Imaging cameras have only finite depth of field and only those objects within that depth range are simultaneously in focus. The depth of field of a camera can be improved by mosaicing a sequence of images taken under different focal settings. In conventional mosaicing schemes, a focus measure is computed for every scene point across the image sequence and the point is selected from that image where the focus measure is highest. We have, however, proved in this paper that the focus measure is not the highest in the best-focused frame for a certain class of scene points. The incorrect selection of image frames for these points causes visual artifacts to appear in the resulting mosaic. We have also proposed a method to isolate such scene points, and an algorithm to compose large depth of field mosaics without the undesirable artifacts.