Comparison of a 43 D cornea with a 45 D cornea. Numbers below the vertical arrows indicate distance from the optical axis in millimeters; numbers to the right of the horizontal arrows indicate the separation between the corneas in micrometers. A typical pupil size of 3.0 mm is indicated. A typical red blood cell has a diameter of 7 µm. Within the pupillary space (ie, the optical zone of the cornea), the separation between the corneas is less than the diameter of a red blood cell.