Zoom tracking apparatus and method in a video camera

A zoom tracking apparatus and method in a video camera. An offset adjustment is achieved by deriving applied voltage of the zoom position sensor only by use of data relating to positions of the zoom lens respectively associated with several optional points and corresponding positions of the focusing lens involving focusing while shifting the zoom lens in one direction. The offset adjustment is, thereby, achieved rapidly and accurately, and an accurate focus upon zoom tracking is obtained. The minimum quantity of trace data is stored in the internal ROM of the control circuit, while a trace tracking of the focusing lens associated with the shift of the zoom lens is achieved in a region where an error may occur possibly upon detecting the position of the zoom lens by use of a focusing evaluation value, thereby capable of achieving the trace tracking accurately.

BACKGROUND OF THE INVENTION 
The present invention relates to a zoom tracking apparatus and method in a 
video camera, and more particularly to a zoom tracking apparatus and 
method in a rear focusing type video camera, capable of achieving an 
accurate offset adjustment and accurate zoom tracking. 
Referring to FIG. 1, there is illustrated a conventional apparatus for 
executing a zoom tracking and an offset adjustment in a rear focusing type 
video camera. 
As shown in FIG. 1, the apparatus includes a zoom position sensor 2 for 
sensing the current position of a zoom lens 1 and a zoom motor driver 3 
for generating a drive signal for driving a zoom motor 4 for shifting the 
zoom lens 1. The zoom position sensor 2 is provided with a variable 
resistor (not shown) receiving a signal from a D/A converter 5. As the 
zoom lens 1 is shifted by the zoom motor 4, the variable resistor of the 
zoom position sensor 2 receives an output signal from the D/A converter 5 
and varies the voltage level of the received signal in accordance with its 
variation in resistance caused by the shift of the zoom lens 1. 
Accordingly, the zoom position sensor 2 detects the position of the zoom 
lens 1 by the variation in voltage. 
The apparatus also includes an A/D converter 6 for encoding the voltage 
applied to the variable resistor of the zoom position sensor 2. The 
encoded signal from the A/D converter 6 is transmitted to a control 
circuit 7 which, in turn, recognizes the current position of the zoom lens 
1 from the received signal. 
A focus sensor 8 which is positioned at an origin and is adapted to sense a 
position of a focusing lens 10. The apparatus further includes a stepping 
motor driver 9, a stepping motor 11 driven by the stepping motor driver 9 
and adapted to shift the focusing lens 10, and a focus detector 12 adapted 
to calculate the number of driven steps of the stepping motor 11. The 
calculated number of driven steps is sent to the control circuit 7 which, 
in turn, recognizes the is position of the focusing lens 10 on the basis 
of the received signal. 
The control circuit 7 comprises a microprocessor equipped with a read only 
memory (ROM) storing various trace data respectively for various object 
distances from each possible position of the zoom lens 1. When the zoom 
lens 1 is shifted for executing a zooming, the focusing lens 10 is 
correspondingly shifted along a trace determined by a proper one of the 
trace data stored in the ROM. 
In operation, however, the actual trace of the focusing lens 10 shifted 
upon shifting the zoom lens 1 has a difference (.+-..alpha.) from the 
theoretical trace determined by the trace data stored in the ROM, based on 
the shift range of the zoom lens 1 and the position of the focus sensor 8, 
as shown in FIG. 3. This difference is called an offset. Accordingly, this 
offset value should be corrected in order to achieve both the automatic 
focus adjustment and the zoom tracking in a rear focusing system. For 
proper offset adjustment the offset value and the shift range of the zoom 
lens 1 must be found. 
Such an offset adjustment is executed by sending data from the control 
circuit 7 to a personal computer (PC) 13a. Now, a conventional offset 
adjustment will be described in conjunction with FIG. 4. 
In accordance with the method illustrated in FIG. 4, first, the zoom lens 1 
is shifted to an optional position regarded as a wide-end, that is, the 
point "a" of FIG. 5 (Step S1). Thereafter, the focusing lens 10 is shifted 
so as to achieve an accurate focusing (Step S2). The focusing lens 10 is 
then shifted again from its current position for obtaining the accurate 
focusing by an optional distance, that is, the distance "" of FIG. 5 (Step 
S3). Then, a position of the zoom lens 1 where the focusing is obtained, 
that is, the position "b" of FIG. 5 is searched while shifting the zoom 
lens 1 toward a tele-end (steps S4 and S5). 
Thereafter, the zoom lens 1 is shifted again toward the wide-end by its 
variable shift range indicated by ".gamma." in FIG. 5 (Step S6). A search 
is then made whether the current position of the zoom lens 1 corresponds 
to the optional position "a" assumed as the wide-end at the first step S1 
(Step S7). 
If the current position of the zoom lens 1 corresponds to the optional 
position "a", it is then determined as corresponding to the position of 
the zoom lens 1 returned at the step S6. When the current position of the 
zoom lens 1 does not correspond to the optional position "a", it does not 
correspond to the position of the zoom lens 1 returned at the step S6. In 
the latter case, the focusing lens 1 is then shifted an optional distance, 
that is, the distance "" of FIG. 5. Thereafter, the above procedure is 
repeated so as to find the tele-end and the wide-end. 
The difference between the actual position of the focusing lens 10 and the 
theoretical position based on the trace data stored in the ROM of the 
control circuit 7 is the offset. For such offset adjustment, however, the 
conventional method requires a lot of time for the zoom lens to be 
reciprocated. This method also produces poor focusing upon zoom tracking 
because the offset value is derived only at the wide-end and the tele-end, 
thereby disabling trace error from being taken into consideration. 
In the conventional zoom tracking, recognition of positions of the zoom 
lens 1 and the focusing lens 10 is achieved in the same manner as 
mentioned above. The zoom tracking is executed by positioning the zoom 
lens 1 such that the zooming is initiated at a position where there is no 
erroneous determination for the trace caused by a read error of the 
control circuit 7 before the user executes the zooming, that is, at the 
point .alpha. of FIG. 6, positioning the focusing lens 10 at a position 
where an accurate focusing is obtained, and then calculating object 
distances a, b and c by use of data about the position of the zoom lens 1 
where the zooming is initiated, that is, the point a of FIG. 6 and data 
about the position of the focusing lens 10. 
Where the position of the object corresponds to the object distances a and 
c, the focusing lens 10 can be shifted along a trace associated with the 
object distances a and c upon shifting the zoom lens 1. Accordingly, the 
zoom tracking is achieved. 
However, where the object to be tracked and imaged by the zoom lens is 
positioned at a position b involving no trace data stored in the internal 
ROM of the control circuit 7, a detection is made for trace data 
associated with positions a and c of the upper and lower objects near to 
the object to be tracked and imaged, among trace data for all possible 
positions stored in the internal ROM of the control circuit 7. Thereafter, 
a calculation is made for the difference Da between the position of the 
focusing lens 10 associated with the upper object position a and the 
position of the focusing lens 10 associated with the lower object position 
c at a zooming start point, that is, the point .alpha. of FIG. 6, and the 
difference da between the position of the focusing lens 10 associated with 
the position b of the object to be tracked and imaged and the position of 
the focusing lens 10 associated with the lower object position c. Based on 
the calculated position differences Da and da, the position db of the 
focusing lens 10 associated with the position b of the object to be 
tracked and imaged is calculated. This calculation can be achieved using 
the following equation: 
##EQU1## 
As the zoom lens 1 is shifted, the focusing lens 10 tracks a trace stored 
in the internal ROM of the control circuit 7 or a trace defined by the 
calculation, thereby achieving a zoom tracking. 
In the conventional rear focusing system, however, the A/D converter 6 
encodes a differential voltage applied to the variable resistance of the 
zoom position sensor and sends the encoded signal to the control circuit 7 
so that the position of the zoom lens 1 can be recognized. the position of 
the focusing lens 10 is recognized using the number of driven steps of the 
stepping motor 11 with reference to the origin. As a result, accurate in 
recognition of the zoom lens position and sufficient resolution of the A/D 
converter are required. 
By one-bit error of the A/D converter 6, the position b of the object to be 
tracked and imaged may be erroneously determined as the upper object 
position a or the lower object position c, as shown in FIG. 7. In this 
case, the focusing lens 10 tracks the trace associated with the upper 
object position a or the lower object position c, thereby resulting in a 
bad focusing. This tendency is gradually increased as the zoom lens 1 
moves toward the tele-end. 
Such a problem can not be solved by checking the shifted steps of the 
focusing lens 10 and the resolution of an 8-bit A/D converter (2.sup.8 
=256). As a result, a quantity of data should be stored in the internal 
ROM of the control circuit. This results in an increase in data storage 
capacity and thereby an over-load of the control circuit. 
SUMMARY OF THE INVENTION 
Therefore, an object of the invention is to solve the above-mentioned 
problems encountered in the prior art and, thus, provide a zoom tracking 
apparatus and method in a video camera, wherein an offset adjustment is 
achieved by deriving applied voltage of the zoom position sensor only by 
use of data about positions of the zoom lens respectively associated with 
several optional points and corresponding positions of the focusing lens 
involving focusing while shifting the zoom lens in one direction, thereby 
capable of achieving the offset adjustment rapidly and accurately and 
obtaining an accurate focus upon zoom tracking. 
Another object of the invention is to provide a zoom tracking apparatus and 
method in a video camera, wherein the minimum quantity of trace data is 
stored in the internal ROM of the control circuit while trace tracking of 
the focusing lens associated with the shift of the zoom lens is achieved 
in a region where an error may occur possibly upon detecting the position 
of the zoom lens by use of a focusing evaluation value, thereby capable of 
achieving the trace tracking accurately. 
In accordance with one aspect, the present invention provides a zoom 
tracking apparatus in a video camera, comprising: a zoom lens for zooming 
an object; a focusing lens for focusing the object; photoelectric 
conversion means for converting a video signal passing through the zoom 
lens and the focusing lens into an electrical signal; evaluation value 
detection means for detecting an evaluation value from the output signal 
of the photoelectric conversion means; position detection means for 
detecting a position of the zoom lens and a position of the focusing lens; 
and control means for performing an offset adjustment by use of position 
data of both the zoom lens and the focusing lens outputted from the 
position detection means and controlling a zoom tracking by use of 
position data of both the zoom lens and the focusing lens corresponding to 
a maximum evaluation value. 
In accordance with another aspect, the present invention provides a zoom 
tracking method in a video camera including a zoom tracking apparatus 
having a zoom lens, a zoom position sensor, an A/D converter for encoding 
an output voltage of the zoom position sensor, control means for 
controlling the overall system of the zoom tracking apparatus, a focusing 
lens, a focus sensor, a focus detector for sending data about a position 
of the focusing lens sensed by the focus sensor to the control means, and 
a personal computer for receiving data from the control means and 
executing a calculation for an offset adjustment, the zoom tracking method 
comprising: a voltage range conforming step for conforming the zoom 
position sensor with the A/D converter in voltage range for zooming from 
the tele-end to the wide-end; a level shifting step for executing a level 
shifting to obtain a constant offset value; an end conforming step for 
conforming the tele-end and wide-end of the zoom position sensor with 
those of the A/D converter, respectively: and a zoom tracking step for 
executing a zoom tracking, the zoom tracking step comprising a first 
object distance calculating step for calculating the object distance, 
based on position data of both the zoom lens and the focusing lens 
associated with a maximum evaluation value obtained where the zoom lens 
passes through a point where there is no erroneous determination for the 
trace based on the object distance caused by a read error of the control 
circuit generated upon reading the position of the zoom lens, a second 
object distance calculating step for calculating the object distance, 
based on position data of both the zoom lens and the focusing lens 
associated with trace data for object distance where the zoom lens passes 
through a region where traces remarkably distinguished from other traces 
are generated irrespective of the read error of the control circuit 
generated upon reading the position of the zoom lens, and a zoom tracking 
execution step for executing a zoom is tracking, based on data about 
object distance obtained by the calculation of the first object distance 
calculating step or the calculation of the second object distance 
calculating step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 8 is a block diagram of a zoom tracking apparatus of a video camera in 
accordance with the present invention. This apparatus includes the 
elements of the conventional zoom tracking apparatus shown in FIG. 1 as 
its constituting elements. In FIG. 8, elements corresponding to those 
shown in FIG. 1 are denoted by the same reference numerals, respectively. 
The zoom tracking apparatus of the present invention further includes a 
charge coupled device (CCD) serving as photoelectric conversion means for 
converting an optical image emerging from the focusing lens 10 into an 
electrical video signal, an amplifier 14 for amplifying an output video 
signal from the CCD 13, and an evaluation value detecting circuit 15 for 
detecting an evaluation value from the video signal amplified by the 
amplifier 14 and sending the detected evaluation value to the control 
circuit 7. The control circuit 7 performs an offset adjustment, based on 
an applied voltage of the zoom position sensor 2. PC 13a derives that 
applied voltage using data on both the positions of the zoom lens 1 
corresponding to several optional points selected while shifting the zoom 
lens 1 only in one direction and the positions of the focusing lens 10 
where correct focusing for each of the optical points is achieved for the 
zoom lens 1. The control circuit 7 also calculates an object distance, 
based on position data of both the zoom lens 1 and the focusing lens 10 at 
the maximum evaluation value. Based on the calculated object distance, the 
control circuit 7 controls zoom tracking. 
The control circuit 7 stores in its internal ROM various trace data for the 
focusing lens 10 associated with various positions of the zoom lens 1. For 
each tele-end-side position of the zoom lens 1 with respect to a reference 
position of the zoom lens 1 where there is no erroneous determination for 
the trace based on the object distance caused by a read error of the 
control circuit 7 generated upon reading the position of the zoom lens 1 
(the point .alpha. of FIG. 14), the control circuit 7 stores the trace 
data of the focusing lens 10 associated with various object distances 
between the maximum object distance d and the minimum object distance g, 
for example, including object distances d, e, f and g. For each 
wide-end-side position of the zoom lens 1 with respect to the reference 
position, the control circuit 7 stores the trace data of the focusing lens 
10 associated with only the maximum and minimum object distances d and g. 
The control circuit 7 also stores the trace data of the focusing lens 10 
associated with object distances h and i generating traces remarkably 
distinguished from other traces irrespective of the read error of the 
control circuit 7 generated upon reading the position of the zoom lens 1. 
As shown in FIG. 8, the evaluation value detecting circuit 15 includes a 
pair of band pass filters 20 and 21 each detecting a radio frequency 
component from the video signal outputted from the amplifier 14 and 
determines whether an accurate focusing is obtained, based on the result 
of the detection. A low pass filter 22 detects a low frequency component 
from the video signal outputted from the amplifier 14 and determines a 
variation in luminance, based on the result of the detection. A switching 
circuit 23 selectively extracts output signals of the band pass filters 20 
and 21 and the low pass filter 22 under the control of the control circuit 
7. A A/D converter 24 converts the signal selectively extracted by the 
switching circuit 23 into a digital signal. A gate array 25 divides the 
digital data from the A/D converter 24 into a plurality of data portions, 
performs a digital integration for the data portions, sums all digital 
values of the integrated data portions to obtain an evaluation value, and 
sends the evaluation value to the control circuit 7. 
FIG. 9 is a flow chart illustrating a zoom tracking method for a video 
camera in accordance with the present invention. The zoom tracking method 
of the present invention includes an offset adjustment step S10 and a zoom 
tracking step S20. The offset adjustment step S10 includes three steps: a 
voltage range conforming step S11 for conforming the zoom position sensor 
2 with the A/D converter 6 in a voltage range for zooming from the 
tele-end to the wide-end; a level shifting step S12 for executing a level 
shifting to obtain a constant offset value; and an end conforming step S13 
for conforming the tele-end and wide-end of the zoom position sensor 2 
with those of the A/D converter 6, respectively. The zoom tracking step 
S20 also includes three steps: a first object distance calculating step. 
S21 for calculating the object distance, based on position data of both 
the zoom lens 1 and the focusing lens 10 associated with the maximum 
evaluation value obtained where the zoom lens 1 passes through a point 
where there is no erroneous determination for the trace based on the 
object distance caused by a read error of the control circuit 7 generated 
upon reading the position of the zoom lens 1; a second object distance 
calculating step S22 for calculating the object distance, based on 
position data of both the zoom lens 1 and the focusing lens 10 associated 
with trace data for object distance where the zoom lens 1 passes through a 
region where traces remarkably distinguished from other traces are 
generated irrespective of the read error of the control circuit 7 
generated upon reading the position of the zoom lens 1; and a zoom 
tracking execution step S23 for executing a zoom tracking, based on data 
on object distance obtained by the calculation of the steps S21 and S22. 
As mentioned above, the offset adjustment for achieving the zoom tracking 
in accordance with the present invention is accomplished by the offset 
adjusting step S10 of FIG. 9. This is offset adjustment will now be 
described in conjunction with FIG. 10. 
At a step S30, first, the PC 13a receives, via the control circuit 76, 
various data: high-level and low-level voltages ZPH and ZPL currently 
applied to the zoom position sensor 2; a voltage Va having a voltage range 
of 5 volts for a zooming from the tele-end to the wide-end; high-level and 
low-level voltages ADH and ADL applied to the A/D converter 6; data 
indicative of the resolution of the A/D converter 6; coded values from the 
A/D converter 6 for the tele-end and wide-end; and data on object 
distance. The PC 13a also receives a coded value a from the A/D converter 
6 corresponding to a position of the zoom lens 1 shifted by the zoom motor 
4 under control of the zoom motor driver 3, and shifted-position data of 
the focusing lens 10 associated with the shift of the zoom lens 1. 
Thereafter, several zoom positions are set and a search for a focus 
associated with each of the zoom positions is made at a step S31. The PC 
13a receives data (A/D-converted code) on zoom position recognized by the 
control circuit 7 for each set zoom position and data about a position of 
the focus lens 10 recognized by the control circuit for each searched 
focus. 
The current shift range of the zoom lens 1 is then calculated in terms of 
the voltage applied to the A/fD converter 6, by use of the high-level and 
low-level voltages ZPH and ZPL currently applied to the zoom position 
sensor 2, the voltage Va having the voltage range of 5 volts for the 
zooming from the tele-end to the wide-end, and the high-level and 
low-level voltages ADH and ADL applied to the A/D converter 6. The 
position value of the zoom lens 1 is also calculated in terms of the 
adjusted shift range of the zoom lens 1. This calculation is executed at a 
step S33 in accordance with the following equation: 
EQU a.sup.1 =a*(ADL-ADL/Resolution)*(TEC-WEC)/Va+TEC 
where, 
a.sup.1 : Calculated zoom Lens Position Code, 
a: Input Zoom Lens Position Code, 
ADH: Applied High voltage to A/D Converter, 
ADL: Applied Low Voltage to A/D Converter, 
TEC: Adjusted Target Tele-End Code from A/D Converter, 
WEC: Adjusted Target Wide-End Code from A/D Converter, 
Resolution: Resolution of A/D Converter, and 
Va: voltage Ranged for Zooming from Adjusted Tele-End to Adjusted W495 
ide-End. 
Thereafter, a determination is made whether the difference between the 
actual trace and the theoretical trace based on stored trace data, (the 
offset value) is constant (Step S34). Based on the result of the 
determination at the step S34, the position of the focusing lens 10 is 
searched in a manner as shown in FIGS. 11a and 11b. 
The checking of the offset value at the step S34 may be achieved by 
deriving an average of all possible positions of the zooming lens 1 and 
deriving a deviation of each of the positions from the average or by 
deriving a partial average of several positions between the wide-end and 
the tele-end. 
When the difference between the actual trace and the theoretical trace has 
been determined to be irregular at the step S34, the level shifting 
direction is determined at a step S35. That is, when the position 
difference of the focusing lens 10 at the wide-end is more than that at 
the tele-end, the level shifting toward the tele-end is determined and 
then executed (Step S36). Where the position difference of the focusing 
lens 10 at the wide-end is less than that at the tele-end, the level 
shifting toward the wide-end is determined and then executed. During the 
level shifting, the difference between the actual trace and the 
theoretical trace is continuously checked until the position difference of 
the focusing lens 10 is constant at all positions between the wide-end and 
the tele-end, as shown in FIG. 11c. When the position difference of the 
focusing lens 10 is constant at all positions, the level shifting is 
stopped and the tele-end voltage, the wide-end voltage, and the offset 
value at this state are derived (Steps S37 and S38). The values can be 
derived by use of the following equations: 
EQU ZTE'(V)=ABTV.+-.Number of Shifted Steps*Vb/(TEC-TWC) 
EQU ZWE'(V)=ZTE'+Vb 
where, 
ZTE': Tele-End Voltage of Zoom Lens, 
ZWE': wide-End voltage of Zoom Lens, and 
ADTV: Voltage Corresponding to TEC. 
The above equations are applied for TEC&lt;WEC. If TEC&gt;WEC, parameters for the 
wide-end and parameters for the tele-end in the equations ate exchanged. 
After completing the above procedure, low-level and high-level voltages of 
the zoom position sensor 2 are derived which are those adjusted so that 
the tele-end and wide-end voltages may be equal to the target tele-end and 
wide-end voltages of the A/D converter 6, respectively (step S39). These 
voltages can be derived by use of the following equations: 
##EQU2## 
where, ZPL': Applied Low-Level Voltage of Zoom Position Sensor, 
ZPH': Applied High-Level Voltage of zoom Position Sensor, 
ADTV: Voltage of A/D converter Corresponding to Tele-End 
ADWV: voltage of A/D converter Corresponding to Wide-End, 
Rx: Resistance Ratio at Tele-End, and 
Ry: Resistance Ratio at Wide-End. 
Based on the offset value derived as mentioned above, a trace error value 
at each position of the zoom lens 1 is then derived (Step S40). In this 
case, the error value corresponds to trace data obtained by correcting the 
actual focusing position data (namely, initial input data) by the offset 
value. 
Thereafter, the applied high-level and low-level voltages of the zoom 
position sensor 2, the offset value and the error value are sent to the 
control circuit 7 which, in turn, performs an offset adjustment, based on 
the received data, as shown in FIG. 12 (Step S41). 
Otherwise, the offset determination executed in the offset adjustment 
procedure for achieving the zoom tracking in accordance with the present 
invention may be achieved by deriving an average of offset values at all 
positions, by sampling a representative offset value, or by deriving an 
average of offset values except for the maximum and minimum offset values. 
The zoom tracking in accordance with the present invention will now be 
described. 
First, an optical image passing through the zoom lens 1 and then the 
focusing lens 10 is sent to the CCD 13 which, in turn, converts the 
received optical image into an electrical signal. This video signal is 
then amplified by the amplifier 14 and sent to a video signal processing 
circuit not shown. The video signal is also sent to the band pass filters 
20 and 21 and the low pass filter 22 so that it can be transmitted to the 
switching circuit 23 in the form of radio frequency components to be used 
for searching the focusing state and a low frequency component to be used 
for searching a variation in luminance. 
The switching circuit 23 selects repeatedly one of the filters 20 to 22 
under control of the control circuit 7, so as to sample output signals 
from the filters 20 to 22 in predetermined intervals. The sampled signals 
are sequentially transmitted to the A/D converter 24 which, in turn, 
converts the received signals into digital signals. The digitalized 
sampling data from the A/D converter 24 is then sent to the gate array 25 
which, in turn, divides the sampling data into a plurality of data 
portions in accordance with a predetermined data division. Thereafter, the 
gate array 25 performs a digital integration for the data portions, sums 
all digital values of the integrated data portions to obtain an evaluation 
value, and sends the evaluation value to the control circuit 7. Based on 
the received evaluation value, the control circuit 7 executes the zoom 
tracking. This procedure will now be described. 
First, the current position of the zoom lens 1, namely, the zooming start 
point is detected by the zoom position sensor 2 (Step S100). Thereafter, a 
determination is made whether the zoom lens 1 is to be shifted toward the 
tele-end or toward the wide-end (Step S101). Where the zoom lens 1 is to 
be shifted toward the tele-end, a determination is made whether the 
current position of the zoom lens 1 corresponds to a wide-end-side 
position with respect to a reference position of the zoom lens 1 where 
there is no erroneous determination for the trace based on the object 
distance caused by a read error generated upon reading the position of the 
zoom lens 1, that is, the point .alpha. of FIG. 14 (Step S102). If the 
current position of the zoom lens 1 corresponds to the wide-end-side 
position, a determination is then made whether the current object distance 
b is present between object distances h and i generating traces remarkably 
distinguished from other traces (Step 103). 
Actually, the reference position where there is no erroneous determination 
for the trace based on the object distance caused by the read error is not 
defined by one point, but defined by a predetermined region, as shown in 
FIG. 15, Accordingly, trace data for various object distances, for 
example, including the object distances d, e, f and g are present in the 
predetermined region. 
Where the current object distance b has not been determined to be present 
between object distances h and i generating traces remarkably 
distinguished from other traces at the step S103, that is, where the trace 
associated with the current object distance b is not present between 
traces respectively associated with the maximum and minimum object 
distances d and g, the stepping motor 11 is driven so that the focusing 
lens 10 can be shifted upward by one step, as shown in FIG. 16 (Step 
S104). Subsequently, a step S105 is executed for detecting an evaluation 
value of one-field-delayed video signal by the evaluation value detector 
15, Based on the result of the detection, a variation in evaluation value 
is checked. 
When an increment in evaluation value has been checked at the step S105, 
the steps S104 and S105 are repeatedly executed so that a variation in 
evaluation value can be checked again while shifting the focusing lens 10 
upward by one step. This procedure is continued until the focusing lens 10 
reaches a point where the evaluation value being incremented is 
decremented. This point is regarded as the point where the maximum focal 
distance is obtained. If the point is detected at the step S106, the 
driving of the stepping motor 11 is then stopped in order to stop the 
shift of the focusing lens 10 (Step S107). 
However, when the zoom lens 1 has been determined to be shifted toward the 
wide-end at the step S101 (FIG. 17), when the current position of the zoom 
lens 1 has been determined, at the step S102, to correspond to a 
tele-end-side position with respect to the reference position where there 
is no erroneous determination for the trace based on the object distance 
caused by the read error (FIG. 18), or when the current object distance b 
has been determined to be present between object distances h and i 
generating traces remarkably distinguished from other traces at the step 
103 (FIG. 19), basic data Da and da for calculating the trace are 
calculated using data about traces of both the zoom lens 1 and the 
focusing lens 10 associated with various object distances, for example, 
including the object distances d, e, f, g, h, and i, which data are stored 
in the internal ROM of the control circuit 7 (Step 110) (FIGS. 17 to 19). 
In other words, among the trace data associated with the object distances 
d, e, f, g, h, and i stored in the internal Ron of the control circuit 7, 
the data Da and the data da are detected which are indicative of the 
position difference of the focusing lens 10 resulting from the position 
difference between the upper and lower objects nearest to the object to be 
imaged and the position difference of the focusing lens 10 calculated 
using the difference between the position of the object to be imaged and 
the position of the lower object nearest to the object to be imaged. When 
a command for shifting the zoom lens 1 toward the tele-end is applied 
after execution of the step S107 (Step S108), the zoom motor 4 is driven 
so that the zoom lens 1 can be shifted toward the tele-end to execute a 
zooming (Step 109). On the other hand, when a command for shifting the 
zoom lens 1 toward the wide-end is applied after execution of the step 
S107, the zoom motor 4 is driven so that the zoom lens 1 can be shifted 
toward the wide-end to execute a zooming (Step 111). 
After executing the zooming, the current position of the zoom lens 1 is 
detected again by the zoom position sensor 2 (step S112). Thereafter, a 
determination is made whether the zoom lens 1 is being shifted toward the 
tele-end or toward the wide-end (Step S113). When the zoom lens 1 is being 
shifted toward the tele-end, a determination is made whether the current 
position of the zoom lens 1 corresponds to a tele-end-side position with 
respect to a reference position of the zoom lens 1 where there is no 
erroneous determination for the trace based on the object distance caused 
by a read error generated upon reading the position of the zoom lens 1, 
(the point .alpha. of FIG. 16) (Step S114). If the current position of the 
zoom lens 1 does not correspond to the teleend-side position, a 
determination is then made whether the current position of the zoom lens 1 
corresponds to the reference position (Step S115). When the current 
position of the zoom lens 1 does not correspond to the reference position, 
a determination is then made whether the current object distance b is 
present between object distances h and i generating traces remarkably 
distinguished from other traces (Step 116). Where the current object 
distance b is not present between the object distances h and i, a 
variation in evaluation value is checked by the evaluation value detector 
15 (Step S117). 
If an increment in evaluation value has been checked at the step S117, 
position data of the focusing lens 10 being shifted toward the tele-end in 
accordance with the shift of the zoom lens 1 is then stored in the 
internal ROM of the control circuit 7 (Step S118). If not, a determination 
is made whether the focusing lens 10 has reached a point where the 
evaluation value being incremented is decremented at the step S117. When 
the point is detected at the step S117, the shift state of the focusing 
lens 10 is then detected (Step S119). If the focusing lens 10 is being 
shifted, the driving of the stepping motor 11 is stopped in order to stop 
the shift of the focusing lens 10 (Step S120). If not, the shift speed of 
the focusing lens 10 is determined taking into consideration data on 
positions of both the zoom lens 1 and the focusing lens 10 at the maximum 
evaluation value and the shift speed of the zoom lens 1 (Step S121). 
Thereafter, the stepping motor 11 is driven so that the focusing lens 10 
can be shifted upwards, as shown in FIG. 16 (Step S122). 
Accordingly, as the zoom lens 1 is shifted toward the tele-end, the 
focusing lens 10 is shifted along the trace shown in FIG. 16. When the 
zoom lens 1 has been determined, at the step S115, to reach the reference 
position of the zoom lens 1 where there is no erroneous determination for 
the trace based on the object distance caused by a read error (the point 
.alpha. of FIG. 16), data Da is calculated which is indicative of the 
position difference of the focusing lens 10 using the difference between 
the positions e and f of the upper and lower objects nearest to the object 
to be imaged at the position of the zoom lens 1 associated with one of the 
points involving the maximum evaluation value and stored in the internal 
ROM of the control circuit 7 at the step S118, which point is present in 
the reference position range (range .alpha. of FIG. 16) where there is no 
erroneous determination for the trace based on the object distance caused 
by a read error, as shown in FIG. 16 (Step S123). At the step S123, data 
da is also calculated which is indicative of the position difference of 
the focusing lens 10 using the difference between the position b of the 
object to be imaged and the position f of the lower object nearest to the 
object to be imaged. Also, the number of objects to be imaged is 
calculated. 
When the zoom lens 1 reaches a target position, that is, the point of FIG. 
16 after executing the above procedure, data Db is calculated which is 
indicative of the position difference of the focusing lens 10 using the 
difference between the positions e and f of the upper and lower objects 
nearest to the object to be imaged and stored in the internal RON of the 
control circuit 7. Based on the calculated data Db, data db is calculated 
which is indicative of the position of the focusing lens 10 associated 
with the position of the object to be tracked at the shifted position of 
the zoom lens 1 (Step S124). Thereafter, a step S125 is executed for 
calculating the difference between the position of the focusing lens 10 
calculated at the step S123 and the position of the focusing lens 10 
associated with the position b of the object to be tracked and calculated 
at the step S124, and the shift direction. Based on the calculated 
position difference and the shift direction, the shift speed of the 
focusing lens 10 is calculated (Step S126). Subsequently, the stepping 
motor 11 is driven so that the focusing lens 10 can be shifted along the 
trace to be tracked (Step 127). 
On the other hand, when the zooming start point has been determined to 
correspond to the tele-end-side position with respect to the reference 
position where there is no erroneous determination for the trace based on 
the object distance caused by the read error (the point .alpha. of FIG. 
17) and the zoom lens 1 has been determined to be shifted toward the 
wide-end at the step S113 (FIG. 16), a determination is made whether the 
current position of the zoom lens 1 corresponds to the reference position 
(Step S128). When the current position of the zoom lens 1 does not 
correspond to the reference position, that is, when the target position of 
the zoom lens 1 is at the tele-end side with respect to the reference 
position (the point .alpha. of FIG. 17) the procedure proceeds to the step 
S124. At step 124, the position difference db of the focusing lens 10 at 
the target position of the zoom lens 1 (the point of FIG. 17) is 
calculated base on: the position difference Db of the focusing lens 10 
calculated using the difference between the positions e and f of the upper 
and lower objects nearest to the object to be imaged at the target 
position (the point of FIG. 16) and the data detected at the step S108, 
that is, the position difference Da of the focusing lens 10 calculated 
using the difference between the positions e and f of the upper and lower 
objects nearest to the object to be imaged at the zooming start point; and 
the position difference da of the focusing lens 10 calculated using the 
difference between the position b of the object to be imaged and the 
position f of the lower object nearest to the object to be imaged. After 
executing the step S124, the step S125 and the steps following the step 
S125 are executed. 
When the target position of the zoom lens 1 has been determined, at the 
step S128, to be at the wide-end side with respect to the reference 
position, a step S129 is executed for finding a point involving the 
maximum evaluation value in the reference position range (range .alpha. of 
FIG. 17) where there is no erroneous determination for the trace based on 
the object distance caused by a read error. The position difference Da of 
the focusing lens 10 is then calculated using the difference between the 
positions d and g of the upper and lower objects nearest to the object to 
be imaged at the point involving the maximum evaluation value and the 
position difference da of the focusing lens 10. The difference da is 
calculated using the difference between the position b of the object to be 
imaged and the position g of the lower object nearest to the object to be 
imaged at the point involving the maximum evaluation value. Thereafter, 
the procedure proceeds to the step S124 so as to calculate the position 
difference Db of the focusing lens 10 using the difference between the 
positions d and g of the upper and lower objects nearest to the object to 
be imaged at the target position of the zoom lens (the point .gamma. of 
FIG. 17). The position difference db of the focusing lens 10 is also 
calculated at step 124 using the difference between the position b of the 
object to be imaged and the position g of the lower object nearest to the 
object to be imaged at the target position of the zoom lens 1, based on 
the calculated position difference Db of the focusing lens 10. After 
executing the step S124, the step S125 and the steps following the step 
S125 are executed. 
When the zooming start point of the zoom lens 1 has been determined to 
correspond to the tele-end-side position with respect to the reference 
position where there is no erroneous determination for the trace based on 
the object distance caused by the read error (the point .alpha. of FIG. 
18) and the zoom lens 1 has been determined to be shifted toward the 
tele-end at the step S113 (FIG. 18), the procedure proceeds to the step 
S124. At step 124, the data calculated at the step S123 is detected. The 
position difference Da of the focusing lens 10 is calculated at step 123 
using the difference between the positions e and f of the upper and lower 
objects nearest to the object to be imaged at the zooming start point and 
the position difference da of the focusing lens 10 calculated using the 
difference between the position b of the object to be imaged and the 
position f of the lower object nearest to the object to be imaged. The 
position difference Db of the focusing lens 10 is calculated at step 124 
using the difference between the positions e and f of the upper and lower 
objects nearest to the object to be imaged at the target position (the 
point of FIG. 18). The position difference db of the focusing lens 10 is 
calculated at step 124 using the difference between the position b of the 
object to be imaged and the position f of the lower object nearest to the 
object to be imaged at the target position of the zoom lens 1, based on 
the calculated position difference Db. After executing the step S124, the 
step S125 and the steps following the step S125 are executed. 
When the zoom lens 1 is to be shifted toward the tele-end at the step S113, 
when the current position of the zoom lens 1 has been determined, at the 
step S114, to be at the tele-end-side with respect to the reference 
position where there in no erroneous determination for the trace based on 
the object distance caused by the read error, and when the current object 
distance b has been determined to be present between object distances h 
and i generating traces remarkably distinguished from other traces at the 
step 116 (FIG. 19), the procedure proceeds to the step S124. At step 124 
the data calculated at the step S123 is detected. The position difference 
Da of the focusing lens 10 is calculated in step 123 using the difference 
between the positions h and i or the upper and lower objects nearest to 
the object to be imaged and the position difference da of the focusing 
lens 10 calculated using the difference between the position b of the 
object to be imaged and the position i of the lower object nearest to the 
object to be imaged. The position difference Db of the focusing lens 10 is 
calculated in step 124 using the difference between the positions h and i 
of the upper and lower objects nearest to the object to be imaged at the 
target position (the point of FIG. 18). The position difference db of the 
focusing lens 10 is calculated in step 124 using the difference between 
the position b of the object to be imaged and the position f of the lower 
object nearest to the object to be imaged at the target position of the 
zoom lens 1, based on the calculated position difference Db. After 
executing the step S124, the step S125 and the steps following the step 
S125 are executed. 
As apparent from the above description, the present invention provides a 
zoom tracking apparatus and method in a video camera, capable of executing 
an offset adjustment rapidly and accurately. In accordance with the 
present invention, the quantity of data to be stored can be reduced. By 
virtue of such a reduction in data storage capacity, it is possible to 
reduce the use of RON equipped in a control circuit. The present invention 
also prevents bad focusing caused by position detection error in a zoom 
lens. As a result, it is possible to use A/D converters for the control 
circuit and thereby reduce the cost. 
Although the preferred embodiments of the invention have been disclosed for 
illustrative purposes, those skilled in the art will appreciate that 
various modifications, additions and substitutions are possible, without 
departing from the scope and spirit of the invention as disclosed in the 
accompanying claims.