1.  Field of the Invention
The present invention relates to a camera system which can emit a flash as supplemental illumination upon taking a photograph. 
2.  Description of the Related Art
Conventionally, the TTL auto flash scheme has been used to control the flashing device of a film camera of the monocular reflex type.  Some of these film cameras emit a preflash prior to a main flash upon taking a photograph to measure reflected light from an object and thereby obtain information for the main flash. 
On the other hand, digital cameras have an image sensor such as CCDs placed at the position of the photographic film, thus being dimmed with difficulty using light diffused on the surface of the image sensor.  Accordingly, a well-known digital camera of the TTL auto flash type is employed.  The camera emits a preflash prior to a main flash upon taking a photograph, measures reflected light from an object, calculates the flash amount required for the main flash in accordance with the measurement result of the preflash, and then emits the main flash with the calculated flash amount.  Some of these digital cameras emit a preflash under mirror lockup to measure the reflected light of the preflash relative to the flux of light diffused on the surface of the shutter curtain.  Based on this measurement result, it is calculated how many times the main flash amount should be greater than the preflash amount.  Some other digital cameras allow a photometric sensor provided in the viewfinder optical system to measure the reflected light of the preflash under mirror lockdown and calculate the intensity of the main flash through computation for flashing at the resulting intensity. 
Some of these film cameras employing the TTL auto flash and digital cameras are configured particularly to provide the information about the reflectance of an object in accordance with the measurement result of reflected light upon emitting a preflash and the object-distance information from the photographic lens or the focusing section to optimize the main flash amount.  That is, what these cameras do first is obtain information about whether the object has a standard reflectance, a low reflectance, or a high reflectance.  Then, in accordance with the information obtained, the cameras assign weights to divided dimming regions to calculate the main flash amount or control the dimming sensor. 
Publication of Japanese Patent No.  3136658 (hereinafter denoted as Patent Document 1) describes a camera with a flash controlling device which employs the measurement result of reflected light upon emitting a preflash, the object-distance information, and the object-distance information detection error to determine the region used for dimming. 
On the other hand, Japanese Unexamined Patent Application Publication No.  2003-66504 (hereinafter denoted as Patent Document 2) describes a camera which corrects the main flash amount in accordance with the measurement result of reflected light upon emitting a preflash, the object-distance information, and the object-distance information detection accuracy. 
However, as described above, the following problems were found with the cameras which obtained information regarding the object reflectance from the measurement result of reflected light upon emitting a preflash and the object-distance information to assign weights to the dimming regions or calculate the dimming correction amount.  That is, an error present in the object-distance information would lead to an error in the reflectance information, causing weights to be improperly assigned to the dimming regions.  This would cause the dimming correction amount to be improperly calculated, thus resulting in an improperly set flash amount. 
To obtain the object-distance information, a method is known which allows an encoder to detect the rotational position or the amount of rotation of the focusing ring of the photographic lens.  Also known is such a method which employs defocus degree information detected by an auto focusing sensor. 
However, the above-described methods do not directly measure distances, thus causing the resulting object-distance information to include a relatively significant error.  In particular, a photographic lens with a short focal length is greater in depth of field than one with a longer focal length, thus providing a smaller rotational angle to the focusing ring relative to a change in photographic distance (object distance).  Accordingly, when a photographic lens with a short focal length is used, the error included more significant which is contained in the object-distance information obtained from the rotational position of the focusing ring will be more significant. 
When focus is achieved through auto-focusing, the error of the object-distance information also becomes significant because the object distance falls within a wider range for the same defocusing amount being allowed.  For example, when all distances farther than a certain distance fall within the depth of field, no object-distance information is obtained on distances farther than that distance. 
Furthermore, a wide angle lens with a short focal length provides lower optical image magnification, which often causes the object to be reduced in size.  Accordingly, when a wide angle lens with a short focal length is employed with a bright portion present in the background, the auto focusing sensor is more likely to detect the bright portion in the background to focus the background.  In this case, information on an object distance farther than the distance of the actual main object is output, causing the error to be more significant. 
This point is considered in Patent Documents 1 and 2. 
In Patent Document 1, the dimming region is determined using the object-distance information detection error stored in the photographic lens with the object-distance information.  This object-distance information detection error is stored as information unique to the photographic lens in combination with the object-distance information.  When the photometric value of reflected light in a region upon emitting a preflash is greater than or lower than a predetermined value based on the distance detection error, the region is not allowed to contribute to dimming.  In other words, the region is excluded from contributing to the process of dimming.  The wide angle lens prevents the region from being accidentally excluded according to the object-distance information having a significant error, because the region is not easily excluded.  A region is not easily excluded because, when the object-distance information is on farther distance sides, detection errors are significant. 
On the other hand, in Patent Document 2, the dimming correction amount is calculated as the reflectance correction value based on the preflash measurement result and the object-distance information.  In this calculation, the method of calculating the reflectance correction value is changed with the product between the focal length and the optical image magnification employed as a parameter.  That is, the correction can be readily made when the value of the product between the focal length and the optical image magnification is large (i.e., when the focal length is long and the photographic distance is short).  On the contrary, when the product between the focal length and the optical image magnification is small (i.e., when the focal length is short and the photographic distance is long), the correction is made with greater difficulty.  As described above, when the wide angle lens is employed and the photographic distance is relatively long, the object-distance information is expected to include a significant error.  However, Patent Document 2 avoids making an inappropriate dimming correction based on erroneous object-distance information in the manner mentioned above. 
As described above, both Patent Documents 1 and 2 employ the wide angle lens and provide control such that a positive corrections are not to be made using the object-distance information when the photographic distance is relatively long, thereby preventing improper control. 
However, as described above, the wide angle lens has a large depth of field during the use of auto focusing, and falls within a wide distance range for the same amount of defocusing allowed, thus causing the stopping position of the lens focusing ring to vary within a certain range.  In addition, in the presence of a brighter portion in the background as mentioned above, there is a high possibility that the background is focused because the portion may exert a stronger effect.  This also raises a problem in the dimming level being varied. 
Suppose that the lens stops at a relatively close distance side position of the focusing ring.  In this case, the object-distance information corresponds to a short distance.  In this case, according to the configuration of Patent Document 1, a relatively small value is output as the aforementioned object-distance information detection error, thus allowing a dimming region to be relatively easily excluded.  Suppose that the lens stops at a relatively distant position of the focusing ring, and the same object as mentioned above (where the lens has stopped at a relatively close distance side position of the focusing ring) is photographed at the same distance.  In this case, the object-distance information corresponds to a long distance, and the object-distance information detection error becomes significant, which makes the exclusion of the dimming region difficult. 
Also, in Patent Document 2, when the lens has stopped at a relatively close distance side position of the focusing ring, the object-distance information indicates a value corresponding to a short distance with the optical image magnification being relatively large, thus allowing a correction to be readily made to the amount of dimming.  However, when the lens has stopped at a relatively distant side position and the same object as mentioned above (where the lens has stopped at a relatively close distance side position of the focusing ring) is photographed at the same distance, the optical image magnification is decreased, which makes the correction of the amount of dimming difficult. 
As described above, both the Patent Documents 1 and 2 have a problem in that the flash amount varies as a result of variations in stopping positions of the focusing ring. 
Suppose that a scene of a man standing with no background is photographed using a wide angle lens.  In this case, the area of the man is often small due to a reduced optical image magnification.  Using the conventional TTL auto flash (the conventional technique of performing neither region exclusion or auto dimming correction as described in Patent Documents 1 and 2) to photograph such a scene would cause overexposure due to the reduced area of the object reflecting the illuminated light.  To photograph such a scene, the flash amount would be appropriately set only if the region exclusion and the auto dimming correction as described in Patent Documents 1 and 2 work properly. 
However, when the wide angle lens is used as described above, the stopping position of the lens varies significantly because of auto focusing, which causes large variations in the object-distance information obtained.  Accordingly, the scene may be taken properly in some cases, whereas overexposure may be caused in other cases due to the region exclusion and the auto dimming correction not working properly.  Thus, there was a problem in that use of a wide angle lens caused variations in the amount of dimming and, thus, resulted in unstable control.