the optical system is BF, 0.10<dA / BF<0.60 13. The optical system according to claim 1, wherein the following condition is satisfied:

14. Let dB be the center thickness of the fifth lens, and BF be the back focus of the optical system. 0.05<dB / BF<0.40 14. The optical system according to claim 1, wherein the following condition is satisfied:

15. Let du be the center distance from the surface vertex of the rear group closest to the object side to the surface vertex of the rear group closest to the image side, and BF be the back focus of the optical system. 0.50<du / BF<1.50 15. The optical system according to claim 1, wherein the following condition is satisfied:

16. When the refractive index of the fourth lens is NA, 50<νdA<60 1.45<NA<1.60 16. The optical system according to claim 1, wherein the following condition is satisfied:

17. When the refractive index of the fourth lens is NA, 15<νdA<40 1.55<NA<1.75 16. The optical system according to claim 1, wherein the following condition is satisfied:

18. When the focal length of the optical system is f and the focal length of the third lens is fn, 0.20<-fn / f<0.70 10. The optical system according to claim 6, wherein the following condition is satisfied:

19. 19. The optical system according to claim 1, wherein the first lens is disposed closest to the object side.

20. 20. The optical system according to claim 1, wherein the front group is composed of, arranged in order from the object side to the image side, the first lens, the third lens having negative refractive power, and the second lens.

21. 20. The optical system according to claim 1, wherein the fourth lens is disposed adjacent to the object side or the image side of the fifth lens.

22. 22. An imaging apparatus comprising: an optical system according to claim 1; and an imaging element that captures an optical image formed by the optical system.

Images