Retinal afterimages are so cool. Basically, your visual system is an active process. For example, a small object close to you and a large object far away can have the same retinal image size. How then do you know that the two images are in fact different from one another? Because of distance and depth cues. Even though the retinal image is the same size, you can tell the one that’s further away is larger because to make that retinal image size from that distance, it has to be big.
A really cool thing to try out is how this affects afterimages. You know when you stare at something for a long time and then when you look away you can see the afterimage? Well, stare at a huge black dot from far away for a while. Then, look at a blank white space in front of you. The image you see is a lot smaller. Now look across the room – the image is a lot bigger.