Focus distance estimation from photographed faces: a test of PerspectiveX using 1709 frontal and profile photographs from DSLR and smartphone cameras

As focus distance (FD) sets perspective, it is an important consideration for the forensic analysis of faces in photographs, including those used for craniofacial superimposition. In the craniofacial superimposition domain, the PerspectiveX algorithm has been suggested for FD estimation. This algorithm uses a mean value of palpebral fissure length, as a scale, to estimate the FD. So far, PerspectiveX has not been validated for profile view photographs or for photographs taken with smartphones. This study tests PerspectiveX in both front and profile views, using multiple DSLR cameras, lenses and smartphones. In total, 1709 frontal and 1709 profile photographs of 10 adult participants were tested at 15 ground truth FDs using three DSLR cameras with 12 camera/lens combinations, five smartphone back cameras and four smartphone front cameras. Across all distances, PerspectiveX performed with a mean absolute error (MAE) of 11% and 12% for DSLR photographs in frontal and profile views, respectively, while errors doubled for frontal and profile photographs from smartphones (26% and 27%, respectively). This reverifies FD estimation for frontal DSLR photographs, validates FD estimates from profile view DSLR photographs and shows that FD estimation is currently inaccurate for smartphones. Until such time that FD estimations for facial photographs taken using smartphones improves, DSLR or 35 mm film images should continue to be sought for craniofacial superimpositions. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00414-023-03078-y.