The problem is just as insoluble for our visual systems as it is for algebra students. We cannot determine two unknown quantities from one equation. There are an infinite number of solutions. The cactus in front of you could be just inches away and tiny, or it could be miles away and very, very large. The visual system relies on a host of cues that help determine which is really the case. But we can create visual illusions that demonstrate our visual system does not always produce the right answer.

Next you will learn more about imaging optics and the distance/size problem!