Perception of Slope in Photographs

Perceptions of characteristics of space such as slope, distance, and depth are frequently inaccurate, both in the real world and in pictures. We carried out experiments to study factors that influence the accuracy of perceived slope in photographs. Slopes varied in angle from 5° to 45° inclinations against the horizontal, and in the information available to the observer (outline shape and texture characteristics). We found that perceived slope is correlated with real slope (r=0.99), but that observers consistently overestimate slope. The latter depends not only on the available information, but also on the focal length of the lens with which slopes were photographed. Overestimation is less pronounced for the wide-angle lens compared to the standard lens. A comparison of free viewing and viewing from the correct station-point showed that the latter leads to less overestimation of slope. Since the viewing distance was too far under free viewing, the results are compatible with geometrical optics. In a further experiment the effects of magnification and minification were studied by deliberately viewing the photographs from fixed points closer or further away than the station-point; this led to an increase and decrease in overestimation, respectively. Finally, results are frequently dependent on task characteristics: magnitude judgements of photographs without an anchoring point can only be accurate to a level of scale. Thus using an action-based matching task may lead to more accurate slope perception. We therefore carried out a comparison experiment using a matching task to check for the generality and action-dependence of our results. Practical implications for the use of photographs as surrogates for natural viewing are discussed.