Objects project smaller images on our retinas as they move away from us, which can make it hard to decide if an item is truly small or just far away (as we see in this photograph). Forced perspective photography uses this ambiguity to great effect, while eliminating many of the habitual strategies that our brain uses to distinguish size from distance, such as stereopsis (our visual system can calculate the depth in a scene from the slight differences between our left and right retinal images) and motion parallax (as we move, objects closer to us move farther across our field of view than distant objects do).
Tall and Venti
Is your cuppa joe half empty or half full? It depends on your outlook—and on a little twist on the Jastrow illusion, named after Polish-born American psychologist Joseph Jastrow. In this classic illusion, two identical arches positioned in a certain configuration appear to have very different lengths. Magician Greg Wilson and writer and producer David Gripenwaldt realized that Starbucks coffee sleeves have the perfect shape for an impromptu demonstration of the Jastrow illusion, so now you can amaze your office mates at your next coffee break. All you need to do is align the coffee sleeves as in the accompanying photograph and—presto!—your tall cup sleeve is now venti-sized! Your brain compares the upper arch's lower right corner with the lower arch's upper right corner and concludes, incorrectly, that the upper sleeve is shorter than the lower sleeve. We would like to thank magician Victoria Skye for her demonstration of the Jastrow illusion with Starbucks coffee sleeves.