Vertical diplopia on downgaze with bifocals

To investigate if jump or diplopia is experienced at the dividing line of bifocal lenses and if the belief in jump may be, in part, the result of leading questions.

Ten subjects were asked an open question and 10 a closed question relating to jump for six bifocal lenses and three eye movement conditions.

All subjects failed to see any effects other than near blur with saccadic movements. Many subjects observed vertical diplopia with round 38 segments at the bifocal edge during pursuit movements given an open choice of responses. With pursuit movements, when asked a closed leading question, all subjects saw jump with 38 round segments and approximately 50% saw jump with C28 segments.

Bifocal "jump" is shown to be diplopia if an open question is asked of subjects. This is as would be predicted from simple geometrical optics. Both jump and diplopia are shown to be difficult to perceive with C segment and E-style lenses, even though theoretically present. The relevance of diplopia, as opposed to jump, in adaptation to bifocals is considered.