This investigation is designed to illustrate the ability of the visual system to compensate for changes in retinal image size with viewing distance,i.e the mechanism of size constancy. The accuracy of this mechanism will be evaluated under a number of different viewing conditions and thereby investigating the various cues for judging distance.
‘Size constancy’ is the perception of an object as having a fixed size, regardless of the change in size of the retinal image & the visual angle which accompanies changes in distances (i.e. as an objects moves into the distance, the retinal image decreases yet the object size remains the same). It is thus possible to accurately judge the size of objects by compensating for these changes in retinal image size.
In order to compensate for such changes, we must be able to judge distances accurately & this is done via several cues. Binocular cues include: stereopsis
Stereopsis relies on receiving two slightly different views of the world & hence two different retinal images, due to our eyes existing two slightly different positions laterally. This allows for good depth perception, particularly for near vision. Convergence is another binocular cue in which the eyes adduct. The closer the object, the more our eyes converge. Accommodation is accompanied by convergence.
Monocular cues rely on pictorial clues existing within the retinal image. This allows us to perceive a sense of depth inside of a 2-dimensional object, & relies on certain assumption about the way the world is organised. Monocular cues include: linear perspective (texture gradients)
occlusion or interposition
shading or shadows
Linear perspective is an example of a size cue. The further away an object is, the smaller the retinal image becomes for example a railways track becomes thinner & thinner as it recedes into the distance (figure 1) By...