Abstract:
An imaging optical system includes: an object-side lens group having, in order from its object side, an object-side positive meniscus lens that is convex on its object side and has positive refracting power, and an object side negative meniscus lens that is convex on its object side and has negative refracting power; and image-side lens group having, in order from its object side, an image-side negative lens, an image-side first positive lens and an image-side second positive lens. The image-side negative lens and the image-side first positive lens adjacent thereto form together a cemented doublet.

Description:
This application claims benefit of Japanese Application No. 2008-027164 filed in Japan on Feb. 7, 2008, the contents of which are incorporated by this reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates generally to an imaging optical system, and more particularly to an imaging apparatus comprising an imaging optical system. 
   There has been an imaging optical system known so far in the art, which uses five lenses to make improvements in optical performance (see Patent Publications 1 and 2). The publications disclose an imaging optical system that comprises an aperture stop for limiting axial light beams, a lens group that is located on an object side with respect to that aperture stop and consists of two lenses, and a lens group that is located on an image side with respect to that aperture stop, wherein a lens system comprises, in order from its object side, a positive single lens, a negative single lens, a cemented doublet of a negative lens and a positive lens, and a positive single lens. 
   Patent Publication 1: JP(A) 9-133859 
   Patent Publication 2: JP(A) 2000-180719 
   With such imaging optical system as disclosed in Patent Publications 1 and 2, however, the thickness (the axial distance from the object side surface of the lens nearest to the object side to the image side surface of the lens nearest to the image side) grows large relative to the back focus. For this reason, intentional use of such an imaging optical system for interchangeable lenses of single-lens reflex cameras will work against size reductions of the whole imaging apparatus including the imaging optical system. 
   In view of such problems with the prior art, one object of the present invention is to provide an imaging optical system that works for making sure the given back focus and reducing thickness, and makes it easy to ensure brightness and optical performance. Another object is to provide an imaging apparatus comprising such an imaging optical system. 
   SUMMARY OF THE INVENTION 
   According to the invention, the aforesaid objects are accomplishable by the provision of an imaging optical system, comprising: 
   an aperture stop for limiting an axial light beam, 
   an object-side lens group that is located on an object side of the aforesaid aperture stop and has negative refracting power, and 
   an image-side lens group that is located on an image side of the aforesaid aperture stop and has positive refracting power, wherein: 
   the aforesaid object-side lens group comprises two lenses: in order from its object side, 
   an object-side positive meniscus lens that is convex on its object side and has positive refracting power, and 
   an object-side negative meniscus lens that is convex on its object side and has negative refracting power, and 
   the aforesaid image-side lens group comprises three lenses: in order from its object side, an image-side negative lens, an image-side first positive lens, and an image-side second positive lens, and 
   the aforesaid image-side negative lens and the aforesaid image-side first positive lens form together a mutually cemented doublet, preferably with the satisfaction of the following conditions (1), (2) and (3):
 
2.3 ≦|f 1 /f|≦ 5.3  (1)
 
0.8 ≦f 2/ f≦ 1.0  (2)
 
0.55 ≦L/f≦ 0.85  (3)
 
where f is the focal length of the whole imaging optical system,
 
   f 1  is the focal length of the object-side lens group in the aforesaid imaging optical system, 
   f 2  is the focal length of the image-side lens group in the aforesaid imaging optical system, and L is the axial distance from the vertex of the lens surface located in, and nearest to the object side of, said object-side lens group to the vertex of the lens surface located in, and nearest to the object side of, said image-side lens group. 
   The aperture stop is almost centrally located in the imaging lens system to set up the aforesaid lens layout so that the refracting powers of the lenses in the imaging optical system are symmetrically distributed, working for correction of distortion, Petzval s sum, spherical aberrations, etc. 
   Two positive lenses: the image-side first positive lens L 22  and the image-side second positive lens L 23  are located on the image side of the aperture stop so that an exit pupil can easily be spaced away from an image plane while aberration performance is kept intact, making it easy to hold back deterioration of image quality due to shading. 
   The cemented doublet of the image-side negative lens L 21  and the image-side first positive lens L 22  is used for the image-side lens group so that correction of chromatic aberrations, etc. can be facilitated. 
   The object-side positive lens L 11  and the object-side negative lens L 12  on the object side with respect to the aperture stop are each configured into a meniscus shape convex on its object side, whereby while letting the object-side positive lens L 11  keep functioning to cancel out aberrations occurring from the object-side negative lens L 12 , the angle of incidence of light rays on the respective lens surfaces in the object-side lens group can easily be kept so small that aberrations at the object-side lens group can easily be reduced. 
   The imaging optical system of the invention sets up a retrofocus type with the object-side lens group of negative refracting power and the image-side lens group of positive refracting power, and satisfies conditions (1), (2) and (3) for the purpose of making sure the given back focus, slimming-down, brightness, and optical performance. 
   Conditions (1), (2), and (3) define the preferable focal length of the object-side lens group, the preferable focal length of the image-side lens group, and the preferable position relation of the object- and image-side lens groups, respectively. 
   Abiding by the lower limit of 2.3 to condition (1) enables the negative refracting power of the object-side lens to be kept from growing too strong, working for the maintenance of optical performance even when the object-side lens group is constructed of two lenses. 
   Abiding by the upper limit of 5.3 to condition (1) allows the object-side lens group to have negative refracting power, working for making sure the retrofocus type lens group arrangement has the given back focus. 
   Abiding by the lower limit of 0.8 to condition (2) enables the positive refracting power of the image-side lens to be kept from growing too strong, working for the maintenance of optical performance even when the image-side lens group is constructed of three lenses. 
   Abiding by the upper limit of 1.0 to condition (2) makes sure the image-side lens group has positive refracting power, working for the slimming-down of the lens system. 
   Abiding by the lower limit of 0.55 to condition (3) makes sure the distance from the vertex of the lens surface nearest to the object side to the image-side lens group, helping locate the lenses in the object-side lens group in the number necessary for correction of aberrations. It also helps keep the symmetry of the lenses with respect to the aperture stop. 
   Abiding by the upper limit of 0.85 to condition (3) enables the object-side positive lens L 11  nearest to the object side to draw near to the image-side lens group. This in turn makes it possible to curtail the distance between the entrance pupil and the entrance surface of the lens system and reduce the diameters and thicknesses of the lenses in the object-side lens group, working for thickness and diameter reductions. It also works for correction of various aberrations because of a decrease in the effective diameter of the object-side lens group: even with brightness ensured, aberrations can easily be held back. 
   In the imaging optical system of the invention, the aforesaid cemented doublet is configured into a meniscus shape concave on the aperture stop side, and it is desired that the image-side second positive lens L 23  located in, and nearest to the image side of, the aforesaid image-side lens group be convex on its image side surface. 
   The cemented doublet located on the image side with respect to the aperture stop is configured into a meniscus shape concave on the aperture stop side, and the image-side second positive lens L 23  on the image side thereof is convex on its image side surface, whereby it is easy for that image-side lens group to have positive refracting power and, at the same time, to make correction of axial aberrations and off-axis aberrations and achieve diameter reductions. 
   It is desired that the aforesaid image-side negative lens L 21  be a double-concave negative lens, and the aforesaid image-side first positive lens L 22  be a double-convex positive lens. 
   The aforesaid lens arrangement favors correction of various aberrations inclusive of chromatic aberrations by a reduced number of lenses, making surer size reductions and optical performance. 
   It is desired that at least one of the aforesaid image-side first positive lens L 22  or the aforesaid image-side second positive lens L 23  have an aspheric convex surface having positive refracting power that becomes weak from the optical axis to the periphery. 
   The aforesaid arrangement makes it easy to correct undercorrected spherical aberrations, field curvature (that is apt to become minus) and barrel distortion, all likely to occur as the retrofocus type imaging optical system decreases in size, and works more for size reductions and making sure optical performance. 
   It is desired that the object-side surface of the aforesaid image-side second positive lens L 23  have the aforesaid aspheric convex shape. 
   It is desired that the image-side surface of the aforesaid image-side second positive lens L 23  have the aforesaid aspheric convex shape. 
   The centers of both on-axis and off-axis light beams are reasonably away from the image-side second positive lens L 23 . Therefore, if one or both of the object- and image-side surfaces of that image-side second positive lens L 23  are configured into the aforesaid aspheric convex shape, it is then easier to work for correction of aberrations. 
   It is desired for the aforesaid object-side negative meniscus lens L 12  to satisfy the following condition (4).
 
2.4≦( R 12 f+R 12 r )/( R 12 f−R 12 r )≦3.2  (4)
 
where R 12   f  is the paraxial radius of curvature of the object-side lens surface of the aforesaid object-side negative meniscus lens, and
 
   R 12   r  is the paraxial radius of curvature of the image-side lens surface of the aforesaid object-side negative meniscus lens. 
   Preferably, the object-side negative meniscus lens L 12  is configured in such a way as to satisfy condition (4) so as to obtain a sensible tradeoff between making sure the negative refracting power and a balance among aberrations. 
   Condition (4) defines the preferable shape factor of the object-side negative meniscus lens L 12 . 
   By abiding by the lower limit of 2.4 to condition (4), the curvatures of the object- and image-side surfaces of the object-side negative meniscus lens L 12  are so maintained that even when the object-side lens group is located near to the aperture stop, the angles of incidence of both on-axis and off-axis light beams on that negative lens can be kept small, favoring correction of aberrations. 
   By abiding by the upper limit of 3.2 to condition (4), the curvatures of the object- and image-side surfaces of the object-side negative meniscus lens L 12  are prevented from growing too large so that spherical aberrations, coma, and field curvature at the object-side negative meniscus lens L 12  can easily be reduced. 
   It is desired that the aforesaid object-side positive meniscus lens L 11  and the aforesaid object-side negative meniscus lens L 12  be each a single lens; the paraxial radius of curvature of the image-side surface of the aforesaid object-side positive meniscus lens L 11  be larger than the paraxial radius of curvature of the object-side surface of the aforesaid object-side negative meniscus lens L 12 ; and the aforesaid object-side positive meniscus lens L 11  satisfy the following conditions (5) and (6).
 
−5.0≦( R 11 f+R 11 r )/( R 11 f−R 11 r )≦−1.3  (5)
 
0.2 ≦D 1/ L≦ 0.4  (6)
 
where R 11   f  is the paraxial radius of curvature of the object-side lens surface of the aforesaid object-side positive meniscus lens L 11  in the object-side lens group,
 
   R 11   r  is the paraxial radius of curvature of the image-side lens surface of the aforesaid object-side positive meniscus lens L 11  in the object-side lens group, and 
   D 1  is the axial distance of the aforesaid object-side lens group from the vertex of the lens surface located nearest to the object side to the vertex of the lens surface located nearest to the image side. 
   It is preferable that to slim down the lens system, the object-side positive meniscus lens L 11  is brought near to the aperture stop, and to make sure the given back focus, the object-side negative meniscus lens L 12  is spaced away from the image-side lens group. 
   Therefore, in order to offer a reasonable tradeoff between slimming down the object-side lens group itself and the function of correcting various aberrations occurring from the object-side positive meniscus lens L 11 , it is preferable that the object-side positive meniscus lens L 11  and the object-side negative meniscus lens L 12  are each a single lens; the paraxial radius of curvature of the image-side surface of the object-side positive meniscus lens L 11  is larger than the paraxial radius of curvature of the object-side surface of the object-side negative meniscus lens L 12  so that the effect on correction of off-axis light beams can be enhanced; and the aforesaid conditions (5) and (6) are satisfied. 
   Condition (5) defines the preferable shape factor of the object-side positive meniscus lens L 11 , and condition (6) defines the axial thickness of the object-side lens group. 
   By abiding by the lower limit of −5.0 to condition (5), it is easy to make sure the object-side positive meniscus lens L 11  has a given height of incidence of light rays and have a function of canceling out aberrations occurring from the negative lens. 
   By abiding by the upper limit of −1.3 to condition (5), the object-side positive meniscus lens L 11  is allowed to have refracting power on the object-side surface; so the lenses can easily have symmetry with respect to the aperture stop, placing aberrations in a well-balanced state. 
   Abiding by the lower limit of 0.2 to condition (6) makes sure the object-side lens group have the given axial thickness, working for making sure the positive refracting power necessary for correction of aberrations occurring from the object-side positive meniscus lens L 11 , etc. 
   Abiding by the upper limit of 0.4 to condition (6) prevents the object-side lens group from growing too thick so that the object-side negative meniscus lens L 12  can easily be positioned near to the object, working for making sure the given back focus resulting from the retrofocus arrangement. 
   It is also desired to satisfy the following conditions (7) and (8).
 
−2.0 ≦f 12/ f≦− 0.5  (7)
 
−2.3 ≦f 11/ f 12≦−1.4  (8)
 
where f 11  is the focal length of the object-side positive meniscus lens L 11 , and
 
   f 12  is the focal length of the object-side negative meniscus lens L 12 . 
   To achieve slimming-down and well-balanced aberrations while making sure the object-side lens group has negative refracting power, it is preferable to satisfy the aforesaid conditions (7) and (8). 
   Conditions (7) and (8) define the preferable focal lengths of the object-side positive and negative meniscus lenses L 11  and L 12 , respectively. 
   Abiding by the lower limit of −2.0 to condition (7) allows the object-side negative meniscus lens L 12  to have negative refracting power, working for maintaining the effect on keeping the back focus long, correction of Petzval s sum, etc. 
   Abiding by the upper limit of −0.5 to condition (7) allows the object-side negative meniscus lens L 12  to have a reasonable refracting power to ease off a load of the object-side positive meniscus lens L 11  on correction of aberrations, easily preventing the object-side positive meniscus lens L 11  from growing too thick. 
   Abiding by the lower limit of −2.3 to condition (8) helps the object-side positive meniscus lens maintain its refracting power, working for making sure the function of correcting various aberrations. 
   Abiding by the upper limit of −1.4 to condition (8) works for reducing the thickness of the object-side positive meniscus lens L 11 , reducing aberrations occurring from the object-side positive meniscus lens L 11  itself, and making sure the given back focus. 
   It is desired for the aforesaid cemented doublet to satisfy the following conditions (9) and (10).
 
−4.5 ≦R 21 f/D 2122≦−1.7  (9)
 
0.35 ≦R 21 f/R 22 r≦ 0.80  (10)
 
where R 21   f  is the paraxial radius of curvature of the object-side lens surface of the aforesaid image-side negative lens L 21 ,
 
   D 2122  is the axial thickness of the aforesaid cemented doublet from the vertex of the lens surface located nearest to the object side to the vertex of the lens surface located nearest to the image side, and 
   R 22   r  is the paraxial radius of curvature of the image-side lens surface of the aforesaid image-side first positive lens L 22 . 
   By configuring the cemented doublet into a meniscus shape concave on the aperture stop side, the angles of incidence of on-axis and off-axis light beams on the object- and image-side surfaces of the cemented doublet can easily be kept small in a well-balanced state. 
   It is then preferable for the shape of the cemented doublet to satisfy the aforesaid conditions (9) and (10). 
   Abiding by the lower limit of −4.5 to condition (9) allows that concave surface to have a reasonable refracting power, working for correction of spherical aberrations and Petzval s sum and making sure the given back focus. It also makes sure the axial thickness of the cemented doublet, working for correction of astigmatism, chromatic aberrations, etc. 
   Abiding by the upper limit of −1.7 to condition (9) allows the negative refracting power of that concave surface to be prevented from growing too strong, easily holding back higher-order spherical aberrations. 
   Abiding by the lower limit of 0.35 to condition (10) makes sure the image-side surface of the cemented doublet has positive refracting power so that it can easily share positive refracting power with the image-side second positive lens L 23 , facilitating correction of various aberrations. 
   Abiding by the upper limit of 0.80 to condition (10) prevents the positive power of the image-side surface of the cemented doublet from growing too strong, favoring correction of coma, etc. It also works for making sure the quantity of rim light. 
   It is also desired for the aforesaid cemented doublet to satisfy the following conditions (11) and (12).
 
0.40 ≦D 23/ D 2≦0.60  (11)
 
0.50 ≦f 23/ f≦ 0.90  (12)
 
where D 2  is the axial distance of the aforesaid image-side lens group from the vertex of the lens surface located nearest to the object side to the vertex of the lens surface located nearest to the image side,
 
   D 23  is the axial thickness of the aforesaid image-side second positive lens L 23 , and 
   f 23  is the focal length of the aforesaid image-side second positive lens L 23 . 
   Making sure the image-side second positive lens L 23  has the given positive refracting power works for making sure the retrofocus type has the given back focus. 
   Conditions (11) and (12) define the preferable thickness and refracting power of that image-side second positive lens L 23 , respectively. 
   Abiding by the lower limit of 0.40 to condition (11) allows the image-side second positive lens L 23  to make sure the given axial thickness, working for making sure the positive refracting power. 
   Abiding by the upper limit of 0.60 to condition (11) works for reductions in the thickness and diametrical size of the imaging optical system. 
   Abiding by the lower limit of 0.50 to condition (12) allows the image-side second positive lens L 23  to have a reasonable refracting power. This is preferable because coma, etc. can easily be reduced. 
   Abiding by the upper limit of 0.90 to condition (12) makes sure the reasonable positive refracting power. This is preferable because the given back focus is obtained, and the function of spacing the exit pupil away from the image plane is achieved as well. 
   It is desired for the aforesaid image-side second positive lens L 23  to have a convex shape in such a way as to satisfy the following condition (13).
 
0.5&lt;( R 23 f+R 23 r )/( R 23 f−R 23 r )&lt;1.0  (13)
 
where R 23   f  is the paraxial radius of curvature of the object-side lens surface of the aforesaid image-side second positive lens L 23 , and
 
   R 23   r  is the paraxial radius of curvature of the image-side lens surface of the aforesaid image-side second positive lens L 23 . 
   The image-side second positive lens L 23  is apt to have strong positive refracting power; it should preferably have a shape capable of reducing various aberrations. 
   Condition (13) defines the preferable shape factor of that image-side second positive lens L 23 . 
   Abiding by the lower limit of 0.5 to condition (13) allows the angle of incidence of off-axis light beams to be kept small, working for correction of aberrations. It also helps the image-side second positive lens L 23  to decrease in size. 
   Abiding by the upper limit of 1.0 to condition (13) allows the positive refracting power to be shared by both surfaces of the image-side second positive lens L 23 , working for correction of spherical aberrations. 
   Desirously, the imaging apparatus of the invention comprises the aforesaid imaging optical system, an imaging device that is located on the image side of the aforesaid imaging optical system to convert an image formed through the aforesaid imaging optical system into electric signals, and an optical path splitter member that is interposed between the aforesaid imaging optical system and the aforesaid imaging device to split an optical path into an optical path for forming an image for observation and an optical path for forming the image to be taken. 
   The imaging optical system of the invention is easy to make sure the given back focus; so it is preferably used for interchangeable lenses for single-lens reflex cameras or the like. 
   In short, advantage is taken of the long back focus to locate between the imaging optical system and the imaging device the optical path splitter member such as a quick return mirror and a half-silvered mirror (by which the optical path is split in both the time split mode and the light quantity split mode), thereby forming the optical path for forming the image to be taken and the optical path for forming an image for observation. 
   The image for observation may be viewed as follows. An image formed on a focusing screen may be viewed directly or via a viewing optical system. Alternatively, the image for observation may be taken by means of the imaging device to show that image on a display device, which may in turn be viewed directly or via an optical system. 
   Preferably, the aforesaid requirements should be satisfied in combinations of two or more. 
   Of condition (1), the lower and upper limit values should more preferably be set at 2.313 and 5.294, respectively. 
   Of condition (2), the lower and upper limit values should more preferably be set at 0.814 and 0.955, respectively. 
   Of condition (3), the lower and upper limit values should more preferably be set at 0.567 and 0.843, respectively. 
   Of condition (4), the lower and upper limit values should more preferably be set at 2.476 and 3.179, respectively. 
   Of condition (5), the lower and upper limit values should more preferably be set at −4.0, esp. −3.3 and −1.7, esp. −1.9, respectively. 
   Of condition (6), the lower and upper limit values should more preferably be set at 0.22, esp. 0.23 and 0.39, esp. 0.38, respectively. 
   Of condition (7), the lower and upper limit values should more preferably be set at −1.8, esp. −1.7 and −0.6, esp. −0.7, respectively. 
   Of condition (8), the lower and upper limit values should more preferably be set at −2.0, esp. −1.8 and −1.5, esp. −1.55, respectively. 
   Of condition (9), the lower and upper limit values should more preferably be set at −4.1, esp. −3.9 and −1.9, esp. −2.0, respectively. 
   Of condition (10), the lower and upper limit values should more preferably be set at 0.4, esp. 0.42 and 0.7, esp. 0.65, respectively. 
   Of condition (11), the lower and upper limit values should more preferably be set at 0.43, esp. 0.46 and 0.57, esp. 0.53, respectively. 
   Of condition (12), the lower and upper limit values should more preferably be set at 0.55, esp. 0.6 and 0.85, esp. 0.77, respectively. 
   Of condition (13), the lower and upper limit values should more preferably be set at 0.65, esp. 0.75 and 0.99, esp. 0.98, respectively. 
   According to the invention, there can be an imaging optical system provided that works for making sure the given back focus and slimming-down and is capable of making sure the desired brightness and optical performance. There can also be an imaging apparatus provided that comprises such an imaging optical system. 
   Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
   The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is illustrative in lens arrangement section of Example 1 of the inventive imaging optical system. 
       FIG. 2  is illustrative, as in  FIG. 1 , of Example 2 of the inventive imaging optical system. 
       FIG. 3  is illustrative, as in  FIG. 1 , of Example 3 of the inventive imaging optical system. 
       FIG. 4  is an aberration diagram for Example 1 upon focusing on an object point at infinity. 
       FIG. 5  is an aberration diagram for Example 2 upon focusing on an object point at infinity. 
       FIG. 6  is an aberration diagram for Example 3 upon focusing on an object point at infinity. 
       FIG. 7  is a sectional view of a single-lens reflex camera in which the inventive imaging optical system is used as an interchangeable lens. 
   

   DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Examples 1, 2 and 3 of the imaging optical system are now explained. Sectional views of the optical systems of Examples 1, 2 and 3 are presented in  FIGS. 1 ,  2  and  3 . 
   Throughout  FIGS. 1 ,  2  and  3 , G 1  represents the object-side lens group, G 2  the image-side lens group, S the aperture stop, F an optical low-pass filter, and I the image plane of a CCD. Note here that the single plane-parallel plate shown in  FIGS. 1 ,  2  and  3  is indicative of various filters such as a low-pass filter, an infrared cut filter, and an ultrasonic vibration filter adapted to vibrate dusts off. 
   The imaging optical system of Example 1 is built up of, in order from its object side, the object-side lens group G 1  of negative refracting power, the aperture stop S and the image-side lens group G 2  of positive refracting-power, as shown in  FIG. 1 . 
   The object-side lens group G 1  is made up of, in order from its object side, a positive meniscus lens that is convex on its object side and acts as the object-side positive meniscus lens L 11  and a negative meniscus lens that is convex on its object side and acts as the object-side negative meniscus lens L 12 , and the image-side lens group G 2  is made up of a cemented doublet of a double-concave negative lens acting as the image-side negative lens L 21  and a double-convex positive lens acting as the image-side first positive lens L 22 , and a double-convex positive lens acting as the image-side second positive lens L 23 . 
   Two aspheric surfaces are used at both surfaces of the double-convex positive lens in the image-side lens group G 2 . 
   The imaging optical system of Example 2 is built up of, in order from its object side, the object-side lens group G 1  of negative refracting power, the aperture stop S and the image-side lens group G 2  of positive refracting power, as shown in  FIG. 2 . 
   The object-side lens group G 1  is made up of, in order from its object side, a positive meniscus lens that is convex on its object side and acts as the object-side positive meniscus lens L 11  and a negative meniscus lens that is convex on its object side and acts as the object-side negative meniscus lens L 12 , and the image-side lens group G 2  is made up of a cemented doublet of a double-concave negative lens acting as the image-side negative lens L 21  and a double-convex positive lens acting as the image-side first positive lens L 22 , and a double-convex positive lens acting as the image-side second positive lens L 23 . 
   Two aspheric surfaces are used at both surfaces of the double-convex positive lens in the image-side lens group G 2 . 
   The imaging optical system of Example 3 is built up of, in order from its object side, the object-side lens group G 1  of negative refracting power, the aperture stop S and the image-side lens group G 2  of positive refracting power, as shown in  FIG. 3 . 
   The object-side lens group G 1  is made up of, in order from its object side, a positive meniscus lens that is convex on its object side and acts as the object-side positive meniscus lens L 11  and a negative meniscus lens that is convex on its object side and acts as the object-side negative meniscus lens L 12 , and the image-side lens group G 2  is made up of a cemented doublet of a double-concave negative lens acting as the image-side negative lens L 21  and a double-convex positive lens acting as the image-side first positive lens L 22 , and a double-convex positive lens acting as the image-side second positive lens L 23 . 
   Two aspheric surfaces are used at both surfaces of the double-convex positive lens in the image-side lens group G 2 . 
   Enumerated below are numerical data in the respective examples. 
   Referring to the numerical data on the lenses in the respective examples, r is the radius of curvature of each lens surface, d is the thickness or spacing of each lens, nd is the d-line refracting index of each lens, υd is the d-line Abbe constant of each lens, K is the conic coefficient, A 4 , A 6 , A 8  and A 10  are the aspheric coefficients, and E±N is ×10 ±N . 
   Each aspheric configuration is given by the following equation, using each aspheric coefficient in each example.
 
 Z= ( Y   2   /r )/[1+{1−(1+ K )·( Y/r ) 2 } 1/2   ]+A 4 ×Y   4   +A 6 ×Y   6   +A 8 ×Y   8   +A 10 ×Y   10  
 
where Z is the coordinates in the optical axis direction, and Y is the coordinates in the vertical direction to the optical axis.
 
   Numerical Example 1 
   
     
       
             
           
             
             
             
             
             
             
           
             
           
             
           
             
           
             
             
             
           
             
           
             
             
             
             
           
         
             
                 
             
             
               Unit mm 
             
             
                 
             
           
           
             
               Surface data 
             
           
        
         
             
                 
               Surface number 
               r 
               d 
               nd 
               νd 
             
             
                 
                 
             
             
                 
                1 
               17.064 
               2.80 
               1.80610 
               33.27 
             
             
                 
                2 
               36.465 
               0.13 
             
             
                 
                3 
               16.356 
               0.95 
               1.69680 
               55.53 
             
             
                 
                4 
               7.966 
               5.57 
             
             
                 
                5 (Stop) 
               ∞ 
               7.70 
             
             
                 
                6 
               −10.584 
               0.83 
               1.75520 
               27.51 
             
             
                 
                7 
               84.666 
               3.36 
               1.69680 
               55.53 
             
             
                 
                8 
               −16.611 
               0.15 
             
             
                 
                9 (Aspheric) 
               670.610 
               4.20 
               1.74250 
               49.30 
             
             
                 
               10 (Aspheric) 
               −13.770 
               29.75 
             
             
                 
               11 
               ∞ 
               4.57 
               1.51633 
               64.14 
             
             
                 
               12 
               ∞ 
               1.06 
             
             
                 
               Image plane 
               ∞ 
             
             
                 
                 
             
           
        
         
             
               Aspheric data 
             
             
                 
             
           
        
         
             
               9th Surface 
             
             
               K = −999.0134, A4 = −1.7520E−05, A6 = 5.9396E−08, 
             
             
               A8 = −1.9307E−09, A10 = 1.1985E−12 
             
             
               10th Surface 
             
             
               K = 0.4098, A4 = 3.1377E−06, A6 = 2.1734E−08, A8 = −5.7979E−10, 
             
             
               A10 = −1.2876E−11 
             
             
                 
             
           
        
         
             
               Various data 
             
             
                 
             
           
        
         
             
                 
               Focal length 
               25.38 
             
             
                 
               F-number 
               2.9 
             
             
                 
               Half angle of view 
               24.9 
             
             
                 
               Image height 
               11.150 
             
             
                 
               Full lens length 
               57.720 
             
             
                 
               BF 
               32.034 
             
             
                 
                 
             
           
        
         
             
               Lens data 
             
           
        
         
             
                 
               Lens 
               Start surface 
               Focal length 
             
             
                 
                 
             
             
                 
               L11 
               1 
               37.380 
             
             
                 
               L12 
               3 
               −23.373 
             
             
                 
               L21 
               6 
               −12.411 
             
             
                 
               L22 
               7 
               20.204 
             
             
                 
               L23 
               9 
               18.220 
             
             
                 
                 
             
           
        
       
     
   
   Numerical Example 2 
   
     
       
             
           
             
             
             
             
             
             
           
             
           
             
           
             
           
             
             
             
           
             
           
             
             
             
             
           
         
             
                 
             
             
               Unit mm 
             
             
                 
             
           
           
             
               Surface data 
             
           
        
         
             
                 
               Surface number 
               r 
               d 
               nd 
               νd 
             
             
                 
                 
             
             
                 
                1 
               14.179 
               2.80 
               1.80610 
               33.27 
             
             
                 
                2 
               27.380 
               0.13 
             
             
                 
                3 
               12.478 
               0.95 
               1.78650 
               50.00 
             
             
                 
                4 
               6.504 
               4.23 
             
             
                 
                5 (Stop) 
               ∞ 
               6.37 
             
             
                 
                6 
               −15.597 
               0.83 
               1.76182 
               26.52 
             
             
                 
                7 
               27.085 
               3.36 
               1.69680 
               55.53 
             
             
                 
                8 
               −36.000 
               0.15 
             
             
                 
                9 (Aspheric) 
               356.666 
               4.20 
               1.74250 
               49.30 
             
             
                 
               10 (Aspheric) 
               −11.783 
               29.75 
             
             
                 
               11 
               ∞ 
               4.57 
               1.51633 
               64.14 
             
             
                 
               12 
               ∞ 
               1.06 
             
             
                 
               Image plane 
               ∞ 
             
             
                 
                 
             
           
        
         
             
               Aspheric data 
             
             
                 
             
           
        
         
             
               9th Surface 
             
             
               K = 233.0929, A4 = −2.9412E−05, A6 = −2.3456E−07, 
             
             
               A8 = 7.1848E−09, A10 = −8.4208E−11 
             
             
               10th Surface 
             
             
               K = 0.3428, A4 = 2.0611E−06, A6 = −2.4195E−07, A8 = 3.9420E−09, 
             
             
               A10 = −5.3603E−11 
             
             
                 
             
           
        
         
             
               Various data 
             
             
                 
             
           
        
         
             
                 
               Focal length 
               25.50 
             
             
                 
               F-number 
               2.9 
             
             
                 
               Half angle of view 
               24.8 
             
             
                 
               Image height 
               11.150 
             
             
                 
               Full lens length 
               55.047 
             
             
                 
               BF 
               32.033 
             
             
                 
                 
             
           
        
         
             
               Lens data 
             
           
        
         
             
                 
               Lens 
               Start surface 
               Focal length 
             
             
                 
                 
             
             
                 
               L11 
               1 
               33.326 
             
             
                 
               L12 
               3 
               −18.575 
             
             
                 
               L21 
               6 
               −12.884 
             
             
                 
               L22 
               7 
               22.678 
             
             
                 
               L23 
               9 
               15.437 
             
             
                 
                 
             
           
        
       
     
   
   Numerical Example 3 
   
     
       
             
           
             
             
             
             
             
           
             
           
             
           
             
           
             
             
             
           
             
           
             
             
             
             
           
         
             
                 
             
             
               Unit mm 
             
             
                 
             
           
           
             
               Surface data 
             
           
        
         
             
               Surface number 
               r 
               d 
               nd 
               νd 
             
             
                 
             
             
                1 
               31.235 
               2.80 
               1.74000 
               28.30 
             
             
                2 
               91.658 
               0.13 
             
             
                3 
               25.747 
               0.95 
               1.48749 
               70.23 
             
             
                4 
               10.936 
               9.82 
             
             
                5 (Stop) 
               ∞ 
               6.92 
             
             
                6 
               −8.559 
               0.83 
               1.68893 
               31.07 
             
             
                7 
               500.006 
               3.36 
               1.60300 
               65.44 
             
             
                8 
               −13.578 
               0.15 
             
             
                9 (Aspheric) 
               123.829 
               4.20 
               1.67790 
               55.34 
             
             
               10 (Aspheric) 
               −13.569 
               29.88 
             
             
               11 
               ∞ 
               4.57 
               1.51633 
               64.14 
             
             
               12 
               ∞ 
               1.06 
             
             
               Image plane 
               ∞ 
             
             
                 
             
           
        
         
             
               Aspheric data 
             
             
                 
             
           
        
         
             
               9th Surface 
             
             
               K = −80.9211, A4 = −1.8642E−05, A6 = 1.3284E−07, 
             
             
               A8 = −3.2044E−09, A10 = −1.5415E−11 
             
             
               10th Surface 
             
             
               K = −0.4375, A4 = 5.0738E−06, A6 = 4.0676E−08, A8 = 4.5114E−10, 
             
             
               A10 = −4.7840E−11 
             
             
                 
             
           
        
         
             
               Various data 
             
             
                 
             
           
        
         
             
                 
               Focal length 
               24.49 
             
             
                 
               F-number 
               2.9 
             
             
                 
               Half angle of view 
               25.7 
             
             
                 
               Image height 
               11.150 
             
             
                 
               Full lens length 
               61.425 
             
             
                 
               BF 
               32.257 
             
             
                 
                 
             
           
        
         
             
               Lens data 
             
           
        
         
             
                 
               Lens 
               Start surface 
               Focal length 
             
             
                 
                 
             
             
                 
               L11 
               1 
               62.791 
             
             
                 
               L12 
               3 
               −39.838 
             
             
                 
               L21 
               6 
               −12.207 
             
             
                 
               L22 
               7 
               21.976 
             
             
                 
               L23 
               9 
               18.265 
             
             
                 
                 
             
           
        
       
     
   
   Aberration diagrams for Examples 1, 2 and 3 upon focusing on an object point at infinity are presented in the form of  FIGS. 4 ,  5  and  6 . In the diagrams here for spherical aberrations and chromatic aberrations of magnification, the solid line, broken line and one-dotted line are indicative of the quantities (mm) of aberrations with respect to the C-line, d-line and F-line, respectively; in the diagrams for astigmatisms here, the solid line S and broken line M are indicative of the sagittal image plane (mm) and the meridional image plane (mm), respectively, and in the diagrams for distortions, the solid line is indicative of distortion (%) with respect to the d-line. 
   The aberration diagrams are drawn with an image circle larger than the aforesaid image height. The technique of shifting the imaging device in the vertical plane to the optical axis of the imaging optical system to reduce influences of shake of the imaging apparatus goes mainstream; however, the optical system of the invention is designed to achieve good image quality even when such shake is corrected. 
   Tabulated below are the values of conditions (1) to (13) in each of the above examples. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Condition 
               Example 1 
               Example 2 
               Example 3 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               (1) 
               3.444 
               2.314 
               5.293 
             
             
                 
               (2) 
               0.876 
               0.815 
               0.954 
             
             
                 
               (3) 
               0.675 
               0.568 
               0.842 
             
             
                 
               (4) 
               2.899 
               3.178 
               2.477 
             
             
                 
               (5) 
               −2.759 
               −3.148 
               −2.034 
             
             
                 
               (6) 
               0.292 
               0.366 
               0.232 
             
             
                 
               (7) 
               −0.921 
               −0.728 
               −1.627 
             
             
                 
               (8) 
               −1.599 
               −1.794 
               −1.576 
             
             
                 
               (9) 
               −2.526 
               −3.723 
               −2.043 
             
             
                 
               (10)  
               0.637 
               0.433 
               0.630 
             
             
                 
               (11)  
               0.492 
               0.492 
               0.492 
             
             
                 
               (12)  
               0.718 
               0.605 
               0.746 
             
             
                 
               (13)  
               0.960 
               0.936 
               0.802 
             
             
                 
                 
             
           
        
       
     
   
   In each example, focusing from far distances to near distances may be implemented by various ways, for instance, by letting out the imaging optical system on its entirety, letting out the imaging optical system while changing the spacing between the object-side lens group and the image-side lens group or letting out the image-side lens group alone. 
     FIG. 7  is a sectional view of the single-lens reflex camera that operates as an electronic imaging apparatus wherein the inventive imaging optical system is used and a small-format CCD or C-MOS or the like is used as the imaging device. In  FIG. 7 , reference numeral  1  is the single-lens reflex camera,  2  the imaging optical system mounted in a lens barrel and  3  a mount portion of the lens barrel that enables the imaging optical system  2  to be attached to or detached from the single-lens reflex camera  1 . For that mount, a screw type mount, a bayonet type mount or the like may be used. In this example, the bayonet type mount is used. 
   Reference numeral  4  is the imaging device surface,  5  is a quick return mirror interposed between the lens system and the imaging device surface  4  on an optical path  6  through the imaging lens system  2 ,  7  a finder screen located in an optical path taken by light reflected off the quick return mirror  5 ,  8  a penta prism,  9  a finder, and E the eye of the viewer (eye point). 
   The inventive imaging optical system exemplified by Example 1, 2, and 3, for instance, may be used as the imaging optical system  2  of the single-lens reflex camera  2  arranged as mentioned above.