Ok, not sure if it qualifies as an illusion, but when I took the pic below I’m pretty sure, as I think the angle indicates, that the cap of the pen was further from the lens then the body of the pen. So why does it appear so much larger? It doesn’t appear this way to my eyes when I reset the scene. Thanks.
Just guesses…
- The cap really is larger (it has to fit over the body of the pen)
- The structure of the pen, where it goes from thick to thin in a couple of stages, could give a sense of false perspective. That is, a normal cylinder that was facing away from you would seem smaller at the far end, due to perspective. Because the pen is smaller at the end, it fools your eye into correcting for the perspective, making it seem bigger at the end. Since that is the clue as to the size of the pen cap, the pen cap looks bigger.
As muttrox said, the cap really is bigger. And the two objects point toward different perspective vanishing points on different horizons. This skewing of perspective might exaggerate the difference in size.
It looks to me the photo was taken with a fairly long focal length lens, or taken with a standard lens and cropped tightly. Either way, the effect is to decrease the effect of perspective. If you move the camera closer to the subject and use a wide-angle lens, the background object would appear much smaller relative to the foreground object and look more natural.
Longer focal length lenses (~100mm) are quite common for close-up work as they allow greater distance between lens and subject for the same size image. Get too close and lighting becomes a problem.
For me, the thing that makes the cap look bigger is the gold ring at its mouth. There is a corresponding gold ring on the pen, but it appears that that ring is sized to snap into the ring at the mouth of the cap.
When you see the pen and its cap side by side, it appears that the cap is bigger because the ring is larger than the pen’s ring. One assumes that the rings are identical, when in reality one is small enough to fit within the other.