Image stabilizing optical system

This specification discloses an image stabilizing photo-taking lens system having a lens system in which a plurality of lens units are disposed and at least one lens spacing is changed to change the focal length, and a variable vertical angle prism unit whose vertical angle is changed and driven to refract the optic axis immediately rearwardly of a predetermined lens surface which satisfies the condition that .vertline.Fa.vertline.<10 Ft, where Fa is the combined focal length from the front lens surface of the lens system to the predetermined lens surface, and Ft is the focal length of the entire lens system at the telephoto end thereof.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to a magnification changing optical system, 
particularly a compact magnification changing optical system, suited to 
correct image vibration caused by the tilting of the optical system and 
achieve image stabilization. 
Related Background Art 
When photographing on a vehicle, a ship or an aircraft which is moving, or 
when photographing while walking, vehement trembling is apt to be given to 
an image thereby making the image difficult to see and therefore, a normal 
(or good) image cannont be obtained particularly when exposure time is 
long. 
Generally, when photographing is to be effected with a lens of long focal 
length mounted on a single-lens reflex camera, it is usual to fix the 
camera to a tripod, but this impairs maneuverability and therefore it is 
often the case that the camera is used while being held in hand, and in 
some cases, image vibration prevents the obtainment of a good image. Also, 
recently, the enhancement of the magnification of photo-taking lenses 
mounted on video cameras has posed the remarkable problem that good images 
are not obtained. 
Various anti-vibration optical systems or image stabilizing optical systems 
have been proposed to prevent such image vibration, and there is well 
known, for example, a construction in which bellows with transparent 
liquid of a predetermined refractive index enclosed therein are provided 
forwardly of a photo-taking lens and the angle formed between the 
incidence surface and emergence surface of the bellows is controlled so 
that the angle of light refraction of an optical wedge formed by the 
bellows may correct the angle of inclination of the photo-taking lens. 
However, if the photo-taking lens is a zoom lens, particularly of high 
magnification zoom, the diameter of the bellows disposed forwardly of the 
lens becomes large and the bellows themselves are a hindrance and also, it 
is difficult to change the vertical angle at a high speed, and this also 
gives rise to the problem that it becomes difficult to follow a high 
frequency well. 
In order to eliminate such a problem, it has been separately proposed to 
dispose bellows between the afocal portion, specifically the magnification 
changing portion, of a zoom lens and a relay lens, but another problem has 
resulted from this. That is, the problem is that in a video camera or a 
still video camera, which uses a solid state image pickup device such as 
CCD or MOS in the image receiving portion thereof, the light receiving 
surface of the device is high in reflectance and therefore part of an 
imaging light beam is reflected thereby and travels back along the optical 
path, is reflected by the incidence surface or the emergence surface of 
the bellows or both, enters the solid state image pickup device again and 
forms ghost or flare, thus deteriorating the quality of image. Of course, 
in the case of silver salt film, a similar problem arises to one degree or 
another. Also, a technique whereby a variable vertical angle prism is 
disposed in an objective lens is disclosed in U.S. application Ser. No. 
291,263 (filed on Dec. 28, 1988) abandoned in favor of U.S. patent 
application Ser. No. 701,326 (filed May 7, 1991), which is commonly 
assigned with the present application. 
SUMMARY OF THE INVENTION 
The present invention eliminates the above-noted problems, and makes the 
apparatus compact and does not deteriorate the quality of image. 
A feature of the present invention is that when a variable vertical angle 
prism unit whose vertical angle is changed and driven to refract the optic 
axis is to be disposed in a lens system wherein at least one lens spacing 
in a plurality of lens units is changed to thereby change the focal 
length, the variable vertical angle prism is disposed at a position which 
satisfies the condition that .vertline.Fa.vertline.&lt;10 FT, where Fa is the 
combined focal length from the front lens surface of the lens system to 
the surface immediately front of the variable vertical angle prism unit, 
and Ft is the focal length of the entire system at the telephoto end 
thereof. This condition has been found from the optical analysis of lenses 
including various zoom lenses which will be described later having been 
effected on a power arrangement which does not cause a substantially 
problematic degree of ghost or flare on the image plane. The zoom lenses 
used are all comprised of lenses, but the present invention is also 
applicable to zoom lenses including spherical lenses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a first embodiment of the present invention. In FIG. 1, it is 
to be understood that an object is on the left side, and a first lens unit 
f.sub.1 of positive refractive power is moved back and forth in the 
direction of the optic axis for focusing. The reference character f.sub.2 
designates a second lens unit of negative refractive power having the 
magnification changing function which is moved in the direction of the 
optic axis by a driving mechanism, not shown. The reference character 
f.sub.3 denotes a third lens unit of negative refractive power for 
negating the movement of the image plane. The third lens unit f.sub.3 is 
moved with the second lens unit f.sub.2 and independently along a locus 
shown in the figure. The reference numeral f.sub.4 designates a so-called 
relay lens unit having the imaging action. I denotes the image plane on 
which the image receiving surface of a solid state image pickup device is 
located. SP designates a stop for determining the brightness of the 
system. 
V denotes a variable vertical angle prism unit, which is shown on an 
enlarged scale with a driving portion in FIG. 2. The reference numerals 10 
and 11 designate a light transmitting entrance end plate and a light 
transmitting exit end plate. The reference numeral 12 denotes a bendable 
thin film cylinder constituting bellows, and the reference numeral 13 
designates light transmitting liquid having a desired refractive index 
which is enclosed in the space defined by the thin film cylinder 12 and 
the end plates 10 and 11. Accordingly, when for example, the end plate 10 
is fixed and the edge of the end plate 11 is pressed, the end plate 11 is 
inclined and an optical wedge of a vertical angle .theta. is formed. 
The reference numeral 14 denotes an actuator having its driving shaft 
flexibly connected to the end plate 11. Although only one actuator 14 is 
shown, it is to be understood that for example, three actuators are spaced 
apart by 120.degree. from one another and provided about the optic axis or 
one more actuator is provided for the end plate 11 through a gimbal 
mechanism so that desired optical wedges may be formed with respect to all 
radial directions. The reference numeral 15 designates an inclination 
angle detecting sensor which may be, for example, a gyro or acceleration 
sensor. It is to be understood that a plurality of such inclination angle 
detecting sensors 15 are provided about the optic axis, and they can 
detect the inclination of a photo-taking lens in any direction. The 
reference numeral 16 denotes a microcomputer which calculates the amount 
of outward and inward movement of the driving shaft of each actuator 14 on 
the basis of magnification change information taking into account the 
output of each inclination angle detecting sensor 15 and the position at 
which the variable vertical angle prism unit is disposed, and controls 
each actuator 14 by an output signal. A soft elastic resin plate may be 
employed instead of the liquid and thin film cylinder. 
Turning back to FIG. 1, the variable vertical angle prism unit V is fixed 
to the movable inner cylinder, not shown, of the second lens unit f.sub.2 
immediately in front of the second lens unit f.sub.2 and thus, it is moved 
with the second lens unit, and this leads to the advantage that even if a 
magnification change is effected, the amount of revision of the vertical 
angle .theta. of the prism may be small and can be easily controlled. 
FIGS. 3 and 4 show zoom lenses similar to the zoom lens of FIG. 1, but it 
is to be understood that the position of disposition of the variable 
vertical angle prism unit V is fixed unlike the case of FIG. 1. Although 
each lens unit is shown as a lens component, it is to be understood that 
actually, each lens unit is comprised of one or more lens components. 
FIG. 5 shows another embodiment of the present invention which corresponds 
to a fourth numerical value example which will be described later. The 
fourth lens unit f.sub.4 is divided into three sub-lens units, and the 
variable vertical angle prism unit V is fixed between second and third 
sub-lens units f.sub.42 and f.sub.43. There is usually a relatively great 
air space in the relay lens unit, and this leads to the advantage that the 
full lens length need not be extended to provide the variable vertical 
angle prism unit. Also, the prism unit is in the fixed lens unit, and this 
leads to the ease with which the prism unit is fixed and the simplicity of 
the structure of a lens barrel mechanism or the like. Further, optically, 
this is a location at which the degree of sensitiveness when the vertical 
angle is changed becomes high and therefore, the range of change in the 
vertical angle becomes small, and this is useful to quicken the driving 
and advantageous to make the system compact. FIG. 6 shows a fifth 
embodiment in which the variable vertical angle prism unit V is fixedly 
disposed between the zoom lens and the image plane I. A space is already 
secured as the back focal length between the zoom lens and the image 
plane, and this leads to the advantage that the full lens length does not 
become great. 
FIG. 7 shows a sixth embodiment which has, in succession from the object 
side, a first lens unit f.sub.1 of positive refractive power, a second 
lens unit f.sub.2 of negative refractive power, a third lens unit f.sub.3 
of positive refractive power and a fourth lens unit f.sub.4 of positive 
refractive power and in which the second lens unit is moved to effect a 
magnification change and the fluctuation of the image plane resulting from 
a magnification change is corrected by the fourth lens unit being moved 
and a variable vertical angle prism unit V is disposed between the first 
lens unit and the second lens unit of the optical system in which the 
fourth lens unit is moved to effect focusing. As in the previously 
described first embodiment, the prism unit is adapted to be moved with the 
second lens unit, and this is useful to make the system compact and light 
in weight, and the difference in the amount of variation in the vertical 
angle of the prism resulting from a magnification change is small and the 
control of the prism unit becomes easy. 
FIGS. 8 and 9 show a seventh and an eighth embodiment, respectively. In 
these embodiments, the construction of the zoom lens is similar to that 
shown in FIG. 7, but the variable vertical angle prism unit V is disposed 
forwardly or rearwardly of the third lens unit f.sub.3, and the third lens 
unit f.sub.3 is located near the stop SP and therefore, a light beam 
passing therethrough is stopped, and this leads to the advantage that the 
outer diameter of the prism unit can be made small. Also, the fact that 
the prism unit is near the fixed lens unit leads to the advantage that the 
fixed structure of the prism unit becomes simple. Further, this location 
is high in the sensitivity given to the movement of an image when the 
vertical angle of the prism is changed, and this leads to the effect that 
the range of the vairable angle can be made small. 
The embodiment of FIG. 10 is one in which the prism unit V is disposed 
between the zoom lens and the image plane, and has the same advantage as 
that of the FIG. 6 embodiment. 
The refractive power (1/f) arrangements and the principal point spacings 
(e.sub.W and e.sub.T) of each respective lens unit at the wide angle end 
and the telephoto end in Embodiments 1-9 corresponding to FIGS. 1 and 3-4 
will be shown below. Also, the correspondence relations between the 
respective embodiments and the conditional expression will be shown below. 
______________________________________ 
Embodiment 
f.sub.1 f.sub.2 
f.sub.3 
f.sub.4 
f.sub.41 
f.sub.42 
f.sub.43 
______________________________________ 
1 38.66 -7.6 -35.96 
20.91 
2 33.36 -7.6 -40.64 
18.69 
3 33.42 -7.6 -35.94 
20.04 
4 33.42 -7.6 -35.94 22.88 
32.89 
44.81 
______________________________________ 
e.sub.1W 
e.sub.2W 
e.sub.3W 
e.sub.41 
e.sub.42 
e.sub.1T 
e.sub.2T 
e.sub.3T 
______________________________________ 
1 11.0 23.28 18.85 28.61 
6.71 17.80 
2 5.65 27.91 12.65 23.14 
11.00 12.07 
3 5.65 23.28 17.35 23.20 
6.30 16.78 
4 5.65 23.28 3.40 5.77 24.08 23.20 
6.30 2.83 
______________________________________ 
Embodiment f.sub.1 
f.sub.2 f.sub.3 
f.sub.4 
______________________________________ 
5 33.42 -7.6 -35.94 21.09 
6 37.41 -7.35 25.42 18.80 
7 36.05 -7.35 25.42 18.80 
8 32.91 -7.35 25.42 18.80 
9 32.91 -7.35 25.42 18.80 
______________________________________ 
e.sub.1W 
e.sub.2W e.sub.3W 
e.sub.1T 
e.sub.2T 
e.sub.3T 
______________________________________ 
5 5.65 23.28 19.16 
23.20 6.30 18.59 
6 11.00 22.42 16.87 
27.86 5.56 18.28 
7 9.50 27.01 16.87 
26.36 10.16 17.68 
8 6.50 22.42 22.00 
23.35 5.56 23.59 
9 6.50 22.42 16.87 
23.35 5.56 18.28 
______________________________________ 
Condi- 
tional 
expres- 
Embodiment 
sion 1 2 3 4 5 6 7 8 9 
______________________________________ 
.vertline.Fa.vertline./ 
0.49 0.19 0.10 0.18 0.13 0.60 0.27 0.77 0.13 
Ft 
______________________________________ 
In the above-described embodiments, a curvature may be given to the outer 
side of the end plate of the prism unit to thereby endow it with a lens 
action. 
Also, a material absorbing and cutting infrared light can be mixed with the 
liquid in the prism unit to thereby make the prism unit act also as an 
infrared cut filter, and if an infrared cut layer is deposited by 
evaporation on the surface of the end plate of the prism unit, the costs 
of material and assembly can be reduced as compared with a case where an 
infrared cut filter is provided separately. 
On the other hand, the present invention can be utilized in a tracking 
system in addition to an anti-vibration system. 
The above-described present invention is a photo-taking lens of variable 
magnification and yet achieves the excellent effect that the variable 
vertical angle prism unit is not made bulky and the quality of image is 
not deteriorated particularly by ghost or flare.