Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-08-07
2003-12-09
Mack, Ricky (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S696000
Reexamination Certificate
active
06661585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens control apparatus, zoom lens control method, program, and storage medium and, more particularly, to a zoom lens control apparatus which moves the second lens unit for correcting a focal plane along with movement of the first lens unit for performing magnification operation, a zoom lens control method applied to the zoom lens control apparatus, a program for causing a computer to execute the zoom lens control method, and a storage medium which stores the program.
2. Related Background Art
Image pickup apparatuses such as a video camera which incorporates an inner focus type lens system have been conventionally available.
FIG. 3
is a view schematically showing an inner focus type lens system.
In
FIG. 3
, the lens system comprises a fixed lens group
101
serving as a front lens group, a zoom lens (magnification lens)
102
serving as a lens group for performing magnification operation, an aperture stop
103
, a fixed lens group
104
, a focus lens
105
serving as a lens group with a focus adjustment function (focusing function), and an image pickup element
106
formed from a CCD. The focus lens
105
also has a so-called compensation function of compensating for movement of the focal plane caused by magnification operation of the zoom lens
102
.
As is well known, the focus lens
105
has both the compensation function and focus adjustment function in the lens system having the arrangement shown in FIG.
3
. Even with the same focal length, the position of the focus lens
105
for focusing the lens on the plane of the image pickup element
106
changes depending on the object distance. When the position of the focus lens
105
for focusing the lens on the plane of the image pickup element
106
is plotted along with changes in focal length at each object distance, characteristics as shown in
FIG. 4
are obtained.
FIG. 4
shows the focus lens position by a locus (curve) as a function of the focal length for each object distance (e.g., 80 cm, 3 m, or ∞). During magnification operation, a locus shown in
FIG. 4
is selected in accordance with the object distance. When the zoom lens
102
is driven to change the focal length, the focus lens
105
is moved along the locus, thus realizing zooming without any blur.
In a front focus type lens system, an independent compensation lens is arranged for the magnification lens, and the magnification lens and compensation lens are coupled via a mechanical cam ring. For example, a manual zooming dial is attached to the cam ring, and the focal length is manually changed. Even if the dial is quickly changed, no blur occurs in this operation as far as the focus lens is in focus because the cam ring rotates following the dial operation and the magnification lens and compensation lens move along the cam groove of the cam ring.
In the control of the inner focus type lens system having the above-mentioned features, characteristic information about a plurality of loci shown in
FIG. 4
is generally stored in some format in a lens control microcomputer. A locus is selected in accordance with the object distance, and movement of the focal plane by magnification operation is corrected along the selected locus in zooming. The position of the focus lens
105
is controlled by reading out the position of the focus lens
105
with respect to the position (focal length) of the zoom lens
102
from the lens control microcomputer. For this purpose, the performance of an actuator which drives the focus lens
105
is important. As is apparent from
FIG. 4
, when the zoom lens
102
moves at a constant speed or almost constant speed at the same object distance, the moving speed and moving direction of the focus lens
105
momently change. In other words, the actuator of the focus lens
105
must respond to the speed with a high precision of about 1 Hz to several hundred Hz.
An example of the actuator having this performance is a stepping motor, which is generally being used for the focus lens
105
of the inner focus lens system. The stepping motor rotates in perfect synchronism with a stepping pulse output from the lens control microcomputer or the like, and keeps the stepping angle per pulse constant. The stepping motor can, therefore, obtain a high speed response characteristic, high stop precision, and high position precision. Further, the stepping motor keeps the rotation angle constant with respect to the number of stepping pulses. The stepping pulse can be directly used as an increment type encoder, and no special position encoder needs to be added to the lens system.
To perform magnification operation while keeping an in-focus state by using the stepping motor, locus information shown in
FIG. 4
must be stored in the lens control microcomputer or the like, as described above. Locus information is read out in accordance with the position or moving speed of the magnification lens, and the focus lens is moved based on the information. Alternatively, a function which expresses the position of the focus lens
105
by using the object distance and the focal length of the zoom lens
102
as variables may be adopted.
A method of performing compensation calculation between loci and calculating the standard moving speed of the focus lens in the use of the locus data table will be explained.
FIG. 5
is a graph showing an example of the locus characteristic used in a conventional locus tracing method applied when magnification operation is executed while an in-focus state is maintained using a stepping motor. The locus characteristic is stored in the lens control microcomputer.
In
FIG. 5
, Z
0
, Z
1
, Z
2
, . . . , Z
6
represent positions of the magnification lens (zoom lens); and a
0
, a
1
, a
2
, . . . , a
6
and b
0
, b
1
, b
2
, . . . , b
6
, typical loci indicating focus positions for respective object distances. p
0
, p
1
, p
2
, . . . , p
6
represent intermediate loci each calculated based on the two loci, and are calculated by
p
(n+1)
=|
p
(n)
−a
(n)
|/|b
(n)
−a
(n)
|×|b
(n+1)
−a
(n+1)
|+a
(n+1)
(1)
For example, a point p
1
is calculated by equation (1), a ratio with which a point p
0
internally divides a line segment (b
0
−a
0
) is obtained, and a point which internally divides a line segment (b
1
−a
1
) in accordance with this ratio is set as p
1
. When the focus lens
105
is located at the point p
0
, the standard moving speed of the focus lens
105
for maintaining an in-focus state can be obtained from the position difference (p
1
−p
0
) between the points p
1
and p
0
, and the time taken to move the zoom lens
102
from the position Z
0
to the position Z
1
.
Calculation of the position of the focus lens
105
when the stop position of the zoom lens
102
is not a position (zoom boundary position) on stored typical locus data will be explained.
FIG. 6
is a locus graph for explaining an interpolation method in the direction of the magnification lens position. Some of data in
FIG. 5
are extracted, and magnification lens position data are arbitrarily set.
In
FIG. 6
, the ordinate and abscissa respectively represent the positions of the focus lens
105
and zoom lens
102
. Typical locus positions (discrete positions of the focus lens
105
for discrete positions of the zoom lens
102
) stored in the lens control microcomputer are represented by zoom lens positions Z
0
, Z
1
, . . . , Z
k−1
, Z
k
, . . . , Z
n
, and focus lens positions a
0
, a
1
, . . . a
k−1
, a
k
, . . . , a
n
and b
0
, b
1
, . . . , b
k−1
, b
k
, . . . , b
n
for respective object distances at the zoom lens positions Z
0
, Z
1
, . . . , Z
k−1
, Z
k
, . . . , Z
n
. Assume that the zoom lens position is an intermediate position Z
x
which is not a discrete position (zoom boundary position) on locus data, and a
x
and b
x
represent focus lens positions on locus data for respective object distances at the intermediate
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