Single focal length lens

Abstract

To provide a single focus lens that enables a quick autofocus at a large aperture ratio of F 1.4 or so, and has various aberrations excellently corrected.SOLUTION: A single focus lens comprises, in order from an object side to an image side, a first lens group G1 that has positive power as a whole, and a second lens group G2 that has the positive power as a whole. The first lens group G1 includes, in order from the object side to the image side, a 1A lens group G1A that has negative power as a whole, and a 1B lens group G1B that has the positive power as a whole, and the second lens group G2 includes, in order from the object side to the image side, a 2A lens group G2A, an aperture stop S, and a 2B lens group G2B. Upon focusing from an object at infinity to an object at a close distance, the first lens group G1 is fixed relative to an imaging plane, and the second lens group G2 moves to the object side while changing an interval between the 2A lens group G2A and the 2B lens group G2B. The single focus lens satisfies a prescribed conditional expression.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a single focal length lens suitable for use as a photographic lens in digital still cameras, digital video cameras, cinema cameras, and the like. [Background technology]

[0002] Single-focus imaging optical systems used in optical devices such as digital cameras are required to have a large aperture ratio to accommodate high-speed shooting and expressions that make the most of bokeh. Furthermore, as image sensors become increasingly dense with pixels, they are required to provide even greater correction of aberrations and achieve high performance. Traditionally, imaging optical systems for single-lens reflex cameras have often been of the retrofocus type due to the need to ensure back focus as a mirror movement range. There are known imaging lenses that use the retrofocus type to achieve a long back focus, a large aperture ratio, and high performance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-57802 [Patent Document 2] Japanese Patent Application Publication No. 2019-139158 [Patent Document 3] Japanese Patent Publication No. 2022-71325 Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 discloses an example in which a double Gauss type lens configuration, which is commonly used in standard angle-of-view lenses, has a lens configuration consisting of concave and convex lens elements on the object side that acts as a wide-angle converter. This optical system achieves a large aperture ratio with an FNO of approximately 1.4. However, aberration correction, mainly in the sagittal direction, is insufficient, resulting in large image spread at the peripheral image height. Patent Document 2 discloses an optical system similar to Patent Document 1, with a double Gauss type lens configuration on the object side that acts as a wide-angle converter. This optical system has an increased number of lenses constituting the lens group that acts as a wide-angle converter, enabling closer-up photography than Patent Document 1, but it also has insufficient sagittal aberration correction, resulting in large image spread at the peripheral image height. Patent Document 3 effectively corrects the sagittal aberration caused by the double Gauss type, which was a problem in Patent Documents 1 and 2, by adding a new lens element with a convex surface on the object side immediately after the image side of the aperture stop. Furthermore, when focusing from infinity to a close object, fast autofocus is enabled by moving only the second lens group toward the object. However, it cannot be said that astigmatism and curvature of field have been sufficiently optimized over the entire range of focusing distances.

[0005] The present invention has been made in light of these circumstances, and its object is to provide a single focal length lens that has a large aperture ratio of approximately F1.4, is capable of fast autofocusing, and effectively corrects various aberrations over the entire range of shooting distances. [Means for solving the problem]

[0006] In order to achieve the above object, a single focal length lens embodying the present invention comprises, in order from the object side to the image side, a first lens group G1 having a positive overall power and a second lens group G2 having a positive overall power, the first lens group G1 comprising, in order from the object side to the image side, a 1A lens group G1A having a negative overall power and a 1B lens group G1B having a positive overall power, the 1A lens group G1A and the 1B lens group G1B being separated by the largest air gap within the first lens group G1. the second lens group G2 is composed of, in order from the object side to the image side, a 2A lens group G2A, an aperture stop S, and a 2B lens group G2B, the object side surface of the lens adjacent to the image side of the aperture stop S is convex toward the object side, and when focusing from an object at infinity to an object at a close distance, the first lens group G1 is fixed with respect to the image plane, and the second lens group G2 moves toward the object side while changing the distance between the 2A lens group G2A and the 2B lens group G2B, and the following conditional expression is satisfied: (1) -2.0 <fG1A / fG1B<-1.0 fG1A: Focal length of lens group G1A fG1B:1B lens group G1B focal length

[0007] Furthermore, in a single focal length lens embodying the present invention, the lens closest to the object side in the first lens group G1 is preferably a positive lens, and is characterized in that the following conditional expression is satisfied: (2) 1.0 <fp1 / f<3.0 fp1: The focal length of the lens in the first lens group G1 closest to the object f: focal length of a prime lens when focused at infinity

[0008] Furthermore, a single focal length lens embodying the present invention is preferably characterized by satisfying the following conditional expression: (3) 1.5 <Pext / BFL<2.5 Pext: The distance on the optical axis from the exit pupil to the image, converted into air, when a single focal length lens is focused at infinity BFL: Back focus in air equivalent

[0009] Furthermore, a single focal length lens embodying the present invention is preferably characterized by satisfying the following conditional expression: (4) 0.5 <fG2B / fG2<1.5 fG2B: Focal length of 2B lens group G2B fG2: focal length of the second lens group G2

[0010] In addition, a single focal length lens embodying the present invention is preferably characterized in that the lens adjacent to the image side of the aperture stop S is a cemented lens, and all surfaces of the cemented lens are convex toward the object side. [Effects of the Invention]

[0011] A single focal length lens embodying the present invention can provide a single focal length lens with a large aperture ratio of approximately F1.4, capable of quick autofocusing, and with excellent correction of various aberrations over the entire range of shooting distances. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a lens configuration diagram of a single focal length lens according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a longitudinal aberration diagram of the single focal length lens of Example 1 at an infinite shooting distance. [Figure 3] FIG. 1 is a longitudinal aberration diagram of the single focal length lens of Example 1 at a shooting distance of 650 mm. [Figure 4] FIG. 2 is a diagram showing lateral aberration of the single focal length lens of Example 1 at an infinite shooting distance. [Figure 5] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 1 at a shooting distance of 650 mm. [Figure 6] FIG. 1 is a lens configuration diagram of a single focal length lens according to a second embodiment of the present invention. [Figure 7] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 2 at an infinite shooting distance. [Figure 8] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 2 at a shooting distance of 650 mm. [Figure 9] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 2 at an infinite shooting distance. [Figure 10] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 2 at a shooting distance of 650 mm. [Figure 11] FIG. 10 is a lens configuration diagram of a single focal length lens according to a third embodiment of the present invention. [Figure 12] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 3 at an infinite shooting distance. [Figure 13] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 3 at a shooting distance of 648 mm. [Figure 14] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 3 at an infinite shooting distance. [Figure 15] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 3 at a shooting distance of 648 mm. [Figure 16] FIG. 10 is a lens configuration diagram of a single focal length lens according to a fourth embodiment of the present invention. [Figure 17] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 4 at an infinite shooting distance. [Figure 18] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 4 at a shooting distance of 649 mm. [Figure 19] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 4 at an infinite shooting distance. [Figure 20] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 4 at a shooting distance of 649 mm. [Figure 21] FIG. 10 is a lens configuration diagram of a single focal length lens according to a fifth embodiment of the present invention. [Figure 22] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 5 at an infinite shooting distance. [Figure 23] FIG. 10 is a longitudinal aberration diagram of the single focal length lens of Example 5 at a shooting distance of 650 mm. [Figure 24] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 5 at an infinite shooting distance. [Figure 25] FIG. 10 is a diagram showing lateral aberration of the single focal length lens of Example 5 at a shooting distance of 650 mm. DETAILED DESCRIPTION OF THE INVENTION

[0013] The single focal length lens of the present invention comprises, in order from the object side to the image side, a first lens group G1 having overall positive power and a second lens group G2 having overall positive power; the first lens group G1 comprises, in order from the object side to the image side, a 1A lens group G1A having overall negative power and a 1B lens group G1B having overall positive power; the 1A lens group G1A and the 1B lens group G1B are separated by the largest air gap within the first lens group G1; the second lens group G2 comprises, in order from the object side to the image side, a 2A lens group G2A, an aperture stop S, and a 2B lens group G2B; the object-side surface of the lens adjacent to the image side of the aperture stop S faces the object side convexly; and when focusing from an object at infinity to a close distance object, the first lens group G1 is fixed with respect to the image plane, and the second lens group G2 moves toward the object while changing the distance between the 2A lens group G2A and the 2B lens group G2B; and the following condition is satisfied: (1) -2.0 <fG1A / fG1B<-1.0 fG1A: Focal length of lens group G1A fG1B:1B lens group G1B focal length

[0014] The single focal length lens of the present invention has a so-called retrofocus power arrangement, with the 1A lens group G1A having negative power and the 1B lens group G1B and the second lens group G2 having positive power. Also, by giving the 1A lens group G1A negative power and the 1B lens group G1B positive power, the first lens group G1 functions similarly to a wide converter, achieving a wide angle while maintaining back focus.

[0015] As mentioned above, because the first lens group G1 functions as a wide-angle converter, the axial light beam within the 1B lens group G1B becomes thick, and the axial light beam emerging from the first lens group G1 also becomes thick. For this reason, the aperture stop S can be made smaller in diameter by being located between the lenses in the second lens group G2, where the axial light beam entering it becomes thinner.

[0016] Double Gauss-type lens configurations have been widely adopted for standard-angle prime lenses with large apertures (approximately f / 1.4) due to their symmetrical power distribution before and after the aperture, making distortion and coma easy to correct. However, double Gauss-type lenses inherently have difficulty correcting sagittal coma flare, making it prone to sagittal image spread. Therefore, in this prime lens, the second lens group G2 uses a double Gauss-type lens configuration, and by locating a lens element with its convex surface facing the object immediately after the image side of the aperture stop S, this configuration allows for aberration correction with characteristics different from those of a typical double Gauss lens, while still maintaining a nearly symmetrical arrangement, and reduces sagittal image spread. This results in high imaging performance all the way to the edges of the image.

[0017] When focusing from an object at infinity to a close distance, the first lens group G1, which has a large beam diameter, is fixed relative to the image plane, and the second lens group G2 is moved toward the object, making it possible to reduce the size and weight of the focus group. Furthermore, the rear focus design makes it easy to provide the lens barrel with dust-proof and weather-resistant features.

[0018] When focusing from an object at infinity to an object at a close distance, the second lens group G2 is moved toward the object while changing the distance between the 2A lens group and the 2B lens group, thereby optimizing astigmatism, field curvature, and the like over the entire range of shooting distances.

[0019] Conditional formula (1) stipulates the condition for preventing an increase in the diameter of the second lens group G2 and reducing the size and weight of the focus group by optimizing the height of the axial marginal ray emerging from the first lens group G1 while ensuring back focus. If the upper limit of conditional formula (1) is exceeded and the negative power of the 1A lens group becomes weaker, it becomes difficult to ensure back focus, which is undesirable. If the negative power of the 1A lens group becomes stronger and the lower limit of conditional formula (1) is exceeded, the afocal magnification of the first lens group G1, which functions similarly to a wide-angle converter, becomes larger. In this case, the diameter of the axial ray beam emerging from the first lens group G1 becomes larger, which undesirably increases the diameters of the 1B lens group G1B and the second lens group G2.

[0020] It is more desirable to further limit the lower limit of the above-mentioned conditional expression (1) to -1.5, since this makes it possible to ensure the above-mentioned effects.

[0021] Furthermore, in the single focal length lens of the present invention, it is desirable that the lens element closest to the object side in the first lens group G1 be a positive lens element and satisfy the following conditional expression: (2) 1.0 <fp1 / f<3.0 fp1: The focal length of the lens in the first lens group G1 closest to the object f: focal length of a prime lens when focused at infinity Retrofocus lenses have an asymmetrical power arrangement with negative power on the object side and positive power on the image side, which makes them prone to negative distortion. By placing a positive lens closest to the object in the first lens group G1, positive distortion is generated, making it easier to correct the negative distortion that occurs with the asymmetrical power arrangement of retrofocus lenses. Furthermore, by placing a positive lens closest to the object in the first lens group G1, the diameter of the light beam passing through the first lens group G1 can be reduced, which is advantageous for making the first lens group G1 more compact.

[0022] Conditional expression (2) defines the power of the positive lens located closest to the object in the first lens group G1 and is a condition for achieving good correction of distortion. If the upper limit of conditional expression (2) is exceeded and the power of the positive lens located closest to the object in the first lens group G1 becomes weak, correction of negative distortion becomes insufficient. Furthermore, if the lower limit of conditional expression (2) is exceeded and the power of the positive lens located closest to the object in the first lens group G1 becomes strong, the 1A lens group as a whole has negative power, and the power of the negative lens in the 1A lens group also becomes strong. This undesirably increases the sensitivity of the positive and negative lenses to decentration errors.

[0023] It is more desirable to further limit the lower limit of the above-mentioned conditional expression (2) to 1.5 and the upper limit to 2.6, since this will make it possible to ensure the above-mentioned effects.

[0024] Furthermore, it is desirable that the single focal length lens of the present invention satisfy the following conditional expression. (3) 1.5 <Pext / BFL<2.5 Pext: The distance on the optical axis from the exit pupil to the image, converted into air, when a single focal length lens is focused at infinity BFL: Back focus in air equivalent

[0025] With standard lenses with a large aperture ratio, minimizing vignetting is also important. Reducing vignetting means making the light beam at the peripheral angle of view thicker, which tends to increase the diameter of various parts of the lens system.

[0026] Conditional expression (3) defines the ratio of the back focal length to the axial distance between the exit pupil and the image plane of a fixed focal length lens, and is a condition for reducing the diameter of the 2B lens group G2B. If the axial distance between the exit pupil and the image plane exceeds the upper limit of conditional expression (3), the peripheral light beam passes through a high position in the 2B lens group G2B, making it difficult to reduce the diameter, which is undesirable. If the back focal length exceeds the lower limit of conditional expression (3), the increase in the back focal length is undesirable, which is undesirable, as it is detrimental to reducing the overall length of the entire system.

[0027] It is more desirable to further limit the lower limit of the above-mentioned conditional expression (3) to 1.6 and the upper limit to 2.3, since this will make it possible to ensure the above-mentioned effects.

[0028] Furthermore, it is desirable that the single focal length lens of the present invention satisfy the following conditional expression. (4) 0.5 <fG2B / fG2<1.5 fG2B: Focal length of 2B lens group G2B fG2: focal length of the second lens group G2

[0029] Conditional expression (4) defines the focal length of the 2B lens group G2B relative to the second lens group G2, and is a conditional expression for ensuring good aberration correction while ensuring back focus. In the single focal length lens of the present invention, the first lens group G1, which is a fixed group, and the second lens group G2, which is a movable group, each correct aberrations when focusing. If fG2B becomes small beyond the lower limit of conditional expression (4), it becomes difficult to correct negative distortion in particular, which is undesirable. If fG2B becomes large beyond the upper limit of conditional expression (4), the power arrangement of the retrofocus type becomes weaker, making it difficult to ensure back focus, which is undesirable.

[0030] It is more desirable to further limit the lower limit of the above-mentioned conditional expression (4) to 0.7, as this will make the above-mentioned effect even more certain.It is more desirable to further limit the upper limit of the above-mentioned conditional expression (4) to 1.2, as this will make the above-mentioned effect even more certain.

[0031] Furthermore, the single focal length lens of the present invention is characterized in that the lens adjacent to the image side of the aperture stop S is a cemented lens, and all surfaces of the cemented lens are convex toward the object side.

[0032] By having all surfaces of the cemented lens convex toward the object side, the shape becomes close to aplanatic, which makes it easier to suppress the spherical aberration and coma that occur in cemented lenses. This is also desirable because it reduces the sensitivity of cemented lenses to decentering errors.

[0033] Next, the lens configuration of an example of a single focal length lens according to the present invention will be described. Note that in the following description, the lens configuration will be described in order from the object side to the image side. [Example]

[0034] FIG. 1 is a lens configuration diagram of a single focal length lens according to a first embodiment of the present invention.

[0035] The first lens group G1 is composed of a 1A lens group G1A having negative power overall and a 1B lens group G1B having positive power overall. The 1A lens group G1A is composed of a positive meniscus lens L11 with a convex surface facing the object side, a biconcave negative lens L12, and a negative meniscus lens L13 with a convex surface facing the object side. The 1B lens group G1B is composed of a cemented lens component consisting of a negative meniscus lens L14 with a convex surface facing the image side and a positive meniscus lens L15 with a convex surface facing the image side, a cemented lens component consisting of a biconcave negative lens L16 and a positive meniscus lens L17 with a convex surface facing the object side, and a biconvex positive lens L18. The first lens group G1 is fixed relative to the image plane during focusing.

[0036] The second lens group G2 is composed of a 2A lens group G2A having an overall positive power, an aperture stop S, and a 2B lens group G2B having an overall positive power. The 2A lens group G2A is composed of a positive meniscus lens L21 having a convex surface facing the object side, and a cemented lens component consisting of a biconvex positive lens L22 and a biconcave negative lens L23. The 2B lens group G2B is composed of a cemented lens component consisting of a negative meniscus lens L24 having a convex surface facing the object side and a positive meniscus lens L25 having a convex surface facing the object side, a cemented lens component consisting of a biconcave negative lens L26 and a biconvex positive lens L27, and a biconvex positive lens L28. Both the object-side and image-side surfaces of the biconvex positive lens L28 have a predetermined aspheric shape. The second lens group G2 moves along the optical axis toward the object side while changing the distance between the 2A lens group G2A and the 2B lens group G2B when focusing from a long distance to a short distance. [Example]

[0037] FIG. 6 is a lens configuration diagram of a single focal length lens according to a second embodiment of the present invention.

[0038] The first lens group G1 is composed of a 1A lens group G1A having negative power overall and a 1B lens group G1B having positive power overall. The 1A lens group G1A is composed of a positive meniscus lens L11 with a convex surface facing the object side, a biconcave negative lens L12, and a negative meniscus lens L13 with a convex surface facing the object side. The 1B lens group G1B is composed of a cemented lens component consisting of a biconcave negative lens L14 and a biconvex positive lens L15, a cemented lens component consisting of a biconcave negative lens L16 and a positive meniscus lens L17 with a convex surface facing the object side, and a biconvex positive lens L18. The first lens group G1 is fixed relative to the image plane during focusing.

[0039] The second lens group G2 is composed of a 2A lens group G2A having an overall positive power, an aperture stop S, and a 2B lens group G2B having an overall positive power. The 2A lens group G2A is composed of a positive meniscus lens L21 having a convex surface facing the object side, and a cemented lens component consisting of a biconvex positive lens L22 and a biconcave negative lens L23. The 2B lens group G2B is composed of a cemented lens component consisting of a negative meniscus lens L24 having a convex surface facing the object side and a positive meniscus lens L25 having a convex surface facing the object side, a cemented lens component consisting of a biconcave negative lens L26 and a biconvex positive lens L27, and a biconvex positive lens L28. Both the object-side and image-side surfaces of the biconvex positive lens L28 have a predetermined aspheric shape. The second lens group G2 moves along the optical axis toward the object side while changing the distance between the 2A lens group G2A and the 2B lens group G2B when focusing from a long distance to a short distance. [Example]

[0040] FIG. 11 is a lens configuration diagram of a single focal length lens according to a third embodiment of the present invention.

[0041] The first lens group G1 is composed of a 1A lens group G1A having negative power overall and a 1B lens group G1B having positive power overall. The 1A lens group G1A is composed of a positive meniscus lens L11 with a convex surface facing the object side, a biconcave negative lens L12, and a negative meniscus lens L13 with a convex surface facing the object side. The 1B lens group G1B is composed of a cemented lens component consisting of a negative meniscus lens L14 with a convex surface facing the image side and a positive meniscus lens L15 with a convex surface facing the image side, a cemented lens component consisting of a biconcave negative lens L16 and a positive meniscus lens L17 with a convex surface facing the object side, and a biconvex positive lens L18. The first lens group G1 is fixed relative to the image plane during focusing.

[0042] The second lens group G2 is composed of a 2A lens group G2A having an overall positive power, an aperture stop S, and a 2B lens group G2B having an overall positive power. The 2A lens group G2A is composed of a positive meniscus lens L21 having a convex surface facing the object side, and a cemented lens component consisting of a biconvex positive lens L22 and a biconcave negative lens L23. The 2B lens group G2B is composed of a cemented lens component consisting of a negative meniscus lens L24 having a convex surface facing the object side and a positive meniscus lens L25 having a convex surface facing the object side, a cemented lens component consisting of a biconcave negative lens L26 and a biconvex positive lens L27, and a biconvex positive lens L28. Both the object-side and image-side surfaces of the biconvex positive lens L28 have a predetermined aspheric shape. The second lens group G2 moves along the optical axis toward the object side while changing the distance between the 2A lens group G2A and the 2B lens group G2B when focusing from a long distance to a short distance. [Example]

[0043] FIG. 16 is a lens configuration diagram of a single focal length lens according to a fourth embodiment of the present invention.

[0044] The first lens group G1 is composed of a 1A lens group G1A having negative overall power and a 1B lens group G1B having positive overall power. The 1A lens group G1A is composed of a biconvex positive lens L11, a negative meniscus lens L12 with a convex surface facing the object side, and a negative meniscus lens L13 with a convex surface facing the object side. The 1B lens group G1B is composed of a cemented lens component consisting of a negative meniscus lens L14 with a convex surface facing the image side and a positive meniscus lens L15 with a convex surface facing the image side, a cemented lens component consisting of a biconcave negative lens L16 and a positive meniscus lens L17 with a convex surface facing the object side, and a biconvex positive lens L18. The first lens group G1 is fixed relative to the image plane during focusing.

[0045] The second lens group G2 is composed of a 2A lens group G2A having an overall positive power, an aperture stop S, and a 2B lens group G2B having an overall positive power. The 2A lens group G2A is composed of a positive meniscus lens L21 having a convex surface facing the object side, and a cemented lens component consisting of a biconvex positive lens L22 and a biconcave negative lens L23. The 2B lens group G2B is composed of a cemented lens component consisting of a negative meniscus lens L24 having a convex surface facing the object side and a positive meniscus lens L25 having a convex surface facing the object side, a cemented lens component consisting of a biconcave negative lens L26 and a biconvex positive lens L27, and a biconvex positive lens L28. Both the object-side and image-side surfaces of the biconvex positive lens L28 have a predetermined aspheric shape. The second lens group G2 moves along the optical axis toward the object side while changing the distance between the 2A lens group G2A and the 2B lens group G2B when focusing from a long distance to a short distance. [Example]

[0046] FIG. 21 is a lens configuration diagram of a single focal length lens according to a fifth embodiment of the present invention.

[0047] The first lens group G1 is composed of a 1A lens group G1A having negative power overall and a 1B lens group G1B having positive power overall. The 1A lens group G1A is composed of a positive meniscus lens L11 with a convex surface facing the object side, a negative meniscus lens L12 with a convex surface facing the object side, and a negative meniscus lens L13 with a convex surface facing the object side. The 1B lens group G1B is composed of a cemented lens component consisting of a negative meniscus lens L14 with a convex surface facing the image side and a positive meniscus lens L15 with a convex surface facing the image side, a cemented lens component consisting of a biconcave negative lens L16 and a positive meniscus lens L17 with a convex surface facing the object side, and a biconvex positive lens L18. The first lens group G1 is fixed relative to the image plane during focusing.

[0048] The second lens group G2 is composed of a 2A lens group G2A having an overall positive power, an aperture stop S, and a 2B lens group G2B having an overall positive power. The 2A lens group G2A is composed of a positive meniscus lens L21 having a convex surface facing the object side, and a cemented lens component consisting of a biconvex positive lens L22 and a biconcave negative lens L23. The 2B lens group G2B is composed of a cemented lens component consisting of a negative meniscus lens L24 having a convex surface facing the object side and a positive meniscus lens L25 having a convex surface facing the object side, a cemented lens component consisting of a biconcave negative lens L26 and a biconvex positive lens L27, and a biconvex positive lens L28. Both the object-side and image-side surfaces of the biconvex positive lens L28 have a predetermined aspheric shape. The second lens group G2 moves along the optical axis toward the object side while changing the distance between the 2A lens group G2A and the 2B lens group G2B when focusing from a long distance to a short distance.

[0049] Specific numerical data for each of the embodiments of the imaging optical system of the present invention described above will be shown below.

[0050] In [Surface Data], the surface number is the lens surface or aperture stop number counted from the object side, r is the radius of curvature of each surface, d is the spacing between surfaces, nd is the refractive index for the d-line (wavelength 587.56 nm), and vd is the Abbe number for the d-line.

[0051] An asterisk (*) next to a surface number indicates that the lens surface is aspherical, and BF represents the back focal length.

[0052] The (diaphragm) next to the surface number indicates that an aperture diaphragm is located at that position. The radius of curvature for the plane or aperture diaphragm is marked as ∞ (infinity).

[0053] [Aspherical Data] shows the coefficient values ​​that give the aspherical shape of lens surfaces marked with an * in [Surface Data]. The shape of the aspherical surface is expressed by the following equation, where y is the displacement from the optical axis in a direction perpendicular to the optical axis, z is the displacement (sag) from the intersection of the aspherical surface and the optical axis in the direction of the optical axis, r is the radius of curvature of the reference sphere, K is the Conic coefficient, and A4 and A6 are the fourth- and sixth-order aspherical coefficients, respectively.

[0054] TIFF0007823879000001.tif37157

[0055] [Various data] shows values ​​such as focal length at each shooting distance.

[0056] [Variable Distance Data] shows the variable distance and BF values ​​for each shooting distance.

[0057] [Lens Group Data] shows the surface number of each lens group closest to the object and the composite focal length of the entire group.

[0058] In addition, for all of the values ​​of the following specifications, the focal length f, radius of curvature r, lens surface spacing d, and other length units are given in millimeters (mm) unless otherwise specified; however, this is not a limitation, as optical systems can achieve equivalent optical performance with proportional magnification and proportional reduction.

[0059] Also shown is a list of values ​​corresponding to the conditional expressions in each of these embodiments.

[0060] In the aberration diagrams corresponding to the respective examples, d, g, and C represent the d-line, g-line, and C-line, respectively, and ΔS and ΔM represent the sagittal image surface and meridional image surface, respectively.

[0061] Numerical Example 1 Unit: mm [Face Data] Surface number rd nd vd Object surface ∞ (d0) 1 98.2632 7.2623 1.77250 49.62 2 832.6957 5.6518 3 -2713.4733 2.0000 1.61340 44.27 4 107.7526 3.2094 5 140.3642 2.0000 1.58144 40.89 6 43.8218 18.5516 7 -47.0026 2.0000 1.64769 33.84 8 -378.1420 7.3600 1.77250 49.62 9 -59.6794 1.9179 10 -239.5032 2.0000 1.61340 44.27 11 58.2020 8.4289 1.77250 49.62 12 171.2887 0.3881 13 89.4061 7.5364 1.77250 49.62 14 -226.5904 (d14) 15 54.4693 6.0802 1.92286 20.88 16 154.8088 0.3000 17 41.4418 11.3313 1.59282 68.62 18 -145.4074 1.5000 1.77047 29.74 19 29.8367 (d19) 20 (Aperture) ∞ 1.2000 21 50.2871 1.3000 1.67270 32.17 22 20.8353 6.2055 1.59282 68.62 23 46.9347 6.4661 24 -34.1250 1.0000 1.77047 29.74 25 169.7026 2.5894 1.88100 40.14 26 -300.5876 0.3000 27* 87.5520 6.5742 1.85135 40.10 28* -39.1043 (BF) Image plane ∞ [Aspherical data] 27th page 28th page K 0.00000 0.00000 A4 -1.88464E-06 2.95591E-06 A6 4.66471E-10 -1.62893E-10 [Various data] INF 650mm Focal length 63.22 62.29 F-number 1.46 1.66 Full angle of view 2ω 37.90 34.70 Image height Y 21.63 21.63 Lens length 173.02 173.02 [Variable Interval Data] INF 650mm d0 infinity 477.3161 d14 10.4492 2.0827 d19 9.9203 10.3632 BF 39.5009 47.4244 [Lens group data] Group starting plane focal length G1 1 347.06 G2 15 84.13 G1A 1 -148.22 G1B 7 123.53 G2A 15 218.41 G2B 21 81.39

[0062] Numerical Example 2 Unit: mm [Face Data] Surface number rd nd vd Object surface ∞ (d0) 1 98.0974 7.2939 1.77250 49.62 2 546.5135 7.8521 3 -1000.0000 2.0000 1.61340 44.27 4 279.2533 1.8492 5 238.7823 2.0000 1.58144 40.89 6 42.5965 23.5449 7 -49.0596 2.0000 1.64769 33.84 8 1464.6213 7.9764 1.77250 49.62 9 -61.4519 0.4583 10 -185.9219 2.0000 1.61340 44.27 11 54.8191 5.2406 1.77250 49.62 12 113.1394 0.3000 13 79.5971 8.2451 1.77250 49.62 14 -214.0248 (d14) 15 55.6932 6.4918 1.92286 20.88 16 196.7427 0.3000 17 39.9902 11.0674 1.59282 68.62 18 -119.0499 1.5000 1.77047 29.74 19 28.2909 (d19) 20 (Aperture) ∞ 1.2000 21 42.5150 1.3000 1.67270 32.17 22 21.2373 9.8932 1.59282 68.62 23 43.3562 6.6801 24 -31.7609 1.0000 1.77047 29.74 25 274.5922 2.5247 1.88100 40.14 26 -167.8025 0.3000 27* 110.7591 6.1581 1.85135 40.10 28* -37.4842 (BF) Image plane ∞ [Aspherical data] 27th page 28th page K 0.00000 0.00000 A4 -1.72481E-06 3.08177E-06 A6 2.42889E-10 -5.29963E-10 [Various data] INF 650mm Focal length 64.03 62.51 F-number 1.46 1.64 Full angle of view 2ω 37.21 34.42 Image height Y 21.63 21.63 Lens length 173.03 173.03 [Variable Interval Data] INF 650mm d0 infinity 477.3150 d14 6.8217 0.3000 d19 11.5023 9.5537 BF 35.5260 43.9963 [Lens group data] Group starting plane focal length G1 1 474.71 G2 15 81.91 G1A 1 -165.02 G1B 7 143.71 G2A 15 206.16 G2B 21 77.90

[0063] Numerical Example 3 Unit: mm [Face Data] Surface number rd nd vd Object surface ∞ (d0) 1 100.4237 7.0537 1.77250 49.62 2 922.6578 6.3455 3 -1000.0000 2.0000 1.61340 44.27 4 159.8542 2.4682 5 181.7334 2.0000 1.58144 40.89 6 43.7167 18.2780 7 -44.9137 2.0000 1.64769 33.84 8 -349.8731 7.6198 1.77250 49.62 9 -56.4431 1.2903 10 -204.3271 2.0000 1.61340 44.27 11 57.9629 7.4711 1.77250 49.62 12 161.6266 0.3000 13 88.0894 7.7119 1.77250 49.62 14 -210.4836 (d14) 15 54.7594 6.0658 1.92286 20.88 16 163.8107 0.3000 17 40.9465 11.0299 1.59282 68.62 18 -138.0814 1.5000 1.77047 29.74 19 29.1448 (d19) 20 (Aperture) ∞ 1.2000 21 49.5710 1.3000 1.67270 32.17 22 21.8284 6.1207 1.59282 68.62 23 47.6368 6.9431 24 -33.8372 1.0000 1.77047 29.74 25 161.5324 2.6712 1.88100 40.14 26 -265.2404 0.3000 27* 97.4466 7.4172 1.85135 40.10 28* -38.5989 (BF) Image plane ∞ [Aspherical data] 27th page 28th page K 0.00000 0.00000 A4 -1.70311E-06 2.89178E-06 A6 2.16793E-10 -2.74765E-10 [Various data] INF 648mm Focal length 64.31 62.99 F-number 1.46 1.66 Full angle of view 2ω 37.21 34.29 Image height Y 21.63 21.63 Lens length 170.52 170.52 [Variable Interval Data] INF 648mm d0 infinity 477.3160 d14 9.0941 1.0327 d19 10.1607 9.8214 BF 38.8828 47.2835 [Lens group data] Group starting plane focal length G1 1 330.82 G2 15 84.49 G1A 1 -158.35 G1B 7 124.69 G2A 15 227.31 G2B 21 80.30

[0064] Numerical Example 4 Unit: mm [Face Data] Surface number rd nd vd Object surface ∞ (d0) 1 102.8107 9.8740 1.77250 49.62 2 -8988.5598 2.7805 3 928.8802 2.0000 1.61340 44.27 4 176.5468 3.4361 5 336.9415 2.0000 1.58144 40.89 6 42.8993 22.1052 7 -43.6406 2.0000 1.64769 33.84 8 -256.0342 7.7796 1.77250 49.62 9 -55.8507 0.3026 10 -151.8992 2.0000 1.61340 44.27 11 77.8484 5.0118 1.77250 49.62 12 215.4396 0.3002 13 85.1175 8.0194 1.77250 49.62 14 -210.8736 (d14) 15 51.2676 5.9365 1.92286 20.88 16 121.7976 1.6736 17 40.1219 11.5718 1.59282 68.62 18 -131.6288 1.5000 1.77047 29.74 19 26.0864 (d19) 20 (Aperture) ∞ 1.2000 21 39.5725 1.3000 1.67270 32.17 22 21.1157 6.7915 1.59282 68.62 23 43.2415 7.0199 24 -34.7539 1.0000 1.77047 29.74 25 218.4871 2.5799 1.88100 40.14 26 -214.1827 0.3000 27* 94.7104 6.2631 1.85135 40.10 28* -38.8191 (BF) Image plane ∞ [Aspherical data] 27th page 28th page K 0.00000 0.00000 A4 -2.11332E-06 2.47473E-06 A6 2.94427E-10 -9.48601E-10 [Various data] INF 649mm Focal length 64.21 62.76 F-number 1.46 1.64 Full angle of view 2ω 37.22 34.40 Image height Y 21.63 21.63 Lens length 171.92 171.92 [Variable Interval Data] INF 649mm d0 infinity 477.3154 d14 7.6918 0.3000 d19 10.7286 9.8004 BF 38.7564 47.0764 [Lens group data] Group starting plane focal length G1 1 325.49 G2 15 83.80 G1A 1 -176.92 G1B 7 132.63 G2A 15 496.00 G2B 21 67.12

[0065] Numerical Example 5 Unit: mm [Face Data] Surface number rd nd vd Object surface ∞ (d0) 1 69.6497 7.4637 1.77250 49.62 2 173.2547 3.4165 3 193.0795 2.0000 1.61340 44.27 4 97.7683 3.4080 5 192.7817 2.0000 1.58144 40.89 6 41.4963 23.5306 7 -43.6742 2.0000 1.64769 33.84 8 -249.7141 7.7421 1.77250 49.62 9 -54.5749 0.7100 10 -175.6270 2.0000 1.61340 44.27 11 60.8459 6.0692 1.77250 49.62 12 182.7629 0.8027 13 98.5131 8.2679 1.77250 49.62 14 -171.9095 (d14) 15 58.5301 5.8958 1.92286 20.88 16 188.0450 0.3000 17 42.9018 10.7829 1.59282 68.62 18 -153.1767 1.5000 1.77047 29.74 19 33.6235 (d19) 20 (Aperture) ∞ 1.2000 21 66.0463 1.3000 1.67270 32.17 22 21.9381 6.5781 1.59282 68.62 23 52.0549 6.9517 24 -35.6314 1.0000 1.77047 29.74 25 134.0689 2.5519 1.88100 40.14 26 -531.5398 0.3000 27* 86.2473 7.5508 1.85135 40.10 28* -40.0751 (BF) Image plane ∞ [Aspherical data] 27th page 28th page K 0.00000 0.00000 A4 -2.14813E-06 2.76257E-06 A6 3.58873E-10 -3.50068E-10 [Various data] INF 650mm Focal length 63.76 62.73 F-number 1.46 1.63 Full angle of view 2ω 37.70 34.57 Image height Y 21.63 21.63 Lens length 173.02 173.02 [Variable Interval Data] INF 650mm d0 infinity 477.3164 d14 10.1027 1.4000 d19 9.3076 10.0873 BF 38.2876 46.2106 [Lens group data] Group starting plane focal length G1 1 286.13 G2 15 86.93 G1A 1 -172.90 G1B 7 126.09 G2A 15 173.40 G2B 21 95.61

[0066] [Conditional expression corresponding value] Example 1 Example 2 Example 3 Example 4 Example 5 (1) -2 <fG1A / fG1B<-1 -1.20 -1.15 -1.27 -1.33 -1.37 (2) 1.0 <fp1 / f<3.0 2.27 2.40 2.26 2.05 2.29 (3) 1.5 <Pext / BFL<2.5 1.70 1.90 1.77 1.76 1.78 (4) 0.5 <fG2B / fG2<1.5 0.97 0.95 0.95 0.80 1.10 [Explanation of symbols]

[0067] G1 First lens group G2 Second lens group G1A 1A lens group G1B 1B lens group G2A 2A lens group G2B 2B lens group S aperture stop

Claims

1. a first lens group G1 having a positive overall power and a second lens group G2 having a positive overall power, the first lens group G1 comprising, from the object side to the image side, a 1A lens group G1A having a negative overall power and a 1B lens group G1B having a positive overall power, the 1A lens group G1A and the 1B lens group G1B being separated by the largest air gap within the first lens group G1; the second lens group G2 comprising, from the object side to the image side, a 2A lens group G2A, an aperture stop S, and a 2B lens group G2B, the object-side surface of the lens adjacent to the image side of the aperture stop S facing its convex surface toward the object side; when focusing from an object at infinity to an object at a close distance, the first lens group G1 is fixed with respect to the image plane, and the second lens group G2 moves toward the object side while changing the distance between the 2A lens group G2A and the 2B lens group G2B; and the following condition is satisfied: (1) -2.0<fG1A / fG1B<-1.0 fG1A: focal length of 1A lens group G1A fG1B: focal length of 1B lens group G1B

2. 2. The single focal length lens according to claim 1, wherein the lens closest to the object side in the first lens group G1 is a positive lens, and the following condition is satisfied: (2) 1.0<fp1 / f<3.0 fp1: focal length of the lens in the first lens group G1 closest to the object f: focal length of a prime lens when focused at infinity

3. 2. The single focal length lens according to claim 1, which satisfies the following condition: (3) 1.5<Pext / BFL<2.5 Pext: The distance on the optical axis from the exit pupil to the image, converted into air, when a single focal length lens is focused at infinity BFL: Back focus in air equivalent

4. 2. The single focal length lens according to claim 1, which satisfies the following condition: (4) 0.5<fG2B / fG2<1.5 fG2B: focal length of 2B lens group G2B fG2: focal length of the second lens group G2

5. 2. The single focal length lens according to claim 1, wherein the lens adjacent to the image side of the aperture stop S is a cemented lens, and all surfaces of the cemented lens are convex toward the object side.

Images