At near observation distances (e.g., 1–2 m), not only can depth differences based on binocular disparity be discriminated but the magnitude of the depth separation can be perceived. Previous research has shown evidence of stereoscopic improvements in discriminating real depths at much larger distances (17 m and beyond—e.g., Hirsch & Weymouth,
1948; Jameson & Hurvich,
1959; Teichner, Kobrick, & Wehrkamp,
1955). However stereopsis cannot be understood on the basis of depth discrimination data alone. Perceived depth magnitude, about which little is known at larger distances, is also important. The veridical perception of depth magnitude, across changes in observation distance and despite variations in binocular disparity, is known as stereoscopic depth constancy. As
Equation 1 shows, this would require that horizontal angular disparity be scaled for the observation distance. It has been established that stereoscopic depth constancy does occur for observation distances of 2 m or less (Cormack,
1984; Ono & Comerford,
1977; Wallach & Zuckermann,
1963) but is not perfect (e.g., Johnston,
1991). At these near distances, the cues of vertical disparity, accommodation, and convergence could all be used to scale binocular disparity information and derive depth (Foley,
1985; Foley & Richards,
1972; Gillam, Chambers, & Lawergren,
1988; Gillam & Lawergren,
1983; Gogel,
1977; Mayhew & Longuet-Higgins,
1982; Rogers & Bradshaw,
1993; Wallach & Zuckermann,
1963). O'Leary and Wallach (
1980) have also claimed that perspective and familiar size cues are used to scale depth from binocular disparity at near observation distances (0.4–0.8 m). While larger observation distances will be beyond the range of vertical disparity and oculomotor cues, perspective (and potentially also familiar size)-based information about distance should still be available for disparity scaling. For example, it has been shown that blindfolded people can walk accurately to previously viewed targets (in full cue conditions) located at distances as far away as 12 m (Loomis, Da Silva, Fujita, & Fukusima,
1992). Levin and Haber (
1993) also appear to show that absolute distances can be estimated quite accurately as far away as 20 m. Linear perspective from the ground plane (possibly used in conjunction with eye height) is likely to be an important perspective-based source of information that could provide this absolute distance (Sedgwick,
1986; Thompson, Dilda, & Creem-Regehr,
2007; Wu, He, & Ooi,
2007). However, there are a variety of other perspective-based cues that should also be available in well-lit real-world environments (e.g., texture gradients, relative size, aspect ratio—see Gillam,
1995; Sedgwick,
1986 for a review). The current study examined whether perspective and familiar size-based distance cues could be used to scale binocular disparities ranging from 0 to 5 arcmin between a pair of target points, when the closer of these target points was located at a distance of either 20 or 40 m. Throughout this paper, we will refer to the distance of the closest target point as the
observation distance.