Method of automatically focusing using a quadratic function in camera
View Patent ↗A method of automatically focusing a camera determines a position of a focus lens where an amount of a high frequency in an image signal is a maximum. The amount of a high frequency included in an image signal is measured at intervals of multiple focus motor steps. Three of the measured positions (x 1 , x 2 , x 3 ) and their corresponding measured high frequency amounts (y 1 , y 2 , y 3 ) are identified, where y 2 is the largest of the measured high frequency amounts, and x 1 and x 3 are the nearest measured positions on either side of x 2 . A quadratic function f(x) is determined for the high frequency amount with respect to the focus lens position x. A position at which a derivative ⅆ f ( x ) ⅆ x of the quadratic function f(x) is zero, is determined to be the position corresponding to maximum high frequency amount. Thus, the driving time of the focus motor can be reduced.
1. A method of automatically focusing a camera, comprising the steps of:
measuring a high frequency amount in an image signal according to a position of a focus lens at three or more positions;
identifying at least three positions, x 1 , x 2 , and x 3 along with their corresponding high frequency amounts y 1 , y 2 , and y 3 ; respectively;
determining an absolute value of a difference between y 2 and y 1 ;
determining an absolute value of a difference between y 1 and y 3 ; and
if the absolute value of the difference between y 2 and y 1 is not less than a first threshold value, and if the absolute value of the difference between y 1 and y 3 is not less than a second threshold value, then proceeding with the steps of determining a quadratic function f(x) where f(x) is a high frequency amount in the image signal and x is a position of the focus lens, and determining a position where a derivative
ⅆ
f
(
x
)
ⅆ
x
of the quadratic function f(x) is equal to zero corresponding to largest amount of high frequency.
2. The method of claim 1 wherein the quadratic function is of the form,
f ( x )+ a+bx=cx 2 ,
where a, b, and c are constants.
3. The method of claim 2 , where the step of determining the position corresponding to the largest amount of high frequency further comprises:
setting m=(−b)/2c, where m is the position of the focus lens corresponding to the largest high frequency amount.
4. The method of claim 2 , wherein the step of determining the quadratic function f(x) further comprises:
determining values of the constants.
5. The method of claim 1 wherein y 2 is the highest measured frequency amount out of the three or more amounts measured.
6. The method of claim 1 wherein x 1 and x 3 are measured positions before and after x 2 .
7. The method of claim 1 wherein, out of the three or more positions at which the high frequency amount is measured, x 1 and x 3 are the measured positions that are closest before and after x 2 .
8. The method of claim 1 wherein the three or more measured positions are spaced apart at equal intervals.
9. The method of claim 1 , further comprising the steps of:
determining an absolute value of a difference between y 2 and y 3 ; and
if both the absolute value of the difference between y 2 and y 1 is less than a first threshold value, and the absolute value of the difference between y 2 and y 3 is less than a first threshold value,
then determining a position corresponding to largest amount of high frequency by direct measurement and skipping the step of determining a quadratic function f(x).
10. The method of claim 1 wherein the quadratic function is an approximation.
11. The method of claim 1 wherein the position corresponding to the largest amount of high frequency in an image signal is an approximation.
12. The method of claim 1 wherein the position of the focus lens is an integer multiple of a step unit.
13. A method of automatically focusing a camera, comprising the steps of:
measuring a high frequency amount in an image signal according to a position of a focus lens at three or more positions;
identifying at least three positions, x 1 , x 2 , and x 3 along with their corresponding high frequency amounts y 1 , y 2 , and y 3 ; respectively;
determining an absolute value of a difference between y 2 and y 3 ;
determining an absolute value of a difference between y 1 and y 3 ; and
if the absolute value of the difference between y 2 and y 3 is not less than a first threshold value, and if the absolute value of the difference between y 1 and y 3 is not less than a second threshold value, then proceeding with the steps of determining a quadratic function f(x) where f(x) is a high frequency amount in the image signal and x is a position of the focus lens, and determining a position where a derivative
ⅆ
f
(
x
)
ⅆ
x
of the quadratic function f(x) is equal to zero corresponding to largest amount of high frequency.
14. The method of claim 13 wherein the quadratic function is of the form,
f ( x )= a+bx+cx 2 ,
where a, b, and c are constants.
15. The method of claim 14 where the step of determining the position corresponding to the largest amount of high frequency further comprises:
setting m=(−b)/2c, where m is the position of the focus lens corresponding to the largest high frequency amount.
16. The method of claim 14 , wherein the step of determining the quadratic function f(x) further comprises:
determining values of the constants.
17. The method of claim 13 wherein y 2 is the highest measured frequency amount out of the three or more amounts measured.
18. The method of claim 13 wherein x 1 and x 3 are measured positions before and after x 2 .
19. The method of claim 13 wherein, out of the three or more positions at which the high frequency amount is measured, x 1 and x 3 are the measured positions that are closest before and after x 2 .
20. The method of claim 13 wherein the three or more measured positions are spaced apart at equal intervals.
21. The method of claim 13 wherein the quadratic function is an approximation.
22. The method of claim 13 wherein the position corresponding to the largest amount of high frequency in an image signal is an approximation.
23. The method of claim 13 wherein the position of the focus lens is an integer multiple of a step unit.
24. A method automatically focusing a camera, comprising the steps of:
measuring a high frequency amount in an image signal according to a position of a focus lens at three or more positions;
identifying at least three positions, x 1 , x 2 , and x 3 along with their corresponding high frequency amounts y 1 , y 2 , and y 3 ; respectively;
determining an absolute value of a difference between y 2 and y 1 ;
determining an absolute value of a difference between y 2 and y 3 ;
determining an absolute value of a difference between y 1 and y 3 ; and
if either the absolute value of the difference between y 2 and y 1 is not less than a first threshold value, or the absolute value of the difference between y 2 and y 3 is not less than a first threshold value, and
if the absolute value of the difference between y 1 and y 3 is less than a second threshold value,
then setting x 2 as a position corresponding to largest amount of high frequency.
25. The method of claim 24 wherein y 2 is the highest measured frequency amount out of the three or more amounts measured.
26. The method of claim 24 wherein x 1 and x 3 are measured positions before and after x 2 .
27. The method of claim 24 wherein, out of the three or more positions at which the high frequency amount is measured, x 1 and x 3 are the measured positions that are closest before and after x 2 .
28. The method of claim 24 wherein the three or more measured positions are spaced apart at equal intervals.