Hobbit houses and the Moon trick the brain and eye

Science can't always explain how optical illusions work, writes Jason England.

In an Ames room similarly sized people appear to be quite different. – jeffrey phillips

I recently visited New Zealand for the first time, where I toured “The Shire”, a set from The Lord of the Rings and The Hobbit movie trilogies. The set, which is mostly outdoors, includes many of the small, round doors that lead into the homes of Frodo, Bilbo and their friends. A few of the doors open slightly and upon stepping inside I was immediately reminded of an optical effect used during the filming. The effect tricks the audience’s eye into perceiving the wizard Gandalf as towering over the little hobbits. Although many viewers probably thought computer generated sleight-of-hand was responsible, the juxtaposition is actually an in-camera practical film effect using old optical illusion principles.

The secret lies in the construction of the room. The walls, floor and ceiling are angled in such a manner that a viewer (or camera) looking into the space from the right vantage point will perceive a normal rectangular room. But its actual shape is anything but. One end of the room’s back wall is much closer to the viewer than the other, and the ceiling is steeply sloped. By placing two identically sized people at opposite sides of the room, the one on the left appears tiny, while the one on the right looks like a giant!

Although small versions of these “forced perspective” rooms had been invented before, the first full-size version was created and built by Adelbert Ames in the mid-1940s. Ames was an American lawyer, artist and scientist who began investigating the various ways in which the human eye perceived distance and perspective, and the techniques painters employ to create depth. The more he investigated, the more fascinated he became – which is how the Ames room came about. He left art behind to focus on ophthalmology and psychology, making significant contributions.

As well as the height disparity effect and tricking the eye that the room is rectangular, some early Ames rooms made a third illusion possible: the floor was angled in such a way as to make a large ball placed on the floor appear to roll uphill. As this feature can’t be captured in a photograph, it is often left out of many modern Ames room designs.

The illusion is so strong it persists even when the viewer understands how the room is constructed.

One striking aspect of an Ames room is that the illusion is so strong it persists even when the viewer understands how the room is constructed. In other words, your higher order cognitive abilities cannot override your lower order perceptions. Optical illusion experts refer to this phenomenon as “cognitive impenetrability”.

There isn’t complete agreement as to why an Ames room works. Some experts claim that past experience with normal rectangular rooms is what causes the effect. The brain is forcing itself to interpret the shape of the room as normal because that’s all that it knows. But others have pointed out that individuals from cultures that don’t have rectangular rooms are also fooled by the illusion. The jury is still out.

There are two common events in the night sky where you can see optical phenomena that, while not exactly comparable, are similar to how an Ames room works. The first is in the constellations. Although intellectually we know the stars in the various constellations are hundreds of light-years apart, the absence of any visual cues makes them appear to be essentially equidistant from the Earth and “flattened” against the night sky. In an Ames room, instead of removing any visual cues to distance, the cues are purposefully distorted to fool the eye.

The second example involves an apparent increase in the size of the Moon when it is near the horizon. Many people think this is because the viewer can also see smaller visual cues in the foreground that aren’t usually available when the Moon is directly overhead. The only problem with this explanation is that the Moon also looks larger on the horizon when you’re out at sea. So what’s the explanation? The most likely answer involves a phenomenon called the Ponzo illusion. Whenever you have two objects that appear to be the same size but that are clearly at different distances, the farther object must be larger than the closer object. Imagine a rock in your fingertips and a rock 100 metres away. If they appear to be the same size to you, it’s because the rock 100 metres away is much larger!

In a quirk of human perception, the brain perceives the night sky as if it were a large flattened dome which appears closer directly overhead than it does at the horizon. Subsequently, when the Moon is at the horizon, it appears to be much farther away from us than when it’s overhead. But our brains expect the Moon to appear smaller on the horizon due to this “increase” in distance. When the Moon doesn’t appear smaller, the brain thinks this must be because the Moon is bigger! Like the Ames room, the Moon on the horizon effect is difficult to ignore even when you know the real explanation.

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