The first grade of Binocular Single Vision (BSV), Simultaneous Perception (SP), is tested using two dissimilar pictures, such as a lion and a cage. The tubes are adjusted objectively (by the examiner) and subjectively (by the patient) so that either the lion is perceived to be inside the cage or one image is suppressed. Each slide size has been calculated to subtend a different angle at the nodal point of the eye.