# Patent

## Title
Teleconverter, and imaging system comprising the same

## Abstract

A teleconverter, comprising:

    
    
        a master lens apparatus-side mount on which a master lens apparatus is mounted,
        a camera body-side mount on which a camera body is mounted, and
        a converter lens unit that has a generally negative refracting power for mounting the master lens apparatus thereon to obtain a lens system having a focal length longer than that of the master lens apparatus, wherein:
        the converter lens unit comprises a first lens group on the master lens apparatus side and a second lens group on the camera body side with an on-axis longest air separation interposed there-between, and does not include any lens group other than the first lens group and the second lens group,
        the first lens group has positive refracting power,
        the second lens group has negative refracting power,
        the first lens group consists of a first lens element having positive refracting power, and
        the second lens group comprises, in order from an object side to an image side along an optical path, a second lens element having negative refracting power and adjacent to the first lens element, a third lens element having positive refracting power and adjacent to the second lens element, and a fourth lens element having negative refracting power and adjacent to the third lens element, with satisfaction of the following condition (3):

0.04<d12/D<0.3  (3)

where d12 is an air separation between the first lens group and the second lens group, and

        D is an on-axis thickness of the converter lens unit.

## Background
This is a divisional application of U.S. application Ser. No. 14/673,194, filed on Mar. 30, 2015, which is a divisional application of U.S. application Ser. No. 13/892,253, filed on May 11, 2013, now U.S. Pat. No. 9,075,291, granted on Jul. 7, 2015, and claims the benefit of Japanese Application No. 2012-109239 filed in Japan on May 11, 2012, the contents of which are incorporated by this reference.


BACKGROUND OF THE INVENTION
The present invention relates to a tele-converter that is mounted on a master lens apparatus to obtain a lens system having a focal length longer than that of the master lens apparatus, and an imaging system comprising the same.
So far, there has been a rear teleconverter known in the art that comprises mounts corresponding to the mount of a master lens apparatus and the mount of a camera body, wherein a converter lens is interposed between the master lens apparatus and the camera body via those mounts to extend or elongate the focal length of the whole lens system.
Such rear teleconverters are convenient to and preferably used by those who use single-lens reflex cameras with interchangeable lenses in general and those who carry out telephotography, because of capable of making the focal length longer simply. Such rear teleconverter lenses, for instance, are set forth in Patent Publications 1 to 6.
Patent Publication 1: JP(A) 2004-226648
Patent Publication 2: JP(A) 2005-043788
Patent Publication 3: JP(A) 2005-107261
Patent Publication 4: JP(A) 2009-080176
Patent Publication 5: JP(A) 2010-191211
Patent Publication 6: JP(A) 2011-081111
SUMMARY OF THE INVENTION
In one embodiment, the invention provides a teleconverter comprising:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit that has a generally negative refracting power for mounting said master lens apparatus thereon to obtain a lens system having a focal length longer than that of said master lens apparatus, wherein:
said converter lens unit comprises a first lens group on said master lens apparatus side and a second lens group on said camera body side with an on-axis longest air separation interposed there-between,
said first lens group has positive refracting power, and
said second lens group has negative refracting power, with satisfaction of the following condition (1):

−1.53<f1/f<−0.66  (1)

where f is a focal length of a whole system of said converter lens unit, and

f1 is a focal length of said first lens group.
In one embodiment, the invention provides a teleconverter comprising:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit that has a generally negative refracting power for mounting said master lens apparatus thereon to obtain a lens system having a focal length longer than that of said master lens apparatus, wherein:
said converter lens unit satisfies the following condition (6):

1.1<D/fb<3.0  (6)

where D is an on-axis thickness of said converter lens unit, and

fb is an on-axis distance in air from an exit surface of said converter lens unit to an image plane when said teleconverter lens is attached to said master lens apparatus.
In one embodiment, the invention provides a teleconverter comprising:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit which has a generally negative refracting power for mounting said master lens apparatus thereon to obtain a lens system having a focal length longer than that of said master lens apparatus, wherein:
said converter lens unit comprises a first lens group on said master lens apparatus side and a second lens group on said camera body side with an on-axis longest air separation interposed there-between,
said first lens group has positive refracting power,
said second lens group has negative refracting power,
said converter lens unit consists of five lenses: in order along an optical path, a first positive lens, a first negative lens, a second positive lens, a second negative lens, and a third positive lens,
said first lens group consists of one lens: said first positive lens, and
said second lens consists of four lenses: said first negative lens, said second positive lens, said second negative lens, and said third positive lens.
In one embodiment, the invention provides an imaging system, comprising:
a master lens apparatus,
a camera body, and
a teleconverter attachable between said master lens apparatus and said camera body, wherein:
said camera body includes an imaging device that is located in a position of an image formed by a combined optical system of said master lens apparatus and said teleconveter, and operable to convert the image formed by said combined optical system into electric signals.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent form 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. 1A is a sectional view of the master lens at the wide-angle end, as taken apart along the optical axis.
FIG. 1B is a sectional view of the master lens at the telephoto end, as taken apart along the optical axis.
FIG. 2 is a sectional view of the converter lens of Example 1 as taken apart along the optical axis.
FIG. 3 is a sectional view of the converter lens of Example 2 as taken apart along the optical axis.
FIG. 4 is a sectional view of the converter lens of Example 3 as taken apart along the optical axis.
FIG. 5 is a sectional view of the converter lens of Example 4 as taken apart along the optical axis.
FIG. 6 is a sectional view of the converter lens of Example 5 as taken apart along the optical axis.
FIGS. 7A-7D show a set of aberration diagrams for the
master lens at the wide-angle end.
FIGS. 7E-7H show a set of aberration diagrams for the master lens at the telephoto end.
FIGS. 8A-8D show a set of aberration diagrams for the
converter lens of Example 1 at the wide-angle end.
FIGS. 8E-8H show a set of aberration diagrams for the
converter lens of Example 1 at the telephoto end.
FIGS. 9A-9D show a set of aberration diagrams for the
converter lens of Example 2 at the wide-angle end.
FIGS. 9E-9H show a set of aberration diagrams for the
converter lens of Example 2 at the telephoto end.
FIGS. 10A-10D show a set of aberration diagrams for the converter lens of Example 3 at the wide-angle end.
FIGS. 10E-10H show a set of aberration diagrams for the
converter lens of Example 3 at the telephoto end.
FIGS. 11A-11D show a set of aberration diagrams for the converter lens of Example 4 at the wide-angle end.
FIGS. 11E-11H show a set of aberration diagrams for the
converter lens of Example 4 at the telephoto end.
FIGS. 12A-12D show a set of aberration diagrams for the converter lens of Example 5 at the wide-angle end.
FIGS. 12E-12H show a set of aberration diagrams for the
converter lens of Example 5 at the telephoto end.
FIG. 13 is a sectional view of one embodiment of the imaging system wherein a converter lens/master lens combination is used for a taking lens.



DESCRIPTION OF EXEMPLARY EMBODIMENTS
In recent years, lens-interchangeable camera systems that dispense with any quick-return mirror and make their flange back short have acquired wide popularity because of their small-format, low-profile layout. However, the teleconverters disclosed in Patent Publications 1-6 are tailored to conventional single-lens reflex cameras, laying its base on long flange backs, so they are not compatible with small-format, low-profile new camera systems. The teleconverters of Patent Publications 1-6 all involve much more lenses, adding up to cost.
According to the first aspect of the invention, there is a teleconverter provided that may also be applied to a new camera system having a short flange back to make the focal length of its master lens long in a simple manner. According to the second aspect of the invention, there is a teleconverter provided that works more in favor of cost reductions. According to a further aspect of the invention, there is an imaging system provided that comprises such a teleconverter.
The inventive teleconverter is embodied as follows.
The inventive teleconverter typically comprises:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit which has a generally negative refracting power for mounting said master lens apparatus thereon to obtain a lens system having a focal length longer than that of said master lens apparatus, wherein:
said converter lens unit comprises a first lens group on said master lens apparatus side and a second lens group on said camera body side with an on-axis longest air separation interposed there-between,
said first lens group has positive refracting power, and
said second lens group has negative refracting power, preferably with satisfaction of the following condition (1):

1.53<f1/f<−0.66  (1)

where f is the focal length of said whole converter lens system, and

f1 is the focal length of said first lens group.
The converter lens unit used in the inventive teleconverter is made up of the first lens group and the second lens group with the on-axis longest air separation interposed between them. There is then a power profile such that the first lens group has positive refracting power and the second lens group has negative refracting power so that the converter lens unit itself is of the telephoto type of positive refracting power and negative refracting power. Such a power profile enables the principal point positions to be spaced more away from the center of the converter lens unit. In addition, it makes sure the converter lens unit has a certain magnification while the teleconverter is located within a small space between the master lens apparatus and the camera body.
Condition (1) defines the preferable focal length of the first lens group.
By keeping the focal length of the first lens group against exceeding the upper limit to Condition (1) thereby keeping the positive refracting power of the first lens group moderate, the negative refracting power of the second lens group may be kept small too, for facilitation of aberration correction. This may also lead to a decrease in the lenses count, working in favor of size and cost reductions of the teleconverter.
By keeping the focal length of the first lens group against running short of the lower limit to Condition (1) thereby making sure the first lens group has sufficient positive power, the effect of the telephoto type may be enhanced so that the teleconverter is more advantageously be used with camera bodies having a short flange back. This may also work more in favor of size reductions of the whole imaging system.
Preferably, the inventive teleconverter should satisfy the following Condition (2):

−3.0<f/(B×D)<−1.2  (2)

where B is the magnification of said converter lens unit, and

D is the on-axis thickness of said converter lens unit.
To be well compatible with the camera body having a short flange back, it is preferable to increase the negative refracting power of the whole system that defines the converter lens unit. To take hold of magnification while reducing any increase in the lenses count, on the other hand, it is preferable to impose some moderate limitation on the negative refracting power of the converter lens unit thereby achieving full correction of aberrations with just only a limited space. Condition (2) defines the preferable whole focal length of the converter lens unit; it is preferable to satisfy Condition (2) for the magnification and thickness of the converter lens unit.
As the negative refracting power is kept moderately weak in such a way as not to exceed the upper limit to Condition (2), it may work more in favor of aberration reductions upon the attachment of the converter.
As the moderate negative refracting power is ensured in such a way as not to fall short of the lower limit to Condition (2), it facilitates taking hold of the desired magnification.
The inventive teleconverter should also preferably satisfy the following Condition (3):

0.04<d12/D<0.3  (3)

where d12 is an air separation between said first lens group and said second lens group, and

D is the on-axis thickness of said converter lens unit.
Making sure the air separation between the first lens group and the second lens group may work in favor of correction of field curvatures in general and sagittal field curvature in particular. To this end, it is preferable to satisfy Condition (3).
Not exceeding the upper limit to Condition (3) may work more in favor of size reductions because of reducing full length increases.
Not running short of the lower limit to Condition (3) is preferable for making sure the effect on aberration correction.
Further, the aforesaid converter lens unit should preferably consist of five lenses: in order along the optical path, the first positive lens, the first negative lens, the second positive lens, the second negative lens, and the third positive lens, wherein:
the aforesaid first lens group consists of one lens: said first positive lens, and
said second lens group consists of four lenses: said first negative lens, said second positive lens, said second negative lens, and said third positive lens.
Such a symmetrical layout having a limited lenses count may work more in favor of reductions in various aberrations. In the case of the teleconverter in particular, it is preferable for the second lens group to have strong negative refracting power. For this reason, it is preferable to use two negative lenses and two positive lenses thereby making sure the negative power and, at the same time, aberration reductions.
It is to be noted that as could be seen from the examples given later, if three lenses: the first negative lens, the second positive lens and the second negative lens are cemented to one another, it is then possible to reduce the influences of decentration between them. Alternatively, one of the negative lenses may be cemented to the second positive lens alone.
For correction of coma, the third positive lens should preferably be made up of a single lens. Especially if the third positive lens is in a double-convex shape, it may work in favor of correction of coma.
Preferably, two negative lenses in the second lens group should each be a double-concave lens, and two positive lenses in the second lens group should each be a double-convex lens. This may work much more in favor of making sure the second lens group has negative refracting power, and making sure the positive lenses have a function of canceling various aberrations.
Still further, the inventive teleconverter should preferably satisfy the following Conditions (4) and (5):

15<νd1<35  (4)

50<νd5<100  (5)

where νd1 is the Abbe constant of said first positive lens, and

νd5 is the Abbe constant of said third positive lens.
Both Conditions (4) and (5) are preferably provided for good correction of chromatic aberrations.
Not exceeding the upper limit to Condition (4) may work in favor of cancelation of chromatic aberrations likely to occur at the second lens group of negative refracting power, because the dispersion of the first lens group is enhanced. In addition, this may facilitate making the refractive index of the first lens group high, working in favor of the telephoto layout effect and correction of spherical aberrations.
Running short of the lower limit to Condition (4) is preferable for cutting down the material cost of the first positive lens.
To make the first positive lens satisfy Condition (4), it is preferable for the Abbe constant of the third positive lens not to fall short of the lower limit to Condition (5), because the influence of the third positive lens on chromatic aberration of magnification is easily mitigated.
Not exceeding the upper limit to Condition (5) is preferable for cutting down the material cost of the third positive lens.
The inventive teleconverter comprises:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit which has a generally negative refracting power for mounting said master lens apparatus thereon to obtain a lens system having a focal length longer than that of said master lens apparatus, wherein:
said converter lens unit preferably satisfies the following condition (6):

1.1<D/fb<3.0  (6)

where D is the on-axis thickness of said converter lens unit, and

fb is the on-axis distance in air from the exit surface of said converter lens unit to the image plane when said teleconverter lens is attached to said master lens apparatus.
When it comes to the imaging system in which the camera body has a short flange back, the teleconverter is going to be located in a narrow space. To make sure the desired magnification and implement full correction of aberrations on that presupposition, it is preferable to satisfy Condition (6).
Not exceeding the upper limit to Condition (6) is preferable for slimming down the teleconverter and making sure the moderate magnification.
Not running short of the lower limit to Condition (6) is preferable for taking hold of the space for receiving lenses, and making it easy to achieve aberration correction capability.
Preferably, the inventive teleconverter comprises:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit which has a generally negative refracting power for mounting said master lens apparatus thereon to obtain a lens system having a focal length longer than that of said master lens apparatus, wherein:
said converter lens unit comprises a first lens group on said master lens apparatus side and a second lens group on said camera body side with the on-axis longest air separation interposed there-between,
said first lens group has positive refracting power,
said second lens group has negative refracting power,
said converter lens unit consists of five lenses: in order along the optical path, a first positive lens, a first negative lens, a second positive lens, a second negative lens, and a third positive lens,
said first lens group consists of one lens: said first positive lens, and
said second lens consists of four lenses: said first negative lens, said second positive lens, said second negative lens, and said third positive lens.
Such arrangement corresponds to an embodiment of the second aspect of the invention. Such a symmetrical layout having a lower lenses count may work in favor of reductions in various aberrations. In the case of the teleconverter in particular, it is preferable for the second lens group to have strong negative refracting power. To this end, it is preferable to use two negative lenses and two positive lenses to make sure the negative refracting power and aberration reductions.
The inventive imaging system preferably comprises:
a master lens apparatus,
a camera body,
a teleconverter attachable between said master lens apparatus and said camera body, wherein:
said teleconverter is the inventive one,
said camera body is located in a position of an image formed by a combined optical system of said master lens apparatus and said teleconverter, and includes an imaging device for converting the image formed by said combined optical system into electric signals.
Preferably, the inventive imaging system should satisfy the following Condition (7):

0.6<fb/IH<1.6  (7)

where fb is the on-axis distance in air from the exit surface of said converter lens unit to the image plane when said teleconverter lens is attached to said master lens apparatus, and

IH is the maximum image height of said imaging system.
When the teleconverter is attached to the camera body too, the proper BF value also depends on the size of the imaging plane, corresponding to the flange back length. In particular, it is preferable to provide proper determination of the angle of incidence of off-axis light beams on the imaging device.
As the space of the converter lens unit for receiving lenses is ensured in such a way as not to exceed the upper limit to Condition (7), it may work in favor of making sure the telephoto layout effect while keeping the full length of the imaging system short.
Not running short of the lower limit to Condition (7) may work in favor of making the angle of incidence on the imaging plane small (spacing the exit pupil away from the imaging plane) while keeping the diameter of the exit surface of the rear converter small.
More preferably, the requirements for the respective embodiments of the invention should be satisfied mutually at the same time.
More preferably, the upper and lower limit values of each condition should be limited as follows, because it could work much more.
Of Condition (1), the lower limit value should more preferably be set at −1.51, and the upper limit value should more preferably be set at −0.75, and especially −0.80.
Of Condition (2), the lower and upper limit values should more preferably be set at −2.8 and −1.70, respectively.
Of Condition (3), the lower and upper limit values should more preferably be set at 0.10 and 0.25, respectively.
Of Condition (4), the lower and upper limit values should more preferably be set at 18 and 32, respectively.
Of Condition (5), the lower and upper limit values should more preferably be set at 60 and 85, respectively.
Of Condition (6), the lower and upper limit values should more preferably be set at 1.3 and 2.5, respectively.
Of Condition (7), the lower and upper limit values should more preferably be set at 0.9 and 1.4, respectively.
According to the present invention, the tele-converter capable of making the focal length of a master lens longer in a simple manner could be provided for use even on a new camera system having a short flange back. Also, the teleconverter working in favor of cost reductions could be provided. Further, the invention could provide an imaging system incorporating such a teleconverter.
The teleconverters according to some embodiments of the invention will now be explained with reference to the accompanying drawings.
Reference will first be made to the master lens MSL used with the inventive teleconverter.
FIG. 1 is a sectional view of the master lens MSL. More specifically, FIGS. 1A and 1B are sectional views of the master lens MSL at the wide-angle end and the telephoto end, respectively.
The master lens MSL is a telephoto zoom lens having a focal length variable between about 40 mm and about 150 mm and an Fno of about 2.8.
As shown in FIG. 1, the master lens MSL is made up of, in order from the object side to the image side, the first lens group G1 of positive refracting power, the second lens group G2 of negative refracting power, the third lens group G3 of positive refracting power, the fourth lens group G4 of negative refracting power, and the fifth lens group G5 of positive refracting power. In FIG. 1, S, MI, C, and I stand for an aperture stop, an image side-mount surface, a cover glass, and an image plane, respectively.
The first lens group G1 is made up of, in order from the object side to the image side, a cemented lens SU11 of a negative meniscus lens L11 convex on its object side and a double-convex positive lens L12, and a positive meniscus lens L13 convex on its object side.
The second lens group G2 is made up of, in order from the object side to the image side, a negative meniscus lens L21 convex on its object side, a cemented lens SU21 of a double-concave negative lens L22 and a positive meniscus lens L23 convex on its object side, and a negative meniscus lens L24 convex on its image side.
The third lens group G3 is made up of, in order from the object side to the image side, the 3ath lens group G3a and the 3bth lens group G3b. The 3ath lens group G3a is made up of, in order from the object side to the image side, a double-convex positive lens L3a1, a double-convex positive lens L3a2, and a double-concave negative lens L3a3, and the 3bth lens group G3b is made up of, in order from the object side to the image side, a positive meniscus lens L3b1 convex on its object side, a positive meniscus lens L3b2 convex on its object side, a negative meniscus lens L3b3 convex on its object side, and a double-convex positive lens L3b4.
There is the aperture stop S interposed between the 3ath lens group G3a and the 3bth lens group G3b in the third lens group G3.
The fourth lens group G4 is made up of one double-concave negative lens L41.
The fifth lens group G5 is made up of a double-concave negative lens L51, and a double-convex positive lens L52.
Reference will then be made to how the respective lens groups in the master lens MSL move upon zooming from the wide-angle end to the telephoto end.
During zooming, the first lens group G1, the second lens group G2, the third lens group G3, the fourth lens group G4, and the fifth lens group G5 move independently.
From the wide-angle end to the telephoto end, the first lens group G1 moves just only toward the object side with an increasing spacing between it and the second lens group G2.
From the wide-angle end to the telephoto end, the second lens group G2 moves toward the object side with an increasing spacing between the first lens group G1 and it and a decreasing spacing between it and the third lens group G3. The second lens group G2 is positioned more on the object side at the telephoto end than at the wide-angle end.
From the wide-angle end to the telephoto end, the third lens group G3 moves together with the aperture stop S with a decreasing spacing between the second lens group G2 and it and an increasing spacing between it and the fourth lens group G4. The third lens group G3 is positioned more on the object side at the telephoto end than at the wide-angle end.
From the wide-angle end to the telephoto end, the fourth lens group G4 moves toward the object side with an increasing spacing between the third lens group G3 and it and an increasing spacing between it and the fifth lens group G5. The fourth lens group G4 is positioned more on the object side at the telephoto end than at the wide-angle end.
From the wide-angle end to the telephoto end, the fifth lens group G5 moves toward the image side with an increasing spacing between the fourth lens group G4 and it. The firth lens group G5 is positioned more on the image side at the telephoto end than at the wide-angle end.
Four aspheric surfaces are used: two at both surfaces r20 and r21 of the object-side positive meniscus lens L3b1 in the 3bth lens group G3b, and two at both surfaces r28 and r29 of the double-concave negative lens L41 forming the fourth lens group G4.
The converter lenses TCL in Examples 1 to 5 of the invention will now be explained with reference to FIGS. 2 to 6. More specifically, FIGS. 2 to 6 are sectional views of the converter lenses TCL of Examples 1 to 5 as taken apart along the optical axis.
FIG. 2 is a sectional view of the converter lens TCL of Example 1.
As shown in FIG. 2, the converter lens TCL of Example 1 is made up of, in order from the object side to the image side, the first lens group G1 of positive refracting power, and the second lens group G2 of negative refracting power. In FIG. 2, Mo, MI, C, and I stand for an object side-mount surface, an image side-mount surface, a cover glass, and an image plane, respectively.
The first lens group G1 is made up of, in order from the object side to the image side, the first positive lens or one positive meniscus lens CL11 convex on its image side.
The second lens group G2 is made up of, in order from the object side to the image side, the first negative lens or a double-concave negative lens CL21, a cemented lens SU21 of the second positive lens or a double-convex positive lens CL22 and the second negative lens or a double-concave negative lens CL23, and the third positive lens or a double-convex positive lens CL24.
FIG. 3 is a sectional view of the converter lens TCL of Example 2.
As shown in FIG. 3, the converter lens TCL of Example 2 is made up of, in order from the object side to the image side, the first lens group G1 of positive refracting power, and the second lens group G2 of negative refracting power. In FIG. 3, Mo, MI, C, and I stand for an object side-mount surface, an image side-mount surface, a cover glass, and an image plane, respectively.
The first lens group G1 is made up of, in order from the object side to the image side, the first positive lens or one double-convex positive lens CL11.
The second lens group G2 is made up of, in order from the object side to the image side, the first negative lens or a double-concave negative lens CL21, a cemented lens SU21 of the second positive lens or a double-convex positive lens CL22 and the second negative lens or a double-concave negative lens CL23, and the third positive lens or a double-convex positive lens CL24.
FIG. 4 is a sectional view of the converter lens TCL of Example 3.
As shown in FIG. 4, the converter lens TCL of Example 3 is made up of, in order from the object side to the image side, the first lens group G1 of positive refracting power, and the second lens group G2 of negative refracting power. In FIG. 4, Mo, MI, C, and I stand for an object side-mount surface, an image side-mount surface, a cover glass, and an image plane, respectively.
The first lens group G1 is made up of, in order from the object side to the image side, the first positive lens or one positive meniscus lens CL11 convex on its image side.
The second lens group G2 is made up of, in order from the object side to the image side, a cemented lens SU21 of the first negative lens or a double-concave negative lens CL21, the second positive lens or a double-convex positive lens CL22 and the second negative lens or a double-concave negative lens CL23, and the third positive lens or a double-convex positive lens CL24.
FIG. 5 is a sectional view of the converter lens TCL of Example 4.
As shown in FIG. 5, the converter lens TCL of Example 4 is made up of, in order from the object side to the image side, the first lens group G1 of positive refracting power, and the second lens group G2 of negative refracting power. In FIG. 5, Mo, MI, C, and I stand for an object side-mount surface, an image side-mount surface, a cover glass, and an image plane, respectively.
The first lens group G1 is made up of, in order from the object side to the image side, the first positive lens or one positive meniscus lens CL11 convex on its image side.
The second lens group G2 is made up of, in order from the object side to the image side, the first negative lens or a double-concave negative lens CL21, the second positive lens or a double-convex positive lens CL22, the second negative lens or a double-concave negative lens CL23, and the third positive lens or a double-convex positive lens CL24.
FIG. 6 is a sectional view of the converter lens TCL of Example 5.
As shown in FIG. 6, the converter lens TCL of Example 5 is made up of, in order from the object side to the image side, the first lens group G1 of positive refracting power, and the second lens group G2 of negative refracting power. In FIG. 6, Mo, MI, C, and I stand for an object side-mount surface, an image side-mount surface, a cover glass, and an image plane, respectively.
The first lens group G1 is made up of, in order from the object side to the image side, the first positive lens or one positive meniscus lens CL11 convex on its image side.
The second lens group G2 is made up of, in order from the object side to the image side, the first negative lens or a double-concave negative lens CL21, a cemented lens SU21 of the second positive lens or a double-convex positive lens CL22 and the second negative lens or a double-concave negative lens CL23, and the third positive lens or a double-convex positive lens CL24.
Set out below are a variety of numeral data (surface data, etc.) about the master lens MSL and converter lenses TCL.
The surface data include the radius of curvature r and surface separation d of each lens surface (optical surface) for each surface number, the d-line (587.6 nm) refractive index nd of each lens (optical medium), and the d-line Abbe constant νd of each lens (optical medium). The radius of curvature r and surface separation d are all given in mm. In the surface data, “∞” in the radius-of-curvature column stands for infinity.
Aspheric data include data about aspheric lens surfaces in the lens data. Aspheric surface may be represented by the following formula provided that z defines an optical axis where the direction of propagation of light is taken as positive and y defines a direction orthogonal to the optical axis.

z=(y2/r)/[1+{1−(1+K)·(y/r)2}1/2]+A4y4+A6y6+A8y8+A10y10 

Note here that r is the paraxial radius of curvature, K is the conical coefficient, and A4, A6, and A8 are the aspheric coefficients of the 4th, 6th and 8th degrees. It is also to be noted that the capital “E” indicates that the following numerical value is a power exponent having a base of 10. For instance, “1.0E−5” means “1.0×10−5”.
Various data include focal lengths, F-numbers (FNO), angles of view 2ω (°), variable surface separations d, back focuses BF (in air), full lengths (in air), and image heights, all but F-numbers and angles of view being given in mm. Note here that BF is a distance from the final lens surface to the image plane as calculated on an air basis.
WE and TE are the abbreviations for the wide-angle end and telephoto end, respectively.
The focal length data about the respective lens groups include focal lengths f1 to f5 of the respective lens groups, all given in mm.
It is to be noted that an assortment of data set out in the numeral examples are based on the presupposition of an imaging system comprising a camera body in which a master lens apparatus including the master lens MSL is attached to the teleconverters of Examples 1 to 5, and an imaging plane is located in the position of the image plane. Filters (low-pass filters, wavelength selection filters, dustproof filters, etc.) built in the camera body, and a cover glass for the imaging device are supposed as the plane-parallel plates located just in front of the image plane, providing optical plane-parallel plate equivalents.
















Mater lens






Surface data






Surface No.
r
d
n d
ν d


 1
105.4432
2.400
1.85026
32.27


 2
73.5728
8.379
1.49700
81.54


 3
−383.7411
0.150




 4
94.5553
4.286
1.60300
65.44










 5
289.6284
D1 (Variable)












 6
93.6811
2.000
1.83481
42.71


 7
27.3524
5.724




 8
−61.7968
1.800
1.48749
70.23


 9
30.7192
5.169
1.84666
23.78


10
738.2770
3.915




11
−38.1621
2.000
1.76200
40.10










12
−154.2096
D2 (Variable)












13
40.1727
6.046
1.80400
46.57


14
−257.1582
0.150




15
26.7593
8.420
1.43875
94.93


16
−113.3976
0.569




17
−102.0000
1.800
1.90366
31.32


18
33.8270
4.447




19 (Stop)
∞
1.500




20 (Aspheric surface)
53.0521
2.282
1.74320
49.34


21 (Aspheric surface)
113.0112
0.150




22
28.9682
4.305
1.64000
60.08


23
149.0277
0.150




24
44.8443
1.600
1.90366
31.32


25
20.3711
5.252




26
27.3174
6.191
1.72916
54.68










27
−76.1282
D3 (Variable)












28 (Aspheric surface)
−2331.1195
1.200
1.80610
40.88










29 (Aspheric surface)
22.9849
D4 (Variable)












30
−139.5760
1.600
1.51633
64.14


31
45.7459
2.294




32
56.5885
4.072
1.90366
31.32










33
−75.5666
D5 (Variable)












34 (Image side-mount 
∞
15.200




surface)






35
∞
4.000
1.51633
64.14


36
∞
0.800




Image plane
∞










Aspheric coefficient





20th surface


K = 0.000, A4 = −1.1681E−05, A6 = −6.9531E−09, A8 = 7.8500E−12


21th surface


K = 0.000, A4 = 5.8382E−08, A6 = −1.6466E−10, A8 = −7.4745E−13


28th surface


K = 0.000, A4 = 1.7112E−06, A6 = −5.3162E−09


29th surface


K = 0.000, A4 = −1.6310E−06, A6 = −1.7956E−08





Various data










WE
TE





Focal length
40.80
147.00


FNO.
2.88
2.88


Angle of view 2 ω (° )
29.79
8.28


BF (in air)
32.137
28.750


Full length (in air)
151.882
195.839


Image height
10.82
10.82


D5
1.5000
41.2615


D11
25.4170
1.5000


D25
1.5000
3.5170


D29
3.4776
32.9593


D31
13.4987
10.1120










Focal length about the respective lens groups











f1
113.667


f2
−27.169


f3
32.042


f4
−28.229


f5
71.790

























Converter lens






Numeral example 1






Surface data






Surface No.
r
d
n d
ν d


34 (Object side-mount 
∞
−5.429




surface)






35
−93.2149
2.348
1.92286
20.88


36
−40.5105
4.384




37
34.0886
1.300
1.83400
37.16


38
27.6880
0.239




39
25.1698
6.557
1.74077
27.79


40
−22.0429
1.300
2.00100
29.13


41
47.7726
1.955




42
25.3471
5.048
1.49700
81.54


43
−81.1748
−4.438




44 (Image side-mount 
∞
15.200




surface)






45
∞
4.000
1.51633
64.14


46
∞
0.800




Image plane
∞










Various data










WE
TE





Focal length
57.06
205.60


FNO.
4.03
4.03


Angle of view 2 ω (° )
21.85
6.07


BF (in air)
14.200
14.200


Full length (in air)
165.146
209.103


Image height
10.82
10.82


Magnification
1.40
1.40










Focal length about the respective lens groups











f1
76.012


f2
−34.881

























Converter lens






Numeral example 2






Surface data






Surface No.
r
d
n d
ν d


34 (Object side-
∞
−2.816




mount surface)






35
210.9376
3.945
1.92286
20.88


36
−41.1544
1.816




37
−34.4483
1.300
2.00100
29.13


38
23.8340
0.219




39
21.7769
6.518
1.76182
26.52


40
−21.5952
1.300
2.00100
29.13


41
34.5454
0.699




42
25.1598
3.980
1.52249
59.84


43
−148.9062
−5.511




44 (Image side-
∞
15.200




mount surface) 






45
∞
4.000
1.51633
64.14


46
∞
0.800




Image plane
∞










Various data










WE
TE





Focal length
55.98
201.70


FNO.
3.95
3.95


Angle of view 2 ω (° )
21.86
6.07


BF (in air)
13.127
13.127


Full length (in air)
163.332
207.289


Image height
10.82
10.82


Magnification
1.40
1.40










Focal length about the respective lens groups











f1
37.597


f2
−19.132

























Converter lens






Numeral example 3






Surface data






Surface No.
r
d
n d
ν d


34 (Object side-
∞
−4.077




mount surface)






35
−116.5752
2.326
1.92286
20.88


36
−43.6023
3.683




37
−41.8780
1.300
1.83400
37.16


38
17.8581
7.464
1.72825
28.46


39
−19.0163
1.300
2.00100
29.13


40
50.4433
0.150




41
23.8452
4.405
1.49700
81.54


42
−121.9352
−4.344




43 (Image side-
∞
15.200




mount surface)






44
∞
4.000
1.51633
64.14


45
∞
0.800




Image plane
∞










Various data










WE
TE





Focal length
57.06
205.60


FNO.
4.03
4.03


Angle of view 2 ω (° )
21.85
6.07


BF (in air)
14.200
14.200


Full length (in air)
165.146
209.103


Image height
10.82
10.82


Magnification
1.40
1.40










Focal length about the respective lens groups











f1
74.340


f2
−28.179

























Converter lens 






Numeral example 4






Surface data






Surface No.
r
d
n d
ν d


34 (Object side-
∞
−4.077




mount surface)






35
−222.2349
2.459
1.92286
18.90


36
−49.7456
5.247




37
−25.9802
1.448
1.90366
31.32


38
31.0147
0.654




39
35.7226
8.195
1.69895
30.13


40
−21.6871
0.150




41
−30.8000
4.593
1.90366
31.32


42
56.5450
0.150




43
24.0688
5.284
1.48749
70.23


44
−105.5726
−4.439




45 (Image side-
∞
15.200




mount surface)






46
∞
4.000
1.51633
64.14


47
∞
0.800




Image plane
∞










Various data










WE
TE





Focal length
57.06
205.60


FNO.
4.03
4.03


Angle of view 2 ω (° )
21.85
6.07


BF (in air)
14.200
14.200


Full length (in air)
165.146
209.103


Image height
10.82
10.82


Magnification
1.40
1.40










Focal length about the respective lens groups











f1
68.978


f2
−36.955

























Converter lens






Numeral example 5






Surface data






Surface No.
r
d
n d
ν d


34 (Object side-
∞
−4.396




mount surface)






35
−106.5559
2.347
1.69895
30.13


36
−39.5757
4.471




37
−29.8091
1.300
1.72916
54.68


38
21.9609
0.356




39
21.6356
6.500
1.69895
30.13


40
−23.6427
1.300
2.00100
29.13


41
42.8083
0.708




42
37.7055
5.164
1.51633
64.14


43
−26.8409
−5.000




44 (Image side-
∞
15.200




mount surface)






45
∞
4.000
1.51633
64.14


46
∞
0.800




Image plane
∞










Various data










WE
TE





Focal length
55.95
201.58


FNO.
3.95
3.95


Angle of view 2 ω (° )
21.75
6.04


BF (in air)
13.638
13.638


Full length (in air)
164.632
208.589


Image height
10.82
10.82


Magnification
1.37
1.37










Focal length about the respective lens groups











f1
88.797


f2
−42.891









FIGS. 7 to 12 are sets of aberration diagrams for Examples 1 to 5 of the master lens MSL and converter lens TCL at the A to D wide-angle end (WE) and E to H telephoto end (TE).
In these aberration diagrams, SA, AS, DT, and CC are the abbreviations for spherical aberrations, astigmatisms, distortions, and chromatic aberrations of magnification, respectively. There are the spherical aberrations SA given at the wavelengths: 587.6 nm (d-line; a solid line), 435.8 nm (g-line; a long broken line), and 656.3 nm (C-line; a short broken line). There are the chromatic aberrations of magnification CC given at the wavelengths: 435.8 nm (g-line; a long broken line) and 656.3 nm (C-line; a short broken line) on a d-line basis. The astigmatisms AS are shown by a solid line for that of the sagittal image surface, and by a broken line for that of the meridional image surface. Note here that FNO is indicative of an F-number.
Set out below are the values of Conditions (1) to (7) in Examples 1 to 5.



















Example 1
Example 2
Example 3







Condition (1)
−1.136
−0.811
−1.503



Condition (2)
−1.086
−1.708
−1.715



Condition (3)
0.190
0.092
0.179



Condition (4)
20.88
20.88
20.88



Condition (5)
81.54
59.84
81.54



Condition (6)
1.629
1.507
1.443



Condition (7)
1.312
1.213
1.321
















Example 4
Example 5







Condition (1)
−0.852
−1.086



Condition (2)
−2.054
−2.693



Condition (3)
0.186
0.202



Condition (4)
18.90
30.13



Condition (5)
70.23
64.14



Condition (6)
1.985
1.624



Condition (7)
1.312
1.260










To cut off undesired light such as ghosts and flares, a flare stop other than the aperture stop may be located in the inventive master lens MSL. This flare stop may be located in any desired site, for instance, on the object side of the first lens group, between the first and the second lens group, between the second and the third lens group, between the third and the fourth lens group, between the fourth and the fifth lens group, or between the fifth lens group and the image plane.
Flare rays may also be cut off by a frame member or other member. The flare stop may be printed directly, coated, sealed or otherwise bonded to the optical system. The flare stop may be configured in any desired shape such as circular, oval, rectangular or polygonal shape, or may otherwise be formed by a range surrounded by a function curve. In addition to harmful light beams, light beams such as coma flares around the screen may also be cut.
Each lens may be provided with an anti-reflection coating to reduce ghosts and flares. In particular, an antireflection multicoating is preferable because of the ability to effectively reduce ghosts and flares. An infrared cut coating may optionally be applied to each lens surface, cover glass, etc. Optionally, brightness shading around images may be reduced by shifting the micro-lenses of a CCD. For instance, microlens design for the CCD may be altered in association with the angle of incidence of rays at the respective image heights, and image processing may be applied to correct a decrease in light around images.
In general, an antireflection coating is applied to the surface of a lens in contact with air to prevent occurrence of ghosts and flares. At the cementing surface of a cemented lens, on the other hand, the refractive index of an adhesive is ever higher than that of air. For this reason, the cementing surface has often a reflectivity on a par with, or lower than, that of a single-layer coating; so it is less likely to be coated. If the antireflection coating is purposefully applied to the cementing surface too, however, it is then possible to achieve further ghost and flare reductions, thereby making sure good images. Especially in recent years, high-refractive-index vitreous materials have gained popularity, and have often been used on camera optical systems because of having good action on correction of aberrations. However, when such vitreous materials are used for cemented lenses, there is going to be difficult-to-neglect reflection occurring from the cementing surface. In that case, it is particularly effective to apply the antireflection coating to the cementing surface.
How to make effective use of cementing surface coating is set forth typically in JP(A)'s 2-27301, 2001-324676 and 2005-92115 as well as U.S. Pat. No. 7,116,482. According to the teachings of these publications referring to the surface coating of a cemented lens in the first lens group of positive power in a zoom lens in particular, the cemented lens surface in the first lens group in the embodiment of the invention may be coated too.
The coating material used may optionally be selected from materials having relatively high refractive indices such as Ta2O5, TiO2, Nb2O5, ZrO2, HfO2, CeO2, SnO2, In2O3, ZnO and Y2O3, and materials having relatively low refractive indices such as MgF2, SiO2 and Al2O3 depending on the refractive index of the lens to be used and the refractive index of the associated adhesive, and the coating thickness may be set in such a way as to meet phase conditions.
It is a matter of course that multicoating may be applied to the cementing surface as is the case with the coating of the air contact surface of a lens. If double coatings or multicoatings are optionally combined with film thicknesses, it is then possible to achieve further reflectivity reductions and gain control of the spectral and angular characteristics of reflectivity, etc. It goes without saying that coating on the same idea may also be effectively applied to any cementing surface in other lens group(s).
FIG. 13 is a schematically sectional view of the imaging system in which a combination of the inventive rear converter lens apparatus and master lens apparatus is used for a taking lens. The lens unit is schematically shown, and the aforesaid lens system or the like is used for it.
Referring to FIG. 13, a camera body shown by reference numeral 1 does not include any quick return mirror in the optical path involved. Reference numeral 2 is indicative of a mount in the camera body, where the holder 5 of master lens apparatus MSL or the holder 4 of teleconverter TC is attachable to or detachable from the camera body 1, and reference numeral 3 is a mount in the master lens apparatus MSL, where the holder 5 of master lens apparatus MSL is attachable to or detachable from the camera body 1 or the holder 4 of tele-converter TC.
Further, the holder 4 in the teleconverter TC includes a master lens apparatus-side mount a for attachment of the master lens apparatus MSL and a camera body-side mount b for the attachment or detachment of the camera body, and has an object side-mount surface M1 and an image side-mount surface M2 that, upon attachment, are in engagement with the mount 3 of master lens apparatus MSL and the camera body 1, respectively.
The mount used may be typically of either the screw type, or the bayonet type (used in FIG. 13).
In FIG. 13, P is the imaging plane of an imaging device such as a CCD or C-MOS where a subject image is formed by the master lens apparatus MSL and teleconverter TC. The imaging device is operable to convert the image formed on the imaging plane into electric signals that are then sent to a control circuit 12. Just in front of the imaging plane P there are a cover glass and filters F located.
The imaging system is designed to have the maximum image height IH.
Reference numerals 12, 15, and 18 are indicative of control circuits for regulating or otherwise controlling the aperture stop S, the focal length of master lens apparatus MSL, focusing operation and so on, reference numerals 13a, 13b, 16a, and 16b are indicative of electric contacts for making electric connections between the master lens apparatus MSL, teleconverter TL and camera body 1 for transmission of the aforesaid regulation and control signals, and reference numeral 17 is indicative of a control unit for the aperture stop S. This control unit is electrically connected together with lens control units (not shown) for controlling the respective lens groups to the control circuit 12 of the camera body 1.
Information about the layouts of the respective lens groups in the master lens apparatus MSL may be sent to the camera body 1.
The teleconverter according to the present invention may be used as the optical system (rear teleconverter) of the thus constructed imaging system.
While various embodiments of the invention have been explained, it is to be understood that the present invention is never limited thereto, and embodiments comprising combinations of the essential requirements and limitations are embraced in the category of the invention too.

## Claims

1. A teleconverter, comprising:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit that has a generally negative refracting power for mounting the master lens apparatus thereon to obtain a lens system having a focal length longer than that of the master lens apparatus, wherein:
the converter lens unit comprises a first lens group on the master lens apparatus side and a second lens group on the camera body side with an on-axis longest air separation interposed there-between, and does not include any lens group other than the first lens group and the second lens group,
the first lens group has positive refracting power,
the second lens group has negative refracting power,
the first lens group consists of a first lens element having positive refracting power, and
the second lens group comprises, in order from an object side to an image side along an optical path, a second lens element having negative refracting power and adjacent to the first lens element, a third lens element having positive refracting power and adjacent to the second lens element, and a fourth lens element having negative refracting power and adjacent to the third lens element, with satisfaction of the following condition (3):


0.04<d12/D<0.3  (3)



where d12 is an air separation between the first lens group and the second lens group, and
D is an on-axis thickness of the converter lens unit.

2. A teleconverter, comprising:
a master lens apparatus-side mount on which a master lens apparatus is mounted,
a camera body-side mount on which a camera body is mounted, and
a converter lens unit that has a generally negative refracting power for mounting the master lens apparatus thereon to obtain a lens system having a focal length longer than that of the master lens apparatus, wherein:
the converter lens unit comprises a first lens group on the master lens apparatus side and a second lens group on the camera body side with an on-axis longest air separation interposed there-between, and does not include any lens group other than the first lens group and the second lens group,
the first lens group has positive refracting power,
the second lens group has negative refracting power,
the first lens group consists of a first lens element having positive refracting power, and
the second lens group comprises, in order from an object side to an image side along an optical path, a second lens element having negative refracting power and adjacent to the first lens element, a third lens element having positive refracting power and adjacent to the second lens element, a fourth lens element having negative refracting power and adjacent to the third lens element and a fifth lens element having positive refracting power, with satisfaction of the following condition (3):


0.04<d12/D<0.3  (3)



where d12 is an air separation between the first lens group and the second lens group, and
D is an on-axis thickness of the converter lens unit.

3. The teleconverter according to claim 1 or 2, wherein the second lens is a double-concave lens.

4. The teleconverter according to claim 3, wherein the second lens consists of only one lens.

5. The teleconverter according to claim 1 or 2, wherein the third lens is a double-convex lens.

6. The teleconverter according to claim 1 or 2, wherein the third lens and the fourth lens have a cemented surface.

7. The teleconverter according to claim 1, wherein the lens group having positive power on the camera body side comprises a double-convex lens.

8. The teleconverter according to claim 2, wherein the fifth lens is a double-convex lens.

9. The teleconverter according to claim 1 or 2, which satisfies the following condition (3-1):


0.1<d12/D<0.25  (3-1).

10. The teleconverter according to claim 1 or 2, which satisfies the following condition (3-2):


0.092≦d12/D≦0.202  (3-2).

11. The teleconverter according to claim 1 or 2, which further satisfies the following Condition (1):


−1.53<f1/f<−0.66  (1)


where f is a focal length of a whole system of the converter lens unit, and
f1 is a focal length of the first lens group.

12. The teleconverter according to claim 1 or 2, which further satisfies the following Condition (2):


−3.0<f/(B×D)<−1.2  (2)


where B is a magnification of the converter lens unit, and
D is an on-axis thickness of the converter lens unit.

13. The teleconverter according to claim 1 or 2, which further satisfies the following Condition (4) and (5):


15<νd1<35  (4)



50<νd5<100  (5)


where νd1 is an Abbe constant of the first positive lens, and
νd5 is an Abbe constant of the third positive lens.

14. The teleconverter according to claim 1 or 2, which further satisfies the following Condition (6):


1.1<D/fb<3.0  (6)


where D is an on-axis thickness of the converter lens unit, and
fb is an on-axis distance in air from an exit surface of the converter lens unit to an image plane when the teleconverter lens is attached to the master lens apparatus.

15. The teleconverter according to claim 1 or 2, which further satisfies the following Condition (7):


0.6<fb/IH<1.6  (7)


where fb is an on-axis distance in air from an exit surface of the converter lens unit to an image plane when the teleconverter is attached to the master lens apparatus, and
IH is a maximum image height of the imaging system.