This scene shows a larger man chasing a smaller man, if you look carefully you’ll see that the man is one and the same. At first you may say that this is not an illusion, but let’s analyze for a few moments. Perceptual constancy shows the habit to see familiar objects as having standard shape, size, color, or location regardless of changes in the angle of perspective, distance, or lighting.
What to see?
This kind of illustration is a three-dimensional scene with depth relationships. All you can see in the first place are two men: one man that’s in front of you and another that’s in the background. The one in front of you appears to be closer that the one in the background; this thing also raises a questions mark about the reason why they’re the same yet different.
How it works?
When an image recedes into the distance, it gets smaller, in our case the background image remains the same size as the visual angle. The brain, the visual system to be clearer, thinks that since both figures have the same visual angle, but are at differing distances, the one in the background must be larger. This shows us that what we see is not necessarily what we perceive.
This type of experiment depicts two men on a level surface at differing distances. Even if the man in the background seems smaller, it looks normal if you compare it to the one in front of you; you can also see the same thing if you can get the man in the background next to the one in the front.
The important thing, the one that causes the illusion is the size of the scene. The scene in the front is extremely large compared to the one in the background. So the scene gets smaller and smaller from front to back. This causes our visual system not to relay on the depth relationships and it will perceive the two men at different sizes. This kind of trick is used in movies. If you remove the background, entirely, the scene will be perceived as flat and the illusion will no longer be created.