Focus detection method and focus detection device

1. A focus detection method for detecting a focus state based on object images formed through a photographing optical system in at least one focus detection area, while performing a search operation in which a focusing lens group of said photographing optical system is moved from one toward the other of a near extremity and a far extremity, said focus detection method comprising:

determining at least one support area which overlaps said focus detection area;

determining a position of said focusing lens group at which an in-focus state is detected in said focus detection area to be an in-focus position of said focusing lens group when a brightness of said object is one of equal to and higher than a predetermined value; and

determining a position of said focusing lens group at which said in-focus state is detected in said focus detection area to be the in-focus position, upon an in-focus state being detected both in said focus detection area and in said support area which overlaps said focus detection area when said brightness of said object is lower than said predetermined value.

2. The focus detection method according to claim 1 , wherein said focus detection method is based on a contrast AF system which detects contrasts of an image while moving said focusing lens group stepwise, and determines a position of said focusing lens group at which a peak contrast is obtained to be an in-focus position of said focusing lens group.

3. The focus detection method according to claim 2 , wherein, when said position of said focusing lens group at which said in-focus state is detected in said focus detection area and said position of said focusing lens group at which said in-focus state is detected in said support area are positioned within a predetermined range of stepping positions and when said brightness of said object is lower than said predetermined value, said position of said focusing lens group at which said in-focus state is detected in said focus detection area is determined to be said in-focus position of said focusing lens group.

4. The focus detection method according to claim 1 , wherein said focus detection area overlaps at least two support areas, and

wherein, upon said in-focus states being detected in both said focus detection area and at least one of said at least two support areas, said position of said focusing lens group at which said in-focus state is detected in said focus detection area is determined to be said in-focus position of said focusing lens group when said object brightness is lower than said predetermined value.

5. The focus detection method according to claim 1 , further comprising:

stopping said search operation upon determining said position of said focusing lens group to be said in-focus position of said focusing lens group even if said focusing lens group has not yet reached said other of said near extremity and said far extremity.

6. The focus detection method according to claim 5 , further comprising:

adding conditions in order to determine said position of said focusing lens group to be said in-focus position of said focusing lens group when stopping said search operation before said focusing lens group reaches said other of said near extremity and said far extremity.

7. The focus detection method according to claim 1 , wherein each of at least two focus detection areas overlaps at least one adjacent focus detection area.

8. The focus detection method according to claim 1 , wherein said focusing lens group is moved stepwise by a stepping motor.

9. The focus detection method according to claim 1 , wherein a plurality of focus detection areas and a plurality of support areas are arranged within a predetermined range corresponding to an area defined by a focus frame.

10. The focus detection method according to claim 1 , wherein the position of said focusing lens group at which said in-focus state is detected in said focus detection area is determined to be the in-focus position, upon an in-focus state being detected both in said focus detection area and in said support area at the same time, when said brightness of said object is lower than said predetermined value.

11. The focus detection method according to claim 1 , wherein the focus detection area and the at least one support area which overlaps the focus detection area each have a fixed size.

12. A focus detection device comprising:

an image pickup device which converts an object image that is formed via a photographing optical system into electrical image data;

a lens driving device which moves a focusing lens group of said photographing optical system along an optical axis thereof; and

a controller for detecting a focus state based on image data, converted by said image pickup device, within at least one focus detection area and at least one support area, which overlaps said focus detection area, which are obtained at a plurality of focusing lens group positions while said focusing lens group is moved along said optical axis via said lens driving device,

wherein said controller determines a position of said focusing lens group at which an in-focus state is detected in said focus detection area to be an in-focus position of said focusing lens group when an object brightness is one of equal to and higher than a predetermined value, and

wherein said controller detects a focus state in said focus detection area and said support area, and upon an in-focus state being detected in both said focus detection area and said support area which overlaps said focus detection area, said controller determines a position of said focusing lens group at which said in-focus state is detected in said focus detection area to be an in-focus position of said focusing lens group when said object brightness is lower than said predetermined value.

13. The focus detection device according to claim 12 , wherein the controller determines the position of said focusing lens group at which said in-focus state is detected in said focus detection area to be the in-focus position, upon an in-focus state being detected both in said focus detection area and in said support area at the same time, when said brightness of said object is lower than said predetermined value.

14. The focus detection device according to claim 12 , wherein said focus detection area overlaps at least two support areas, and

wherein, upon said in-focus states being detected in both said focus detection area and at least one of said at least two support areas, said position of said focusing lens group at which said in-focus state is detected in said focus detection area is determined to be said in-focus position of said focusing lens group when said object brightness is lower than said predetermined value.

15. The focus detection device according to claim 12 , wherein the focus detection area and the at least one support area which overlaps the focus detection area each have a fixed size.