Depth of field affects perceived depth-width ratios in photographs of natural scenes

The aim of the study was to find out how much influence depth of field has on the perceived ratio of depth and width in photographs of natural scenes. Depth of field is roughly defined as the distance range that is perceived as sharp in the photograph. Four different semi-natural scenes consisting of a central and two flanking figurines were used. For each scene, five series of photos were made, in which the distance in depth between the central figurine and the flanking figurines increased. These series of photographs had different amounts of depth of field. In the first experiment participants adjusted the position of the two flanking figurines relative to a central figurine, until the perceived distance in the depth dimension equaled the perceived lateral distance between the two flanking figurines. Viewing condition was either monocular or binocular (non-stereo). In the second experiment, the participants did the same task but this time we varied the viewing distance. We found that the participants' depth/width settings increased with increasing depth of field. As depth of field increased, the perceived depth in the scene was reduced relative to the perceived width. Perceived depth was reduced relative to perceived width under binocular viewing conditions compared to monocular viewing conditions. There was a greater reduction when the viewing distance was increased. As photographs of natural scenes contain many highly redundant or conflicting depth cues, we conclude therefore that local image blur is an important cue to depth. Moreover, local image blur is not only taken into account in the perception of egocentric distances, but also affects the perception of depth within the scene relative to lateral distances within the scene.