Method and apparatus for performing auto focus with multiple images having different exposure times

1. An autofocus (AF) adjusting method for an AF adjusting apparatus, the AF adjusting method comprising:

generating a first image signal by photographing an area during a first exposure time;

detecting a brightness saturation area included in the photographed area from the first image signal; and

if the brightness saturation area is not detected from the first image signal, calculating a focus detection evaluation value from the first image signal, and

if the brightness saturation area is detected from the first image signal, generating a second image signal by photographing the area during a second exposure time which is shorter than the first exposure time,

generating a composite image signal by combining the first and second image signals, and

calculating a focus detection evaluation value from the composite image signal.

2. The AF adjusting method of claim 1 , wherein the second exposure time is a maximum possible time during which pixel brightness of the second image signal remains unsaturated.

3. The AF adjusting method of claim 1 , wherein the second exposure time is set to a range of ½ to 1/10 of the first exposure time.

4. The AF adjusting method of claim 1 , wherein the detecting the brightness saturation area included in the photographed area from the first image signal comprises:

counting a number of pixels included in a reference brightness range; and

determining whether a ratio of the counted number of pixels to a total number of pixels is within a reference ratio range,

wherein if the ratio is not within the reference ratio range, the focus detection evaluation value is calculated from the first image signal, and if the ratio is within the reference ratio range, the second image signal is generated.

5. The AF adjusting method of claim 4 , wherein the reference brightness range comprises one or more of a minimum brightness value and a maximum brightness value, and

wherein the reference ratio range comprises one or more of a minimum ratio value and a maximum ratio value.

6. The AF adjusting method of claim 1 , wherein the AF adjusting apparatus comprises a movable focusing lens, and

wherein one or more of the first image signal, the second image signal, and the composite image signal are generated at each position where the focusing lens is moved.

7. The AF adjusting method of claim 1 , wherein the generating the composite image signal is performed by replacing a brightness saturation area of the first image signal with a corresponding area of the second image signal.

8. The AF adjusting method of claim 1 , wherein the first image signal and the second image signal are combined by using the following formula:

P 3( x,y )= P 1( x,y )× w 1+ P 2( x,y )×(1− w 1),

where P1(x, y) refers to a pixel brightness value at a coordinate (x, y) of the first image signal, P2(x, y) refers to a pixel brightness value at a coordinate (x, y) of the second image signal, and P3(x, y) refers to a pixel brightness value at a coordinate (x, y) of the composite image signal.

9. The AF adjusting method of claim 8 , wherein the brightness weight value w1 ranges from 0 to 1 and is determined by the following formula:

1− a ×( P 1( x,y )/ Y max),

where a refers to a positive number, and Ymax refers to a maximum brightness value.

10. An AF adjusting apparatus comprising:

an imaging lens comprising a movable focusing lens for focus control;

an image compositing determiner configured to determine generation of a composite image signal if a brightness saturation area is detected from a first image signal obtained by photographing an area during a first exposure time;

an image composer configured to generate the composite image signal by combining the first image signal and a second image signal obtained by photographing the area during a second exposure time shorter than the first exposure time; and

a focus determiner configured to calculate a focus detection evaluation value based on a composite image signal generated by the image composer and determine a position of the focusing lens.

11. The AF adjusting apparatus of claim 10 , wherein the second exposure time is set to a range of ½ to 1/10 of the first exposure time.

12. The AF adjusting apparatus of claim 10 , further comprising:

a reference parameter storage configured to store a reference brightness range and a reference ratio range; and

an image compositing determiner configured to count a number of pixels included in the reference brightness range and determine whether a ratio of the counted number of pixels to a total number of pixels is within the reference ratio range,

wherein if the ratio is within the reference ratio range, the image compositing determiner is configured to determine that a brightness saturation area is detected from the first image signal, and determine generation of the second image signal.

13. The AF adjusting apparatus of claim 12 , wherein if the image compositing determiner determines generation of the second image signal, the image composer is configured to combine the first image signal and the second image signal.

14. The AF adjusting apparatus of claim 12 , wherein the reference brightness range stored in the reference parameter storage comprises one or more of a minimum brightness value and a maximum brightness value, and

wherein the reference ratio range stored in the reference parameter storage comprises one or more of a minimum ratio value and a maximum ratio value.

15. The AF adjusting apparatus of claim 10 , further comprising:

an imaging device configured to receive light passing through the imaging lens and generate an image signal;

a shutter configured to determine a time during which the imaging device is exposed to the light; and

an imaging device controller configured to drive the imaging device and the shutter to generate one or more of the first image signal, the second image signal, and the composite image signal at each position where the focusing lens is moved.

16. The AF adjusting apparatus of claim 10 , wherein the image composer is configured to generate the composite image signal by replacing a brightness saturation area of the first image signal with a corresponding area of the second image signal.

17. The AF adjusting apparatus of claim 10 , wherein the image composer is configured to combine the first image signal and the second image signal by using the following formula:

P 3( x,y )= P 1( x,y )× w 1+ P 2( x,y )×(1− w 1),

where P1(x, y) refers to a pixel brightness value at a coordinate (x, y) of the first image signal, P2(x, y) refers to a pixel brightness value at a coordinate (x, y) of the second image signal, and P3(x, y) refers to a pixel brightness value at a coordinate (x, y) of the composite image signal.

18. The AF adjusting apparatus of claim 17 , wherein the brightness weight value w1 is inversely proportional to P1(x, y).