People have always been interested in optical illusions. With the invention of photography, illusions experienced a renaissance. Even in sculpturing and painting, the beginnings of three-dimensional spaces could be discerned. In the last century, people began to understand these illusions. Especially optical illusions that affect the sense of sight play a special role in the deception of perception. The human eye perceives two-dimensional images of its environment. Nevertheless, one has the feeling of being able to see three-dimensionally. The relativity of the point of view is particularly important for photography.
How are optical illusions created?
“Illusions arise when our brain tries to recognise the future but the result does not correspond to reality,” Mark Changizi once explained. He is a researcher in the field of computational neuroscience. It sounds a bit crazy, but scientists like Changizi have found that people can see into the future – at least for a fraction of a second. This effect is also known as neuronal delay. When light hits the retina of our eye, it takes about a tenth of a second for our brain to translate the signal into a three-dimensional environment. Based on this, Changizi has discovered that our brain tries to compensate for this neuronal delay. It produces images of what could happen in a tenth of a second in the future. Converging lines, for example, make our brain think that we are moving forward. This mechanism also explains why we are deceived by optical illusions.
The forced perspective
Many three-dimensional geometric objects are captured in a photograph on a two-dimensional layer. This creates distortions that lead to an illusionistic spatial organisation. In architectural photography, optical illusions are used as stylistic devices, for example to achieve optical enlargements and to make a building appear more monumental.
The forced perspective is therefore an optical illusion. Dimensions seem to be shortened and lead viewers astray. Important elements for a successful illusion are the object, the camera position and the viewer’s point of view. For a plausible illusion that is not easy to see through, a few considerations should be made beforehand. Most important is the question of the object(s) to be represented.
For a better understanding, we will explain this using the example of the Leaning Tower in Pisa. Popular pictures show how people lean against the tower with one hand or save it symbolically from falling down. In our example it looks as if the tower could be held in both hands. In this case, we use the effect that the tower in the photo appears much smaller than it actually is. This works as long as the camera is not tilted downwards. This would reveal the spatial distance between the two objects and the viewer would realise that the two objects do not stand next to each other. Considering the size of the tower, however, this is rather secondary. In general, however, the following applies to forced perspectives: a neutral or positive camera orientation is better. With these orientations, it is more difficult to discern spatial depths in photographs, since the objects do not “reveal” their third dimension (depth).
In everyday life, however, we encounter even more optical illusions. In architectural photography, for example, falling lines are important. Anyone who has ever stood close to a skyscraper will have noticed it. In a photo, the edges of the house taper upwards or seem to tilt backwards. This is due to the square photo sensor. This distortion can be avoided with tilt shift lenses.
Optical illusions using focal length
Even different focal lengths can be used for tricks. The relative ratio of foreground to background can be varied. A small example: The starting point is a subject captured with a focal length of 18 mm. Now you move away from the foreground, because you want to keep the object there the same size. With increasing focal length, you notice that while the foreground object remains the same size, the background object seems to be moving forward more and more – thus, the distance between both objects becomes optically smaller. This effect is also described as the Vertigo effect.
Optical illusions can be used for many purposes in photography – there are no limits to your imagination. You can hold the sun in your hands or support the Leaning Tower of Pisa.