Figure 2.

Fixation shift through full-field prisms. For simplicity, we illustrate right eye only with a base-in full-field prism. Note that we assume the flat spectacle frame (orange solid line) is orthogonal to the head direction, and the fronto-parallel plane is orthogonal to the line of sight to the fixation target (green cross mark) with no prism. The solid lines indicate the actual ray path from the fixation target to the retina (through the full-field prism in B-D), and the dashed lines show the apparent path. (A) When a patient with left homonymous hemianopia or right acquired monocular vision (right seeing eye) fixates on a far fixation target with the eye at the primary position of gaze, the fovea aims at the fixation target. (B) The image of the fixation target through the full-field prism is shifted toward the apex of the full-field prism (see dashed blue line and apparent cross image shifted from the fixation direction). The fixation target is now imaged off the fovea. The patient may rotate (C) the eyes and/or (D) the head and eyes together away from the blind side to foveate on the fixation target through the full-field prism, which may negate the field-of-view shift towards the base (red solid line).