Criminal investigations often use photographic evidence to identify suspects. Here we combined robust face perception and high-resolution photography to mine face photographs for hidden information. By zooming in on high-resolution face photographs, we were able to recover images of unseen bystanders from reflections in the subjects' eyes. To establish whether these bystanders could be identified from the reflection images, we presented them as stimuli in a face matching task (Experiment 1). Accuracy in the face matching task was well above chance (50%), despite the unpromising source of the stimuli. Participants who were unfamiliar with the bystanders' faces (n = 16) performed at 71% accuracy [t(15) = 7.64, p39 megapixel cameras routinely. However, as the current study emphasizes, the extracted face images need not be of high quality in order to be identifiable. For this reason, obtaining optimal viewers - those who are familiar with the faces concerned - may be more important than obtaining optimal images.
Animated zoom on the cornea of a high-resolution photographic subject. The zoom begins with a passport photo-style framing of the subject, and ends with a full face close-up of a bystander captured in the subject's corneal reflection. Successive movie frames represent a linear magnification of 6%. Each frame was resized to 720 pixels wide ×540 pixels high using bicubic interpolation to reduce high spatial frequency noise. Contrast was enhanced separately for each frame using the Auto Contrast function in Adobe Photoshop to improve definition. The image sequence was then converted to movie format for viewing.
We thank Stuart Campbell at the Photographic Unit at the University of Glasgow for high resolution photography, Llian Alys at the National Policing Improvement Agency (NPIA UK) for pointing out forensic applications, and an anonymous reviewer for inspiring Experiment 2. Original high-resolution photographs and performance data are available from the corresponding author.
Conceived and designed the experiments: RJ. Performed the experiments: CK RJ. Analyzed the data: CK RJ. Contributed reagents/materials/analysis tools: RJ. Wrote the paper: RJ.
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