Figure P-4 – Ideal Distance between Lens and Image

Harris recommends that the distance between the lens and the image in an optical box be the same as the focal length of the lens. The reason is that the lens brings the eyes closer to parallel when they view any point on the image, just as you see illustrated here. In this application the lens is not creating an image of some distant object that must be focused in the way that a lens in a camera does. So the role of focal length must not be understood in those terms here.

 

Since each eye focuses on the same point, its own lens will gather the dispersed rays coming from that point on the image and refocus them to a point at the back of the eye (the Retina). I have indicated that additional complexity in the illustration. The brain will merge the two identical points and see them as one. Since the eyes are turned parallel, the brain should infer that that constructed point is in the distance and feed that perception to consciousness, so that you "see" it in the distance, rather than on a piece of paper close to you. The principle of depth perception here is binocular convergence.