Bifocal lenses are a common solution for presbyopia. These lenses correct both distance and near vision within a single lens, separated by a visible line. While effective, a specific optical side effect known as “image jump” can be distracting. This phenomenon causes the visual world to shift abruptly when the eye moves from the distance portion to the near portion of the lens. Understanding the cause of this jump and which lens design eliminates it is the first step toward a more comfortable viewing experience.
What Causes the Bifocal Image Jump
Image jump is the displacement of an object’s location that occurs when the line of sight crosses the dividing line of a segmented bifocal. This shift is caused by the prismatic effect introduced by the reading segment, which acts as a plus lens. The base of the prism is located at the segment’s optical center.
As the eye rotates downward and crosses the segment line, it instantly encounters a fixed amount of base-down prism. Light rays are bent toward the base of this prism, which displaces the perceived image upward, creating the sensation of the object “jumping.” The magnitude of this jump is directly related to two factors: the power of the near addition and the distance between the segment’s optical center and the segment line.
Common Segmented Bifocal Styles
The most widely used segmented bifocal is the Flat-Top, often called the D-segment, which features a straight dividing line and a segment shaped like a sideways letter D. The optical center of a standard Flat-Top segment is typically located about 3 to 5 millimeters below the segment line. This offset placement of the optical center directly results in the image jump when the eye moves into the reading area.
Another common style is the Round segment, which has a circular shape and is generally less conspicuous than the Flat-Top. Round segments usually have their optical center at the geometric center of the circle, which can be 11 millimeters below the segment line for a 22mm segment. This greater distance from the line means the Round segment creates the largest amount of image jump among common segmented designs.
The Design That Eliminates Image Jump
The segmented bifocal design that eliminates image jump is the Executive bifocal, also known as the Franklin bifocal. This unique lens is constructed from two distinct lens sections fused together, creating a segment that extends across the entire width of the lens. This design provides an extremely wide field of view for near tasks.
The mechanism that solves the image jump problem is the placement of the near segment’s optical center. In an Executive bifocal, the optical center of the reading portion is located precisely on the dividing line. Because the optical center is aligned directly with the segment line, there is zero prismatic effect induced at the point of transition. The absence of a sudden change in prism power means the image does not experience a displacement or jump when the eye crosses the line. The primary drawback of the Executive lens is its physical profile; because the near and distance powers are ground onto the full surfaces of the lens, it can be noticeably thicker and heavier than other segmented designs. This greater thickness makes it less cosmetically appealing than Flat-Top styles.
Choosing Between Segmented and Progressive Lenses
Segmented bifocals, even the Executive style, provide only two distinct viewing powers: distance and near. Progressive Addition Lenses (PALs) feature a seamless, lineless design that offers a gradual change in power from distance at the top to near at the bottom. This continuous transition means PALs also do not have image jump and are often preferred for their modern aesthetic.
However, the wide field of view provided by segmented lenses like the Executive remains a significant advantage over PALs. Progressive lenses achieve their seamless transition through a “progressive corridor” that can result in narrower intermediate and near viewing areas, often with peripheral distortion. For individuals who require the widest possible, distortion-free field for prolonged close-up work, the Executive bifocal is often the superior choice despite the visible line. The choice between a segmented and a progressive lens depends on lifestyle, occupational needs, and personal preference, making consultation with an eye care professional the best way to determine the correct lens type and fitting.