Smile photograph analysis and its connection with focal length as one of identification methods in forensic anthropology and odontology

To positively identify a highly decomposed body forensic experts often use information obtained from the dentition. In this study authors try to employ a method of comparing incisal borders of the anterior teeth from antemortem (AM) and postmortem (PM) photographs within an experimental scenario with living individuals. The second purpose was to determine how focal length of the lens affects compared smile lines. The research was divided into two stages. In procedure 1, the participants (28 individuals) were asked to provide a photo (for the purpose of the research considered as "antemortem") in which the anterior teeth were visible. A series of experimental photographs (considered as "postmortem") were then taken trying to reproduce the position of the person's head in relation to “AM” picture as faithfully as possible. The procedure 2 (10 individuals) consisted of taking “AM” photos under controlled conditions using an accidental focal length, then taking “PM” photos using a priori established focal lengths: 18 mm, 55 mm and 80 mm. In both procedures, the final stage involved marking and comparing the incisal borders from “AM” and “PM” photographs. Procedure 1 showed that in 82% of cases the compared smile lines are fully compatible. 11% was marked as "tolerable", and only 2% as "insufficient". Procedure 2 proved that the choice of focal length when taking a PM photograph is significant and that the medium focal lengths (range of 55–85 mm) are suggested in case of unknown focal length in AM photo. The results indicate that this method can be reliably applied in forensic cases when AM photographs of deceased are available. •Smile photography is an important tool for identification process.•A detailed procedure of incisal line comparison for identification purpose is provided.•A detailed description of the accuracy of results is provided.•Incisal line comparison delivers satisfactory results in identification process.•The choice of focal length in a postmortem photograph of a smile line is relevant.