Television – Camera – system and detail – Optics
Reexamination Certificate
1996-03-14
2001-02-06
Ho, Tuan (Department: 2712)
Television
Camera, system and detail
Optics
C348S349000, C348S352000
Reexamination Certificate
active
06184932
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lens control apparatus to be preferably used in a video camera.
2. Description of the Related Art
Recently, video cameras or camcorders have become remarkably widespread, and many improvements have been made in performance, function, and operability thereof. Particularly, miniaturization thereof and increase in magnification of zooming are strongly demanded, and many attempts have been made to achieve them.
The reason why miniaturization of the video cameras is realized in these circumstances is that lenses of an internal focusing type which are small and capable of high-magnification zooming are adopted.
FIG. 1
schematically illustrates a configuration of a commonly used lens system of the internal focusing type.
Referring to
FIG. 1
, there are provided the first fixed lenses
101
; the second lenses for varying magnification (hereinafter, referred to as a variator lens); a diaphragm
103
, the third fixed lenses
104
; the fourth lenses
105
(hereinafter, referred to as a focus lens) having both a focusing function and a so-called compensator (focus compensation) function of compensating for a shifting of a focal plane due to a magnification varying, and an image pick-up surface
106
.
According to the lens system constructed as shown in
FIG. 1
, since the focus lens
105
has both the compensator function and focusing function, the position of the focus lens
105
for focusing on the image pick-up surface
106
varies with object distances even if focal lengths are equal. And, it is needless to say that the position of the focus lens
105
varies with the focal lengths even if the object distances are equal.
FIG. 2
is the plot of the position of the focus lens
105
for focusing on the image pick-up surface when the object distances are varied in each of the focal lengths. If a locus shown in
FIG. 2
is selected in accordance with the object distance during magnification varying, and the focus lens
105
is shifted in accordance with the locus, a zooming without defocus becomes possible.
According to a lens system of a for-element focusing type, a compensator lens is provided separately from the focus lens with respect to the variator lens, and the variator lens and compensator lens are coupled by means of a mechanical cam ring. Therefore, when a knob for manual zooming is provided to vary the focal length manually, the cam ring follows the knob to rotate however fast the knob may be actuated, so that the variator lens and compensator lens shift along a groove of the cam ring. Thus, defocus is not caused by the zooming when the focus lens is in focus.
In a zoom control of the lens system of the internal focusing type having characteristics as described above, it is popular that a plurality of locus data shown in
FIG. 2
are stored in a lens control microcomputer in one form or another, the locus of the focus lens is selected in accordance with the positions of the focus lens and variator lens, and the zooming is performed by tracing the selected locus.
Further, since the position of the focus lens with respect to the variator lens is read out from a memory device so as to be utilized for controlling the positions of the lenses, the position of each lens must be read out accurately to some extent. Particularly, as is also apparent from
FIG. 2
, the inclination of the locus of the focus lens varies every moment with the change of the focal length when the variator lens shifts with constant or nearly constant speed. This shows that the shifting speed and shifting direction of the focus lens change every moment. In other words, an actuator of the focus lens must perform accurate speed response from 1 Hz to several hundred Hz.
As an actuator which satisfies the above-described requirement, the use of a stepping motor in the focus lens of the internal-focusing lens system is becoming popular. Since the stepping motor rotates in complete synchronism with stepping pulses output from the lens control microcomputer or the like, and a stepping angle per pulse is constant, it is possible to obtain a high speed response and, stopping accuracy and position accuracy can be obtained.
In addition, the use of the stepping motor offers the following advantage. Since a rotation angle of the motor with respect to the number of stepping pulses is constant, the stepping pulse can be used as an incremental encoder, and there is no need to provide additionally a specific position encoder.
As described above, when the magnification varying is performed while maintaining in-focus with the use of the stepping motor, it is necessary to store the locus data of
FIG. 2
in the lens control microcomputer or the like in one form or another (either the locus itself or a function having a variable of the lens position will do), read out the locus data in accordance with the position or the shifting speed of the variator lens, and then move the focus lens based on the data.
FIGS. 3A and 3B
illustrate an example of the already proposed locus follow-up method.
FIG. 3B
shows a memory table in the lens control microcomputer in which the locus data of
FIG. 3A
are stored. As apparent from
FIG. 3B
, shifting ranges of the variator lens and focus lens are split into a plurality of areas, and focus lens data a
0
, a
1
, . . . , b
0
, b
1
, . . . determined by the variator lens positions z
0
, z
1
, . . . and the object distance are stored in order. In
FIG. 3B
, v represents the variator lens position, n represents the object distance and each of the data Anv (n=0, 1 . . . m; v=0, 1 . . . s) are focus lens position data which are unitarily determined by the variator lens position and object distance.
In
FIG. 3A
, each of z
0
, z
1
, z
2
. . . z
6
represents the variator lens position; each of a
0
, a
1
, a
2
. . . a
6
and each of b
0
, b
1
, b
2
. . . b
6
represent typical loci of the focus lens stored in the lens control microcomputer. And, each of p
0
, p
1
, p
2
. . . p
6
represent the locus of the focus lens calculated from the above-described two loci. The locus is calculated by the following expression:
p
(
n
+1)=|
p
(
n
)−
a
(
n
)|/|
b
(
n
)−
a
(
n
)|·|
b
(
n
+1)−
a
(
n
+1)|+
a
(
n
+1) (1)
The expression (1) shows that when the focus lens is on p
0
, a ratio of a line segment b
0
-a
0
divided internally by p
0
is determined and a point which divides internally a line segment b
1
-a
1
in accordance with the above ratio is taken as p
1
. A standard shifting speed of the focus lens for maintaining in-focus can be found from the position difference between p
1
and p
0
, and the time involved in shifting of the variator lens from z
0
to z
1
.
A case will now be described where there is no restriction such that the variator lens should stop only on the border having the stored typical locus data.
FIG. 4
is a view for explaining an interpolation method of the variator lens position in which a part of
FIG. 3A
is extracted and the variator lens is at the voluntary position.
In
FIG. 4
, the vertical axis represents the focus lens position and the horizontal axis represents the variator lens position, respectively, and the typical locus positions (the focus lens position with respect to the variator lens position) stored in the lens control microcomputer are represented by a
0
, a
1
. . . ak−1, ak . . . an and b
0
, b
1
. . . bk−1, bk . . . bn according to the object positions when the variator lens positions are Z
0
, Z
1
. . . Zk−1, Zk . . . Zn, respectively.
When the variator lens is on Zx which is not the zoom border and the focus lens position is px, ax and bx are determined by the following expressions:
ax=ak−
(
Zk−Zx
)(
ak−ak−
1)/(
Zk−Zk−
1) (2)
bx=bk−
(
Zk−Zx
)(
bk−bk−
1)/(
Zk−Zk−
1) (3)
That is, ax and bx can be determined by internally dividing one of
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ho Tuan
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