Multifocal Designs
Image Jump

Since the reading segment may be thought of as a small plus lens which has been added to a larger lens, then we may also think of it as a set of prisms placed base to base. If we consider only those prisms which are vertically oriented, it would look something like the illustration above. The example shows a 22 mm round bifocal segment where the distance from its center to the periphery is 11mm. The instant the eye enters the segment area (point A above) there is some base down prism induced. To determine how much prism we apply Prentice�s Rule: = F x dec (mm) / 10 ; = 2.00 x 11 / 10 = 2.2 base down. We know that light is always bent toward the base of a prism and the image is displaced toward the apex. In this case, 2.2 diopters of prism is induced displacing the image upward, toward the apex. This is known as image jump since the image appears to �jump� upward the moment the eye enters the segment area. The greater the power of the add and/or the farther the optical center of the segment is from the transition line, the greater the amount of image jump. Since excessive image jump is not desirable, one of the objectives in the development and improvement of multifocal designs has been to minimize image jump.