Blurry backgrounds in photographs can make objects seem smaller than they actually are 鈥� and experiments have given us new clues about what makes this illusion so effective.
Photographers use a technique called tilt shift miniaturisation to make large objects and landscapes look like toy sets. This relies on the fact that our eyes can鈥檛 focus on close-up and distant objects at the same time, so when we focus on an object close to our eyes, the background becomes blurry.
This means that when we see an object with a blurry background in a photograph, our brain is tricked into thinking the object must be close to our faces and therefore miniature, says at the University of York in the UK.
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He says studying illusions like these can reveal how exactly our visual systems work. 鈥淚t shows what our brain is doing without us even realising it,鈥� he says.
To learn more, Baker and his colleagues showed 108 people two images at a time on a computer screen: one of a model train and one of a real train. In some cases, the background of the real train was artificially blurred in different ways. All the images were grayscale as model trains usually look more vivid than real life ones, says Baker.
The participants saw each pair of images for 5 seconds at a time and were asked to say which was of a real train. When they were shown real trains without blurred backgrounds, the participants only gave the correct answer about 67 per cent of the time. 鈥淭he fact it wasn鈥檛 100 per cent is a testament to how close to real life these model trains look and how difficult the task was,鈥� says Baker.
With three different kinds of blur, including one in which the whole image was blurred, the accuracy of the participants was close to 50 per cent. But the participants were even less accurate when the real train was shown with a gradient background blur of the sort used in the tilt shift technique, which made the image blurriest at the edges and less blurry closer to the train. When the photo of a real train was distorted in this way, the subjects chose correctly just 24 per cent of the time.
The participants were also easily fooled by a very crude blur that also covered a lot of the train. They chose the real train 33 per cent of the time in this condition.
鈥淲e think this means that the visual system doesn鈥檛 really care that much about the specifics of the blur,鈥� says Baker; it simply has to obscure most of the background.
He says this may be due to the fact that when we see a blurry background, we want to get rid of it. 鈥淵ou want everything to be in focus, so perhaps we just don鈥檛 have a need for our eyes to be particularly precise in these issues,鈥� he says.
The study shows that we use optical blur as a guide to size, says at the University of Glasgow, UK. 鈥淔iguring out what goes on in illusion is a key way to understand how perception works when all goes well and we perceive the world accurately,鈥� she says. 鈥淲e want to understand how our perceptual systems work for many reasons, not least to figure out how our brains and minds work and therefore understand humans, and other animals, better.鈥�
PLoS One
