Why is it easier to identify someone close than far away?

It is a matter of common sense that a person is easier to recognize when close than when far away. A possible explanation for why this happens begins with two observations. First, the human visual system, like many image-processing devices, can be viewed as a spatial filter that passes higher spatial frequencies, expressed in terms of cycles/degree, progressively more poorly. Second, as a face is moved farther from the observer, the face's image spatial frequency spectrum, expressed in terms of cycles/face, scales downward in a manner inversely proportional to distance. An implication of these two observations is that as a face moves away, progressively lower spatial frequencies, expressed in cycles/face--and therefore, progressively coarser facial details--are lost to the observer at a rate that is likewise inversely proportional to distance. We propose what we call the distance-as-filtering hypothesis, which is that these two observations are sufficient to explain the effect of distance on face processing. If the distance-as-filtering hypothesis is correct, one should be able to simulate the effect of seeing a face at some distance, D, by filtering the face so as to mimic its spatial frequency composition, expressed in terms of cycles/face, at that distance. In four experiments, we measured face perception at varying distances that were simulated either by filtering the face as just described or by shrinking the face so that it subtended the visual angle corresponding to the desired distance. The distance-as-filtering hypothesis was confirmed perfectly in two face perception tasks: assessing the informational content of the face and identifying celebrities. Data from the two tasks could be accounted for by assuming that they were mediated by different low-pass spatial filters within the human visual system that have the same general mathematical description but that differ in scale by a factor of approximately 0.75. We discuss our results in terms of (1) how they can be used to explain the effect of distance on visual processing, (2) what they tell us about face processing, (3) how they are related to "flexible spatial scale usage," as discussed by Schyns and colleagues, and (4) how they may be used in practical (e.g., legal) settings to demonstrate the loss of face information that occurs when a person is seen at a particular distance.