One of the primary jobs of visual perception is to build a three-dimensional representation of the world around us from our flat retinal images. These are rich source of depth cues, but no single one of them can tell us about scale.
The research team from the University of York and Aston University has shown that the human visual system can ‘trick’ the brain into making inaccurate assumptions about the size of objects in the world around them.
The restricted depth of field that every optical system can achieve natural results in image blur gradients, which can be used to gauge visual scale. Participants in the study were shown photos of small-scale railway models that were not blurred, as well as photos of full-scale railway settings with the upper and lower portions of the image blurred.
They created what is commonly referred to as fake tilt-shift miniaturisation by manipulating image blur artificially.
Participants were asked to evaluate each image and select the one that best represented a full-scale, “real” railway scene. The findings showed that participants thought the real trains, which were hazy, were smaller than the models.
Dr. Daniel Baker, from the University of York’s Department of Psychology, said: “In order for us to determine the real size of objects that we see around us, our visual system needs to estimate the distance to the object.”
“To arrive at an understanding of absolute size it can take into account the parts of the image that are blurred out – a bit like the out-of-focus areas that a camera produces – which involves a bit of complicated mathematics to give the brain the knowledge of spatial scale.”
“This new study, however, shows that we can be fooled in our estimates of object size. Photographers take advantage of this using a technique called ’tilt-shift miniaturisation’, that can make life-size objects appear to be scale models.”
The results show how adaptable the human visual system is; sometimes, it can accurately perceive size by taking advantage of what is known as “defocus blur,” but other times, it is susceptible to other factors and is unable to understand the size of actual objects.
Professor Tim Meese, from Aston University, said: “Our results indicate that human vision can exploit defocus blur to infer perceptual scale but that it does this crudely.”
“Overall, our findings provide new insights into the computational mechanisms used by the human brain in perceptual judgments about the relation between ourselves and the external world.”