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Emmert’s law demonstrates how retinal size is determined by a combination of object size and viewing distance. If you look at a bright object on a dark background (or vice versa) and then close your eyes, you see a ghostly afterimage. If you then look at a screen close to you, it looks like a relatively small ghost, but if you look at a screen far away, it looks like a big ghost.

Afterimage is a visual illusion causing retinal impressions to retain a phantom-like image of a previously present stimulus. The afterimage can appear to be “positive” or similar in brightness and/or color to the original image, or “negative” which would show colors complementary and less bright than that of the original image. A common example of an afterimage is the square of light one might see after a camera flashes. Afterimage is believed to be caused by continued activity in the visual system after a stimulus has been removed.

It’s actually really hard to guess how far away a visual object is. As with many other visual problems, our visual system usually solves this one easily. However, images close to the eye and far from the eye arrive in the same place in the retina. Our brains have learned that retinal size is not an indication of actual object size, so we’re always reaching for other cues to figure out how big something is.