Optical system and optical apparatus having the same

An optical system includes, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, the second lens unit being movable along an optical axis to effect focusing, and a third lens unit of positive refractive power, the third lens unit including, in order from the object side, a first lens subunit of positive refractive power, a second lens subunit of negative refractive power and a third lens subunit of positive refractive power, wherein the second lens subunit is movable in directions perpendicular to the optical axis to displace an image formed by the optical system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical systems of the inner focus type 
suited to cameras for film, video cameras, video still cameras or the 
like. 
2. Description of Related Art 
It has been known to provide a lens type suited to the optical systems of 
long focal lengths, comprising, in order from an object side, a first lens 
unit of positive refractive power and a second lens unit of negative 
refractive power, or the so-called "telephoto" type. This lens type is 
advantageous at removing, in particular, spherical aberration and securing 
a fast F-number. 
In, for example, Japanese Laid-Open Patent Applications No. Hei 5-27163, 
No. Hei 5-27164, No. Hei 6-242371, No. Hei 9-159911 and No. Hei 9-203859, 
the use of the telephoto type in optical systems has been proposed. 
For many photographic lenses (optical systems,) focusing is performed by 
moving either the entirety of the complete lens, or one of the constituent 
lenses. In the case of long-focal-length lenses or telephoto lenses, 
because of their having bodies of large size and heavy weight, the former 
focusing method is hardly employed owing to the difficult problem of 
making up the operating mechanism. 
For this reason, most of the telephoto lenses have the focusing provision 
made in one of the lens units. Of the lens units other than the front lens 
unit, a one in the central section is favorable for focusing purposes, 
because its size is relatively small and, moreover, because it is far 
light in weight. The use of this inner focus type has been proposed for 
various lenses. 
For example, Japanese Laid-Open Patent Application No. Sho 55-147606 (U.S. 
Pat. No. 4,348,084) discloses a telephoto lens of the inner focus type 
having a focal length of 300 mm with an F-number of 2.8, and each of 
Japanese Laid-Open Patent Applications No. Sho 59-65820 and No. Sho 
59-65821 discloses another telephoto lens of the inner focus type having a 
focal length of 135 mm with an F-number of 2.8. 
The telephoto lenses of the inner focus type proposed in the above 
applications each comprise, in order from an object side, a first lens 
unit of positive refractive power, a second lens unit of negative 
refractive power and a third lens unit of positive refractive power, 
totaling three lens units, wherein the second lens unit is made axially 
movable for focusing. 
Meanwhile, in shooting with the long-focal-length photographic system 
(optical system), it is generally difficult to suppress the influence of 
the vibrations of the photographic system. When the photographic system 
tilts due to the vibrations, the image on the focal plane changes its 
position out of alignment with a sight line, depending on the tilt angle 
and the focal length of the photographic system. Therefore, the 
photographic apparatus for taking still pictures suffers a problem of 
preventing the image quality from deteriorating by decreasing the exposure 
time to much short values. The motion-picture taking apparatus, too, 
encounters a difficult problem of keeping the composition at a desired 
setting. In such photographic situations, therefore, it becomes necessary 
to compensate for the tilting of the photographic system due to the 
vibrations so that the image on the focal plane remains stationary in 
alignment with the line of sight, that is, the so-called "shake" of the 
image does not occur. 
There have been many methods of compensating for the shake of the image on 
the focal plane. For example, in the video cameras, the image pickup 
element is given a larger effective area than the necessary image frame 
and the image shake is electrically compensated for. Such an electric 
compensation method is widely employed in the video cameras. 
In Japanese Laid-Open Patent Application No. Sho 61-223819, there is 
disclosed a photographic system in which a refractive-type variable angle 
prism is disposed at the frontmost position thereof. When the photographic 
system vibrates, the vertical angle of the variable angle prism is changed 
correspondingly with the vibration of the photographic system, so that an 
image is deflected so as to stabilize the image. 
Another method of compensating for the shake of the image by moving a 
certain lens unit (movable lens unit) of an optical system in directions 
perpendicular to the optical axis was proposed in, for example, Japanese 
Laid-Open Patent Applications No. Sho 50-80147, No. Sho 61-223819, No. Hei 
7-270724 (U.S. Pat. No. 5,646,779) and No. Hei 8-201691. 
With such an optical system, if the relative aperture is increased (or if 
the F-number is made faster), the front lens unit becomes larger 
accordingly. As is well known, the F-number of the optical system is 
defined by 
EQU Fno=f/D 
where Fno: the F-number, 
f: the focal length of the optical system, and 
D: the diameter of an effective light beam coming from an object point at 
infinity on the optical axis. 
Accordingly, the large-relative-aperture telephoto lens (whose focal length 
is long) gets a large diameter for the equivalent F-number in direct 
proportion to the focal length. As the diameter of the front lens unit 
increases, the weight of the optical system increases rapidly. So, this 
tendency has been seen as problematic. Particularly for the photographic 
camera using 35 mm film, in the case of exceeding f=400 mm (semi-angle of 
field .omega.=3.1.degree.), or in the case of the so-called "super" 
telephoto lens, this problem is very serious. In the locations where the 
tripod cannot be used, the hand-held shooting gives the photographer a 
heavy burden, becoming a cause of the image shake and an obstacle to the 
long-time shooting. 
The method of compensating for the shake of the image by decentering a 
certain movable lens unit of the optical system so as to displace the 
image position is amenable to the techniques of minimizing the size of the 
apparatus by selecting a suitable one of the lens units for the movable 
lens unit. 
However, the use of this method leads to taking into account two 
prerequisites, one of which is to properly select or arrange a lens unit 
of small size and light weight so that a lesser decentering movement 
produces a greater effect of displacing the image position, and the other 
of which is to prevent the image quality from deteriorating as far as 
possible, as that lens unit, when being decentered, produces aberrations. 
In general, it is very difficult to balance these conditions. 
Meanwhile, the inner focus type, because of its using a lens unit of small 
size and light weight as the focusing lens, has merits that the management 
is easy, that a fast speed operation becomes possible, and that the 
holding is easy to keep, because the center of gravity of the whole lens 
system little changes between when focused on an infinitely distant object 
and when focused on an object at the minimum distance. 
On the other hand, the use of the inner focus type in the telephoto lens of 
fast F-number increases the range of variation of aberrations with 
focusing. This variation of aberrations, because of being difficult to 
correct well, comes to a cause of lowering the optical performance. 
BRIEF SUMMARY OF THE INVENTION 
The present invention is concerned with the image stabilizing function in 
the optical system which makes one lens unit movable to decenter in 
directions perpendicular to an optical axis. With this regard, an object 
of the invention is to provide an optical system which can manifest the 
image stabilizing function, while still permitting the decentering 
aberrations to be as far suppressed as possible. 
The present invention is concerned also with the inner focus type. Another 
object of the invention is, therefore, to provide an optical system which 
suppresses the variation with focusing of aberrations as far as possible 
over a wide range of object distances. 
To attain the former object, in accordance with an aspect of the invention, 
there is provided an optical system, which comprises, in order from an 
object side, a first lens unit of positive refractive power, a second lens 
unit of negative refractive power, the second lens unit being movable 
along an optical axis to effect focusing, and a third lens unit of 
positive refractive power, the third lens unit including, in order from 
the object side, a first lens subunit of positive refractive power, a 
second lens subunit of negative refractive power and a third lens subunit 
of positive refractive power, wherein the second lens subunit is movable 
in directions perpendicular to the optical axis to displace an image 
formed by the optical system. 
To attain the latter object, in accordance with another aspect of the 
invention, there is provided an optical system, which comprises, in order 
from an object side, a first lens unit of positive refractive power, the 
first lens unit consisting of, with boundaries formed by the widest and 
the second-widest spaces thereof, in order from the object side, a front 
lens subunit of positive refractive power, a middle lens subunit of 
negative refractive power and a rear lens subunit of positive refractive 
power, and a second lens unit of negative refractive power, the second 
lens unit being movable along an optical axis to effect focusing, wherein, 
letting a focal length of the entire optical system be denoted by f, a 
focal length of the front lens subunit be denoted by f1a, and a space 
between the front lens subunit and the rear lens subunit be denoted by 
D1ab, the following conditions are satisfied: 
EQU 0.4&lt;f1a/f&lt;1.1 
EQU 0.035&lt;D1ab/f&lt;0.15. 
These and further objects and aspects of the invention will become apparent 
from the following detailed description of preferred embodiments thereof 
taken in conjunction with the accompanying drawings.