Due to the distance between the eyes, each eye sees the world from a little different angle. These real-world images are projected onto the retina that is flat, or in two dimensions. Stereopsis is the ability of the eye and brain to process images from the real world so that the final perception is not of two different images (one from each eye) but instead of one unified three-dimensional image. Stereopsis is very important for depth perception because the different image from each eye provides clues to distance and size. Some patients, with weak stereopsis, are unable to properly fuse the two images into one unified 3D image, and this results in difficulty operating in the real world, or in some cases can cause double vision. Many types of stereopsis tests are available, most of which present different images to each eye to determine if the patient can properly fuse the images into one.