This option allows you to draw a box around a person's face and tag them when Faces doesn't recognize them. In addition to rescanning for missing faces, the original option of manually adding a missing face is still available. IPhoto's contextual menu offers new options for scanning faces. In a similar vein, if iPhoto initially identified something as a face - and you used the 'x' icon in the face box to tell the software that the object was not actually a face - rescanning those photos for missing faces takes you back to square one: iPhoto will re-identify the objects as faces. Doing so, however, doesn't adjust the algorithm used to scan your entire library, which could result in extra hits of nonface objects being accidentally detected as faces. One important point is that this feature works on a photo-by-photo basis, meaning that you can use it when a face is not recognized. There were easily dozens, if not hundreds, of photos in my iPhoto library where one or more faces weren't recognized when I first tried out iPhoto. The result is that Faces will be more likely to detect, well, faces. In this update, a contextual menu item (available by right-clicking or control-clicking) on a photo allows you to tell iPhoto to rescan the picture and detect missing faces, being less stringent with its recognition algorithm. This was particularly common in photos where someone's face is turned to one side or was shot in low light, or in slightly blurry photos. One of the problems with Faces in its initial release is that it occasionally wouldn't recognize a person's face as a face.

Apple photos rescan for faces