Relative movement of lower lid and line of sight from distant to near fixation

The difference in extent of movement of the lower lid and of the line of fixation of 107 subjects when the eyes were lowered from the primary plane to the reading position were measured using a stroboscopic camera. The difference in the extent of movement between the two is assumed to indicate the amount by which the lower lid would raise a bifocal contact lens before the pupil. The mean difference was found to be 0.8 +/- 0.347 mm. The area of the pupil occupied by the segment based upon each individual pupil size and difference of lid/fixation movement was also calculated. The mean for the above subjects was found to be 16.9 +/- 14.4%. Both the above values are much less than the amounts heretofore predicated for efficient bifocal vision. Other factors, particularly the upper lid's role in the movement of the contact lens translating bifocal, are noted.